FOUS30 KWBC 090758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 9 2024

Day 1
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

Overall pattern evolution will continue to point to a benign
instability axis (MUCAPE ~500-1000 J/kg) being confined closer to
the Gulf with sufficient deep layer shear and moisture presence
(PWATs b/w +1 to +2 deviations) allowing for a small area of
convective development capable of locally enhanced rainfall rates
for flash flood potential. Given the overall antecedent dry
conditions prior, the prospects of flash flooding will likely be
relegated to more urban settings along the Central Gulf coast,
including places such as New Orleans, Biloxi, and Mobile. A few
other Parishes within Southeastern LA also contain areas that
experience a bit easier ability to flash flood given some of the
above urbanization factors, so decided to maintain the previous
SLGT risk inheritance with only some minor modifications made to
the overall risk area.

Latest 00z HREF neighborhood probabilities for >3" of rainfall are
within the low-end 20-25% range with highest prospects likely
between 1-2" when taking relevant CAMs into account. This is
similar to the previous forecast prob fields from the 12z HREF
iteration, and with coordination from the local WFOs along the Gulf
Coast earlier for the SLGT risk, there was no need to deviate from
what was proposed in the last update. Best chance will reside from
training echoes on persistent west-southwest flow during the
diurnal instability maximum, carrying into the evening before the
setup decays prior to the next period of relevant convection for
D2.

Kleebauer


Day 2
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEASTERN
ALABAMA TO UPSTATE SOUTH CAROLINA...

As the previous shortwave trough kicks to the northeast, a stronger
mid-level trough will dig quickly out of the Rockies into the
Southern Plains by Tuesday morning with sights downstream on areas
east of the Mississippi. Increasing meridional flow ahead of the
digging trough will aid in the advection of deep moisture poleward
with the initial surge propagating north out of the Gulf through
the Southeast by Tuesday afternoon, eventually spreading northward
into the Southern and Central Mid Atlantic the second half of the
period. The advection pattern will be sufficient in increasing
regional mid-level instability as profile moisture saturates deeper
into the profile across the areas above. PWAT anomalies on the
order of 2-3 deviations above normal will be common over parts of
GA spreading up through the neighboring Carolinas, eventually
surging into the Central Mid Atlantic and Northeastern U.S by the
end of the D2 cycle. This will prime several areas for heavy
rainfall prospects as the final ingredients come to together for a
locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into Tuesday afternoon and
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through Central GA, aligning with the cold front as it moves
through the region. The eventual heavy rain footprint will spread
northeastward into the Southern Mid Atlantic with locally heavy
rainfall impacting areas as far north as Virginia with some lighter
precip making headway into areas further north. Places across
Southwestern VA down to the I-40 corridor in NC will see anywhere
from 1-2" of rainfall with locally higher further south through the
zone. 2-3" with local totals to 4" will be plausible for the area
extending from Southeastern AL through Central GA into Upstate SC,
an area more prone to flash flood prospects thanks to the
complexity of the local terrain.

This signal has been prevalent over the past series of
deterministic model outputs and is now firmly within the ensemble
mean as bias corrected ENS now supports 2.5-3" through much of the
area outlined above in the 2-3" forecast. The saving grace in this
setup is the time of year allowing for lower surface based
instability parameters and very dry antecedent conditions leading
into the event. This allowed for a SLGT risk to be sufficient for
the setup, a carry over from the previous forecast. The MRGL risk
still extends into VA where the northern periphery of the heavy
rainfall is expected.

Kleebauer


Day 3
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

As the longwave pattern evolves further, the neutral-tilted trough
from previous D2 time frame will enhance further with the mean
trough taking on a more negative tilt allowing for enhanced surface
low strengthening as the surface low progresses northward into New
England and eventually Quebec. PWAT anomalies on the order of 2-3
deviations above normal will quickly progress northward
encompassing much of New England by Wednesday afternoon, even
getting up towards +4 deviations as we move into Wednesday night
over Northern New England from NH through ME. This pattern is
textbook for a corridor of heavy rainfall to spread from the Mid
Atlantic through all of the Northeast with the primary areas of
interest location along and ahead of the cold front that will sweep
eastward during the forecast period. Portions of Western NY state
and Northern PA will eventually change to winter precipitation
allowing for a decay in flash flood concerns, however areas
downstream will be under a respectable deluge of rainfall prior to
the rain threat ceasing in wake of the front.

Totals of 1.5-2.5" of rain are forecast across much of Long Island
up through Southern New England, eventually moving up through ME
as we close out the forecast period. 1-2" is forecast for points
further west, but those areas also have the added snow melt factors
that will play into the flooding concerns as high dewpoints and
heavy rain will promote rapid snow melt for places in the interior
over VT/NH and the neighboring Berkshires in MA. The expectations
are for some creeks and smaller streams to become problematic in
the setup and could cause localized flood concerns during the peak
of the heavy rainfall. The rivers across New England can thankfully
take a decent surge of moisture after a very dry fall, so the
prospects of significant flooding are very low at this point.

The setup is still worthy of a large SLGT risk encompassing much
of New England with the best threats likely over urbanized areas in
LI and Southern New England and over those zones in the interior
where rapid snow melt has historical precedence for localized
flash flood concerns. A MRGL risk will encompass the SLGT risk
across Northeast with an extension down into Central and Eastern NC
as the early portion of the forecast period will still see
moderate to heavy rainfall Wednesday morning before pulling
northward.

Kleebauer
$$
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FOUS30 KWBC 100756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Tue Dec 10 2024

Day 1
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL ALABAMA
THROUGH PORTIONS OF THE WESTERN CAROLINAS...

The overall synoptic pattern remains on track to deliver a swath of
heavier rainfall to portions of the Southeast and Southern Mid
Atlantic during the period. A strong mid- level trough is currently
analyzed over the Four Corners region, digging quickly to the
southeast with sights on the Southern Plains by later this morning.
As the mean trough pivots across the Southern Plains near OK/TX,
increasing meridional flow ahead of the digging trough will aid in
the advection of deep moisture poleward with the initial surge
propagating north out of the Gulf through the Southeast by the
afternoon, eventually spreading northward into the Southern and
Central Mid Atlantic the second half of the period. The advection
pattern will be sufficient in increasing regional mid- level
instability as moisture saturates deeper into the profile across
the areas above. PWAT anomalies on the order of 2-3 deviations
above normal will be common over parts of GA spreading up through
the neighboring Carolinas, eventually surging into the Central Mid
Atlantic and Northeastern U.S by the end of the D1 cycle. This
will prime several areas for heavy rainfall prospects as the final
ingredients come to together for a locally enhanced precip pattern.

The trough to the west will begin tilting more neutral with a
strengthening upper jet positioning allowing for large scale ascent
to increase in earnest as we move into the afternoon hours and
beyond. The main trigger for a line of enhanced convection will
form at the hands of a strong surface cold front that will shove
eastward beneath an intensifying surface low that develops in
conjunction to the favorable upper level evolution. Current
deterministic sfc to 850mb wind field indicates a corridor of
backed flow across much of GA into the Carolinas during Tuesday
evening ahead of the cold front allowing for widespread rainfall to
develop downstream of the approaching front. As the cold front
swings eastward by late Tuesday night through early Wednesday, the
peak rainfall rates will be occurring across the Carolina Piedmont
down through central GA into portions of east-central AL, aligning
with the cold front as it moves through the region. The eventual
heavy rain footprint will spread northeastward into the Southern
Mid Atlantic with locally heavy rainfall impacting areas as far
north as southwestern Virginia with some lighter precip making
headway into areas further north.

Recent 00z HREF output remains steadfast with the prospects of a
widespread 1-3" of rainfall across a large chunk of the Southeast
into the Highlands of the Western Carolinas with the northern
extension of any maxima peeking into southwestern Virginia where
some of the terrain will make use of the southeasterly upslope
component to the wind field. Probability fields in both the
neighborhood and EAS realm are pretty solid when it comes to the
potential with the latter of which indicating a well-defined axis
of 30-50% for at least 2", a signal prevalent historically for
favored SLGT risks or higher for the current impact areas.
Neighborhood probabilities are of course higher in percentile given
how the prob field is calculated, however the most notable output
is the very high 60-80% coverage for >3" and low-end probs of
20-25% for up to 5" across the area southeast of Birmingham up
through the Atlanta metro. This was a small shift northward in
terms of the heaviest QPF axis from previous forecasts and align
well with the latest ECMWF and GFS ML outputs, a testament to
increasing agreement within the deterministic on where the heaviest
rainfall will occur. The saving grace from this setup being a more
prolific outcome is the lack of sufficient surface based
instability that tends to add a greater convective element to the
rainfall rates that shift the signal to a higher-end risk threat.
Thankfully, that is not the case here, so the threat does remain a
bit capped on the upper bound of potential.

The previous SLGT risk was adjusted a bit further northwest across
AL/GA to reflect the change within the axis of heaviest rainfall
putting the Atlanta metro squarely within the SLGT risk forecast.
The SLGT remains firmly in-place across Upstate SC with a general
northward extension into the Appalachian front of southwestern NC
as well as small eastward expansion into the Charlotte metro to
atone for the higher QPF being signaled as of the latest 00z
guidance (All in coordination with the local WFO in
Greenville/Spartanburg). The MRGL risk remains generally in a
similar place compared to previous forecast, but some minor
adjustments to the northwest across AL and northward a bit further
into western VA with the most notable inclusion of the Birmingham
metro where the urban flash flood threat will lie right on the edge
where guidance is depicting the heaviest rainfall.

Kleebauer


Day 2
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

The previous forecast remains largely unchanged as the synoptic
regime anticipated for mid-week continues to trend favorable for
heavy rainfall across much of the Northeast and Mid Atlantic. The
neutral- tilted trough from previous D1 time frame will enhance
further with the mean trough taking on a more negative tilt
allowing for enhanced surface low strengthening as the surface low
progresses northward into New England and eventually Quebec. PWAT
anomalies on the order of 2-3 deviations above normal will quickly
progress northward encompassing much of New England by Wednesday
afternoon, even getting up towards +4 to +5 deviations as we move
into Wednesday night over Northern New England from NH through ME.
This pattern is textbook for a corridor of heavy rainfall to spread
from the Mid Atlantic through all of the Northeast with the
primary areas of interest located along and ahead of the cold front
that will sweep eastward during the forecast period. Portions of
Western NY state and Northern PA will eventually change to winter
precipitation allowing for a decay in flash flood concerns, however
areas downstream will be under a respectable deluge of rainfall
prior to the rain threat ceasing in wake of the front.

Totals of 2-3.5" of rain are forecast across much of Long Island
up through Southern New England, eventually moving up through ME as
we close out the forecast period. 1-2" with locally higher amounts
are forecast for points further west, but those areas also have
the added snow melt factors that will play into the flooding
concerns as high dewpoints and heavy rain will promote rapid snow
melt for places in the interior over VT/NH and the neighboring
Berkshires in MA. The expectations are for some creeks and smaller
streams to become problematic in the setup and could cause
localized flood concerns during the peak of the heavy rainfall. The
rivers across New England can thankfully take a decent surge of
moisture after a very dry fall, so the prospects of significant
flooding are lower than normal at this point.

The previously inherited forecast was largely unchanged as the
setup and subsequent forecast is still worthy of a large SLGT risk
encompassing much of New England with the best threats likely over
urbanized areas in LI and Southern New England and over those
zones in the interior where rapid snow melt has historical
precedence for localized flash flood concerns. A MRGL risk will
encompass the SLGT risk across Northeast with an extension down
into Central and Northeastern NC as the early portion of the
forecast period will still see moderate to heavy rainfall Wednesday
morning before pulling northward.

Kleebauer


Day 3
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer
$$
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FOUS30 KWBC 110803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Dec 11 2024

Day 1
Valid 12Z Wed Dec 11 2024 - 12Z Thu Dec 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL AND
EASTERN LONG ISLAND UP THROUGH MUCH OF NEW ENGLAND...

Potent longwave trough is currently aligned over the Mississippi
Valley with a tilt closer to neutral as of the latest UA analysis.
A vigorous 160kt speed max at 250mb was analyzed over the Ohio
Valley on the leading side of the trough with sampled winds from
the 00z KILN sounding indicating those 160kt winds are verified.
This signals a very potent upper level evolution with an
anticipated holding of strength the next few hrs before a further
strengthening is anticipated later this morning and afternoon as
the trough axis begins tilting more negative allowing for a jet
buckling downstream as the speed max enters New England and
Quebec. At the surface, multiple waves of low pressure are centered
north to south from the eastern Great Lakes, down through the
Southern Appalachians as they lie within the elongated cold front
extending down from the northern low to the Central Gulf coast. The
forecast is for these waves of low pressure to propagate northward
during the next 3-6 hrs before the surface low over the lakes
becomes the dominant SLP feature, strengthening rapidly as the
trough buckles and the low pressure centered gets positioned firmly
within the right entrance region of the very powerful jet over
northeast North America.

The evolution above will lead to a large swath of rainfall to
encompass much of the area along and east of the Appalachian front
with a trailing axis of convection situated over the southern half
of the Carolinas down into FL as the precip aligns along and just
ahead of the surface cold front. Frontal progression will lead to
rainfall shifting eastward overtime, however the environment ahead
of the front will be significantly enhanced with deep layer
moisture advecting poleward well ahead of the mean trough. PWAT
anomalies are already over +2 deviations across a good portion of
the Northeast U.S down through the Southeast with anomalies
expected to climb closer to +3/+4 deviations as me move into the
late-morning afternoon hours over the Northern Mid Atlantic into
New England. Assessment of local bufr soundings across Southern New
England from Central LI up through CT/MA/NH/ME signal a deep, uni-
directional wind field with modest theta_E advection occurring
during the morning and afternoon time frame as the warm-conveyor
points directly through the area with a well-defined warm front
bisecting the area by 18z.

The time frame between 18-00z will be the most significant time
frame with regards to heavy rainfall potential across much of New
England as the cold front begins racing eastward and the warm front
tilts southwest to northeast from Western CT up through Southern
ME. This alignment will become a focus of a strong, cold rain band
that is textbook in these evolving inland synoptic pressure falls
leading to a period where rainfall rates will be a solid 0.5-1"/hr
extending within that convergence zone. The line will advance
eastward eventually as the cold front approaches and kicks the
convergence axis downstream leading to the heavy rain threat
shifting more towards the coastal regions with sights on Eastern
MA/NH/ME towards the 00-03z time frame. Backed flow off the
Atlantic will aid in maintaining the convergent field along and
ahead of the advancing front with heavy rains lingering until the
front back finally pass through and the winds shift west to
northwesterly ushering in colder air and stabilizing the mid-levels
enough to limit the heavier precip. Prior to this occurring, a
solid 2-3" with locally as high as 4" of rainfall are anticipated
in the zone encompassing Central and Eastern LI, much of MA/NH and
the western half of ME. Rapid snow melt will be a problem for
areas located in the Northern Berkshires and Worcester Hills
towards the MA/NH border extending northward as NOHRSC snow depth
analysis pins a solid 2-4+" on the ground currently within those
zones with over a foot in parts of NH/VT/Western ME. This will
allow for potential small stream cresting and ice jam concerns in
the interior that would exacerbate flood concerns during the
heaviest of the precip time frame later this afternoon.

An expanse of Flood Watches are positioned across much of the
above areas, including all of VT as complex terrain and higher snow
depths in the Green Mountains and surrounds leading to higher than
normal flood potential, despite the heaviest rain footprint likely
a bit further east. Considering the above variables and in
coordination with local WFOs, the inherited SLGT risk was
relatively unchanged with the only shift being an extension a bit
further southwest into CT to cover for urban flood concerns within
the axis of expected heavy rainfall during the cold frontal band
being forecasted across much of the 00z CAMs suite. The MRGL risk
still extends around the SLGT and further south into the Central
Mid Atlantic and Carolinas with the primary concerns likely
isolated urban flood potential, especially in the larger metro
corridors like Raleigh, DC/Balt, and Philadelphia.

Kleebauer


Day 2
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

Fairly robust disturbance over the Pacific will translate
southeastward towards the North-Central CA coast by the end of the
week with a persistent IVT pulse oriented southwest to northeast
into the coastal plain over the course of the forecast period
leading to waves of moderate precipitation to impact areas north of
the Bay area. Current progs indicate precip totals on the order of
2-4" with locally higher embedded within the coastal ranges
spanning north of Santa Rosa. Current rates anticipated to be
modest with upwards of 0.5-0.75"/hr at peak intensity lending to a
lower end threat for flash flooding, however totals are significant
enough to warrant a low-end threshold MRGL, mainly for the coastal
urban areas of Northwest CA up through Southwestern OR where
totals will be on the order of 2-4", especially into the windward areas
of the Klamath Range. MRGL was generally maintained from previous
forecast with only some minor modifications to align with the
latest guidance.

Kleebauer
$$
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FOUS30 KWBC 120805
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
305 AM EST Thu Dec 12 2024

Day 1
Valid 12Z Thu Dec 12 2024 - 12Z Fri Dec 13 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA AND SOUTHWESTERN OREGON...

The overall forecast maintained a general continuity in the
expected synoptic evolution of a weak atmospheric river transpiring
across the Northwest CA coast, extending northward into Southwest
OR. IVT signatures between 700-1000 kg/ms will advect into the
coastal plain with some protrusion inland on the initial surge as
the southwest to northeast alignment of the pulse relegates a
majority of the moisture into the coast and adjacent coastal ranges
that will garner the greatest QPF just given the upslope component
of the wind field. A widespread 2-4" with local max of 5-5.5" is
forecast over the period with the general maxima likely within the
coastal King Range as is customary with IVT pulses exhibiting a
southwestern angle of approach across Northern CA. Rainfall rates
on the order of 0.25-0.5"/hr will be most common within these bands
of precipitation, however a few instances of >0.5"/hr might be
realized during the period between 00-12z Saturday when the primary
surge is forecast amongst the 00z deterministic suite. This
correlates well with the anticipated IVT maximum making headway
into the coastal plain which could lead to localized flooding
concerns in the expected zone(s) of impact. As of this juncture,
the heaviest rainfall will lie north of Santa Rosa with the primary
areas of interest likely positioned from the King Range up US101 to
the southern half of the Siskiyou in southwestern OR. With this
alignment, very little change was necessary from the previous MRGL
risk issuance, thus the forecast was relatively unchanged as the
MRGL risk was sufficient in consensus.

Kleebauer


Day 3
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA INTO THE CENTRAL SIERRA FOOTHILLS...

Atmospheric River setup from the previous period will translate
into D3 with a greater inland protrusion of the IVT pulse as the
upper level trough and associated surface low pivot onshore with
an expected wind shift that will mute heavier rain prospects along
the coast and eventually inland as we move into the second half of
the period. The threat is currently very marginal in nature within
the confines of the inland topography with the eastern foothills
of the Sierra's being the furthest east expanse of any heavy rain
threat, but even that is bordering on the lower end of a risk
threshold. This period is more of the transition point in the
pattern as the event will see a general decay in total rainfall and
overall rates as the system moves inland. There is a further south
push of the IVT pulse in the early portions of Saturday with some
modest dynamical element involved as the 500mb vorticity evolution
does bring some significant forcing towards the North- Central CA
coastal plain with impacts to places like Santa Rosa down into the
Bay area. Thankfully the pattern is progressive, but a quick 1-2"
is within the realm of possibility for a mainly urban zone which
could bring about some localized flooding prospects before the
disturbance moves inland and we get a quick end to the chances.

A MRGL risk continues for a good portion of Northwest CA with the
risk area now extending down into the Bay area compared to the
previous D2 alignment. A MRGL risk is also located at the base of
the Sierra foothills for locations below 5500ft.

Kleebauer
$$
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FOUS30 KWBC 130826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 AM EST Fri Dec 13 2024

Day 1
Valid 12Z Fri Dec 13 2024 - 12Z Sat Dec 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

Maintained the Slight Risk area over portions of northwest
California with only minor adjustments needed based on latest
deterministic and ensemble guidance. Rainfall should be growing in
both areal coverage and intensity today as a plume of moisture is
directed from the eastern North Pacific into northwest California.
Event total rainfall amounts through early Saturday morning are
still forecast to be in the 3 to 5 inch range across the Slight
Risk area. By late tonight/early Saturday morning the heavier rain
will shift southward as a strong mid level shortwave approaches the
region. This feature will act to increase large scale ascent,
while also cooling the mid levels enough to allow for some weak
instability to develop within the IVT axis. The 13/00Z HREF
neighborhood probabilities of rainfall exceeding 0.5" in an hour
are over 50% across most of the Slight risk...both across the most
favored upslope areas and also within a narrow southward shifting
axis of stronger convergence. This does not appear to be a high end
event given the progressive nature of the system...although some
creek and stream flooding and a few mudslides will be possible.

Bann


Day 2
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

Rainfall associated with an atmospheric river will be on-going as
the Day 2 period begins at 14/12Z...with the plume of deepest
moisture continuing to be directed in to parts of northwest
California although the axis of heaviest rainfall rates/amount will
be gradually be easing eastward in time with the main shortwave
while being shunted southward ahead of the surface cold front.
southward with time. The higher rates should quickly diminish with
the departure of the shortwave moves...with the risk of heavy rain
over by 18z. Thus this Slight risk is really only for the first few
hours of this day 2 period.

Bann

Day 3
Valid 12Z Sun Dec 15 2024 - 12Z Mon Dec 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.|

Bann
$$
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FOUS30 KWBC 140829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Dec 14 2024

Day 1
Valid 12Z Sat Dec 14 2024 - 12Z Sun Dec 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
COASTAL CALIFORNIA FROM SANTA ROSA INTO SAN Francisco AND SAN JOSE...

Even though a much of the areal coverage of moderate to locally
heavy rainfall associated with an atmospheric river has already
moved on shore...maintained the Slight risk area mainly along the
coast from Santa Rosa southward into San Francisco and San Jose
given an expected up-tick in rainfall rates as a compact shortwave
trough approaches from the west. Short term guidance from the
convective allowing mesoscale guidance shows several hundred Joules
per kilogram of CAPE moving into that portion of California
immediately ahead of the shortwave impulse. The 14/00Z HREF
maintained a nearly 100 pct chance of half-inch per hour rainfall
rates and nearly 40 percent chance of 1 inch per hour rates over
and near the San Francisco Bay area during the early- to mid-
morning hours. The combination of the rainfall rates and the
urbanization of the area still points to an elevated risk of flood
impacts. The overall impacts should be limited by the fairly short
duration of heaviest rainfall...which looks to taper off quickly in
the 15Z to 18Z period.

Bann


Day 2
Valid 12Z Sun Dec 15 2024 - 12Z Mon Dec 16 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Mon Dec 16 2024 - 12Z Tue Dec 17 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent

Bann
$$
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FOUS30 KWBC 170751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Tue Dec 17 2024

Day 1
Valid 12Z Tue Dec 17 2024 - 12Z Wed Dec 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE EAST CENTRAL TO SOUTHEAST COAST OF FLORIDA...

Model consensus is for increasingly active showers late Tuesday
afternoon into Tuesday night/early Wednesday along the east central
to southeast coast of Florida. Low level easterly to southeasterly
flow in an axis of PW values 1.5 to 2+ standard deviations above
the mean expected to persist into the east central to southeast
Florida coast, with models showing an axis of defined boundary
layer convergence in this low level inflow axis parallel to the
east central to southeast coast. There will be potential for very
slow moving cells along the coastal regions, supporting locally
very heavy rainfall amounts and an urban flash flood threat. HREF
neighborhood probabilities are high (50-90%) for 3"+ amounts from
Cape Canaveral, south to Miami, while the 5"+ neighborhood
probabilities are high (60-90%) from near West Palm Beach, south to
Miami. The best hi res overlap, evident in the greatest EAS
probabilities for 2"+ amounts, is from near West Palm, south to
just north of Miami.

Oravec


Day 2
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 3
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec
$$
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FOUS30 KWBC 180823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 AM EST Wed Dec 18 2024

Day 1
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF THE
OHIO AND TENNESSEE VALLEYS ALONG WITH THE SOUTHEAST FLORIDA COAST...

...Parts of the Mid-South to Lower Ohio Valley...
Maintained the Marginal Risk area across parts of western and central
Tennessee into southern Kentucky as models still advertise an
upper-level trough amplifying to the west which allows for a more
curved upper level jet streak on the lee side of the trough. That
configuration may result in some enhanced rainfall rates...an idea
supported by the 18/00Z HREF neighborhood probabilities showing
potential for rainfall rates exceeding an inch per hour between
12-18Z today (mainly in Tennessee). With rainfall blossoming to
the west of the outlook area in the pre-dawn hours this morning
helping to prime the soils a bit more and a general slowing of the
eastward progression across the Tennessee Valley compared with
earlier forecasts...opted to extend the Marginal risk area a bit
farther westward than previously indicated. The HREF probability of
exceeding flash flood guidance remained low but did reflect
somewhat higher probabilities west of the same guidance from the
17/12Z and 17/18Z runs. The decision to expand westward slightly
was also driven by the observed rainfall during the past day or two
over parts of northwest Tennessee and southwest Kentucky which has
resulted in the latest 1-hour and 3- hour flash flood guidance
coming down and the 0-40km soil moisture percentiles per NASA SPoRT
have climbed to 60-80% for most areas.

...Southeast FL...
Post frontal easterly low level flow off the Atlantic will linger
allowing for a possible repeat of convective clusters over and
near the southeast Florida coastline today. The 00Z runs of the
high-res CAMs continued to show onshore low-level moisture
flux/convergence and sufficient deep-layer instability to support
localized downpours. The 18/00Z HREF supports rates of 2+ inch per
hour peaking around 10 percent during the afternoon with and a
50-70 percent 40km neighborhood probabilities of at least 5 inches
of additional rain and 30-50 percent probabilities of 8+ inches.
Given that these numbers are comparable with the 17/12Z and 17/18Z
runs of the HREF...saw little reason to make too many changes other
than nudges to the boundary of the previously issued Marginal risk
area.


Bann

Day 2
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Fri Dec 20 2024 - 12Z Sat Dec 21 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann
$$
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FOUS30 KWBC 210756
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Sat Dec 21 2024

Day 1
Valid 12Z Sat Dec 21 2024 - 12Z Sun Dec 22 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The next, more prominent atmospheric river is forecast to impact
the NorCal coast into southwestern OR by the end of the D2 into D3
time frame with a better aligned IVT pulse anticipated for areas
that saw significant rainfall totals over the past 7-10 days. The
heaviest rain will be confined to the immediate coast with a
southwest to northeast oriented moisture flux expected leading to
upslope flow a bit orthogonal to the coastal terrain extending from
the King Range up through the Siskiyou in southwest OR. Current
forecasted totals within the ensemble suite are between 2-4" with
some deterministic output between 4-5" at max. This is generally
within the confines of a low-end MRGL risk threshold when assessing
historical precedence for these types of events. The main
difference between this event and the previous was the primary
hydrometeor will come in the form of rainfall, even inland as lower
heights will be confined offshore and a flood of warmer air in the
boundary layer will protrude inland. Only the highest elevations of
the Northern Sierra and Mount Shasta will have a higher threat for
winter ptypes, so this points some of the more sensitive areas of
northern CA into play where burn scar remnants are located.

The inherited MRGL from the D4 was relatively maintained for the
areas outlined in CA, but the prospects for flash flooding were
pretty low given the QPF signature for locations north of
southwestern OR, so decided to pull back the northern extension
along the OR coast. The MRGL extends inland to include those more
sensitive areas within the complex terrain, even extending to the
foothills of the Northern Sierra where heavier precip will make its
way by the end of the forecast cycle and likely carrying beyond to
the D4 period.

Kleebauer
$$
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FOUS30 KWBC 220759
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Sun Dec 22 2024

Day 1
Valid 12Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

There is a non-zero opportunity for isolated flash flood concerns
across portions of the eastern FL coastline, mainly along the Space
Coast near Coco Beach up to Cape Canaveral and Titusville. A
developing surface trough off the coast will nose into the
coastline later this evening with flow turning more perpendicular
to the coast allowing for increased frictional convergence in a
small zone within the trough. HREF neighborhood probabilities are
hovering around 20-30% at peak for >3" in any spot within the area
above, very much on the lower end of any flash flood threshold, and
mainly below the current FFG indices for 1/3/6 hr time frames.
More robust deterministic has closer to 4.5-5" over a short period
of time the back end of the forecast period, but most of the CAMs
maintain the heaviest precip offshore. The prospects are very low,
but wanted to make mention that threat is non-zero.

Kleebauer


Day 2
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

Consensus continues to grow for a quick hitting weak to borderline
moderate atmospheric river aimed at southwestern OR down into
northern CA by late Monday into Tuesday morning. Recent ensemble
depictions are in agreement on an IVT pulse centered over the
northern CA coast, protruding inland through the end of the period
with warm air advecting well into the boundary layer and above
yielding a majority rainfall signal across even some of the higher
terrain inland. NAEFS output for PWAT anomalies indicates a tongue
of 2-3 deviations above normal moisture plume being ushered in on
deep layer southwest flow thanks to a shortwave trough undercutting
a broad upper level cyclone located off the PAC Northwest coast.
This look is fairly textbook for a period of prevailing
southwesterly flow running orthogonal to some of the adjacent
coastal ranges that are situated from southwest OR down through
northern CA. The zones most impacted will lie at the foothills of
Mount Shasta and the northern Sierra Nevada for the inland portion,
as well as the King and Siskiyou Ranges that align from OR down
through northwestern CA. Rainfall totals on the order of 3-5" with
locally higher in the highest elevations of the above ranges are
forecast during the time frame beginning 21z Monday through 12z
Tuesday with some carryover into D3 before the main shortwave moves
inland. 2-4" will be common within the coastal plain with locally
higher in areas south of Eureka. Localized flooding prospects are
higher than normal given the stronger IVT pulse being depicted,
also impacting areas that saw decent rainfall as of a week to 10
days prior. The saving grace is the time frame is not very long in
terms of impact, so that helps limit a greater flash flood threat.

The previously inherited forecast was generally maintained outside
some fine tuning across the Sierra Foothills and the northern
periphery of the MRGL risk into OR.

Kleebauer


Day 3
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

..Sierra Foothills..

Atmospheric river from previous period will bleed into the D3 time
frame with the primary shortwave trough quickly propagating inland
with increased forcing upon arrival into the Northern and Central
Sierra, less so for areas back into the coast due to negative
vorticity advection. Locally enhanced rainfall will be plausible
the initial portion of the period with the heaviest likely aligned
along those Foothill regions of the Northern Sierra, mainly within
elevations below 8000ft, although snow levels will fall below
7000ft towards the end of the more significant precipitation time
frame Tuesday morning. Additional totals of 1-3" are possible
within a 6-10 hour window prior to the precip ending leading to a
low-end potential for flash flood concerns just outside the
Valley that extends from Redding down to Sacramento. Higher runoff
capabilities due to terrain orientation and soil moisture anomalies
running closer to normal will present some potential for flood
concerns despite this being an event that doesn't maintain a more
prolonged precipitation signature. Considering limited deviation
from run-to-run amongst guidance with considerable agreement in
timing and magnitude of precipitation, continuity from the previous
forecast was maintained with very little in the way of adjustment
in the inherited MRGL risk across the Sierra Foothills.

..Arklatex and Lower Mississippi Valley..

Surface ridge to the northeast will slowly drift further to the
northeast allowing for a more broad return flow regime to affect
areas across the Western Gulf into the Lower Mississippi Valley.
Southerly flow will advect a bit more unstable air northward with
areal theta_E averages on the increase when assessing the ensemble
means and initial D3 cluster analysis with heavy weight towards the
GEFS and ECENS output. Aloft, a shortwave will dig southeast with
enhanced mid-level energy accompanying aiding in better regional
forcing as we work into Tuesday afternoon and evening. This will
help setup a period of convective initiation across Eastern TX and
points north with the primary axis of heaviest precip likely
aligned southwest to northeast across the Arklatex down into the
eastern Hill Country located east of I-35. Weak jet coupling will
help with the expansion of the precip field across the above areas
with those embedded convective signatures likely situated along and
ahead of an advancing cold front as surface cyclogenesis in-of the
Red River will become a focus for activity in question. Locally
heavy rainfall will be forecast within those stronger convective
cores, however the PWAT anomalies and available surface based
instability will be modest, at best with regards to the setup. This
will likely lead to more spotty instances of >3" of total precip
in any one location with the best areas of interest mentioned above.

The previous MRGL risk was maintained, but did trim some of the
eastern flank of the risk area as probabilities off the blend are
pretty meager and agree with the current ML output basing best
chances to the northwest of where the risk area was drawn. A small
extension to the southwest was done out of prospects for the
traditional southwestern bias in heavier QPF within these types of
setups. This was featured within a few of the regional
deterministic and works well with the proposed elevated SBCAPE
anticipated in that area.

Kleebauer
$$
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FOUS30 KWBC 221925
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
225 PM EST Sun Dec 22 2024

Day 1
Valid 16Z Sun Dec 22 2024 - 12Z Mon Dec 23 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

Update...

The 12Z guidance did not suggest any significant changes were
required, therefore made only minor adjustments to the previous outlook area.

Previous Discussion...

Consensus continues to grow for a quick hitting weak to borderline
moderate atmospheric river aimed at southwestern OR down into
northern CA by late Monday into Tuesday morning. Recent ensemble
depictions are in agreement on an IVT pulse centered over the
northern CA coast, protruding inland through the end of the period
with warm air advecting well into the boundary layer and above
yielding a majority rainfall signal across even some of the higher
terrain inland. NAEFS output for PWAT anomalies indicates a tongue
of 2-3 deviations above normal moisture plume being ushered in on
deep layer southwest flow thanks to a shortwave trough undercutting
a broad upper level cyclone located off the PAC Northwest coast.
This look is fairly textbook for a period of prevailing
southwesterly flow running orthogonal to some of the adjacent
coastal ranges that are situated from southwest OR down through
northern CA. The zones most impacted will lie at the foothills of
Mount Shasta and the northern Sierra Nevada for the inland portion,
as well as the King and Siskiyou Ranges that align from OR down
through northwestern CA. Rainfall totals on the order of 3-5" with
locally higher in the highest elevations of the above ranges are
forecast during the time frame beginning 21z Monday through 12z
Tuesday with some carryover into D3 before the main shortwave moves
inland. 2-4" will be common within the coastal plain with locally
higher in areas south of Eureka. Localized flooding prospects are
higher than normal given the stronger IVT pulse being depicted,
also impacting areas that saw decent rainfall as of a week to 10
days prior. The saving grace is the time frame is not very long in
terms of impact, so that helps limit a greater flash flood threat.

The previously inherited forecast was generally maintained outside
some fine tuning across the Sierra Foothills and the northern
periphery of the MRGL risk into OR.

Pereira/Kleebauer


Day 3
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

Update...

The 12Z guidance did not suggest any significant changes were
required, therefore made only minor adjustments to the previous outlook areas.

Previous Discussion...

..Sierra Foothills..

Atmospheric river from previous period will
bleed into the D3 time frame with the primary shortwave trough
quickly propagating inland with increased forcing upon arrival into
the Northern and Central Sierra, less so for areas back into the
coast due to negative vorticity advection. Locally enhanced
rainfall will be plausible the initial portion of the period with
the heaviest likely aligned along those Foothill regions of the
Northern Sierra, mainly within elevations below 8000ft, although
snow levels will fall below 7000ft towards the end of the more
significant precipitation time frame Tuesday morning. Additional
totals of 1-3" are possible within a 6-10 hour window prior to the
precip ending leading to a low-end potential for flash flood
concerns just outside the Valley that extends from Redding down to
Sacramento. Higher runoff capabilities due to terrain orientation
and soil moisture anomalies running closer to normal will present
some potential for flood concerns despite this being an event that
doesn't maintain a more prolonged precipitation signature.
Considering limited deviation from run-to-run amongst guidance with considerable agreement in timing and magnitude of precipitation,
continuity from the previous forecast was maintained with very
little in the way of adjustment in the inherited MRGL risk across
the Sierra Foothills.

..Arklatex and Lower Mississippi Valley..

Surface ridge to the northeast will slowly drift further to the
northeast allowing for a more broad return flow regime to affect
areas across the Western Gulf into the Lower Mississippi Valley.
Southerly flow will advect a bit more unstable air northward with
areal theta_E averages on the increase when assessing the ensemble
means and initial D3 cluster analysis with heavy weight towards the
GEFS and ECENS output. Aloft, a shortwave will dig southeast with
enhanced mid-level energy accompanying aiding in better regional
forcing as we work into Tuesday afternoon and evening. This will
help setup a period of convective initiation across Eastern TX and
points north with the primary axis of heaviest precip likely
aligned southwest to northeast across the Arklatex down into the
eastern Hill Country located east of I-35. Weak jet coupling will
help with the expansion of the precip field across the above areas
with those embedded convective signatures likely situated along and
ahead of an advancing cold front as surface cyclogenesis in-of the
Red River will become a focus for activity in question. Locally
heavy rainfall will be forecast within those stronger convective
cores, however the PWAT anomalies and available surface based
instability will be modest, at best with regards to the setup. This
will likely lead to more spotty instances of >3" of total precip
in any one location with the best areas of interest mentioned above.

The previous MRGL risk was maintained, but did trim some of the
eastern flank of the risk area as probabilities off the blend are
pretty meager and agree with the current ML output basing best
chances to the northwest of where the risk area was drawn. A small
extension to the southwest was done out of prospects for the
traditional southwestern bias in heavier QPF within these types of
setups. This was featured within a few of the regional
deterministic and works well with the proposed elevated SBCAPE
anticipated in that area.

Pereira/Kleebauer
$$
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FOUS30 KWBC 230758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 23 2024

Day 1 Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The next atmospheric river will make headway into southwest OR and
northwest CA coastal plain with a protrusion of elevated IVT inland
as we work the back end of the period. Consistency among all major deterministic, both global and hi-res indicate a widespread area of
2-4" with locally as high as 5.5" in-of the impacted areas, mainly
north of Santa Rosa up into southwest OR. Latest HREF probs for >3"
are very high (80+%) across areas like the King and Siskiyou Ranges
along with the Foothills of Mount Shasta. Despite the elevation, a
strong warm nose with this event will send snow levels spiking
upwards with the base pushing close to 8000ft MSL for the
rain/snow delineation point. This will create a better heavy
rain threat even away from the coast with areas inland also
maintaining a threat for localized flash flood concerns, especially
as we move into early Tuesday morning. The heaviest rainfall will
likely be within those coastal ranges which are some of the harder
areas to flood, so that will help limit the extensive flash flood
prospects we see with some events. The progressive nature of the AR
regime will also aid in the anticipated impacts, however the threat
is still within the low to medium end of the MRGL risk threshold
leading to a continuance of the MRGL from the previous forecast
issuance. The area(s) with the greatest potential are those that
are still dealing with burn scar aftermath with very sensitive
runoff capabilities. Those are included within the MRGL risk,
especially across northern CA.

Kleebauer


Day 2 Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
SIERRA FOOTHILLS AND EAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

..Sierra Foothills..

Atmospheric river from previous period will bleed into the D2 time
frame with the primary shortwave trough quickly propagating inland
with increased forcing upon arrival into the Northern and Central
Sierra, less so for areas back into the coast due to negative
vorticity advection. Locally enhanced rainfall will be plausible
the initial portion of the period with the heaviest likely aligned
along those Foothill regions of the Northern Sierra, mainly within
elevations below 8000ft, although snow levels will fall below
7000ft towards the end of the more significant precipitation time
frame late Tuesday morning. Additional totals of 1-2" are possible
within a 6-10 hour window prior to the precip ending leading to a
low-end potential for flash flood concerns just outside the Valley
that extends from Redding down to Sacramento. Higher runoff
capabilities due to terrain orientation and soil moisture anomalies
running closer to normal will present some potential for flood
concerns despite this being an event that doesn't maintain a more
prolonged precipitation signature.

The previously inherited MRGL risk was generally maintained,
however some of the risk area was cut out due to provide a gradient
between the Sierra locations that will trend to more winter
precipitation and the areas that will remain liquid through much
of, if not the entire duration of the event.

..Arklatex and Lower Mississippi Valley..

Surface ridge over the eastern CONUS will slowly drift further to
the northeast allowing for a more broad return flow regime to
affect areas across the Western Gulf into the Lower Mississippi
Valley. A steady flux of higher theta_E's will begin moving
northward out of the Western Gulf, carrying as far north as the Red
River before stabilizing as we reach into OK. Aloft, a digging
mid- level shortwave will exit the Central Rockies with sights on
the Southern Plains leading to enhanced left exit region dynamics
and surface cyclogenesis in-of the Red River Valley with a cold
front extending from the base into TX and a small warm front
lifting northward around the Arklatex. The classic mid-latitude
cyclogenesis will create a sector of modest destabilization with
the primary axis aligned from southwest to northeast across east TX
up through the Arklatex, eventually extending northward into AR
and southern MO. The current indications are the best organized
convective schemes will be situated around the small warm sector
along and ahead of the approaching cold front with relative
buoyancy and increasing upstream shear allowing for scattered
convection to develop late Tuesday afternoon through the evening.

As of this time, the deep layer moisture pattern is still
relatively meager compared to some of the more impactful events
that occur in this area of the country, however there is enough
instability and PWAT anomalies creeping between 1-1.5 deviations
above normal to constitute some isolated flash flood concerns,
mainly within those stronger cores. SPC D2 risk includes a targeted
Marginal Risk for severe weather, overlapping the inherited MRGL
risk ERO for the period. Areal rainfall averages will be between
0.5-1" across east TX and 0.75-1.5" across AR, but there is a
growing consensus among the CAMs to have scattered instances of a
quick 2-3" of rainfall within the best convective environment
during the pattern evolution with a 5-10% risk of >3" within the
tail end of the 00z HREF neighborhood probs. This threat is likely
still within the lower end of the MRGL risk threshold, but the
convective premise was enough to maintain general continuity from
the previous issuance.

Kleebauer


Day 3 Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
WASHINGTON AND OREGON...

Another atmospheric river surging off the Pacific will make an
appearance into the Pacific Northwest by Wednesday afternoon
through the end of the D3 cycle. Model guidance is keen on a surge
of moisture represented by fairly elevated IVT signatures on the
order of 600-800 kg/ms within the global ensemble blend. There is
some discrepancy between one of the main deterministic (GFS) and
the other global members carrying more of a 25th percentile outcome
in total precip due to a less robust IVT pulse comparatively. When
assessing the ensemble means from the GEFS compared to the
deterministic, the ensemble sways more in favor the scenario of
greater magnitudes into the IVT channel leading to a more
pronounced atmospheric river regime. This trend leaned more into
maintaining continuity in the inherited MRGL risk across the PAC
NW, although a few changes were made in the proposal.

The first change was to scale back on the eastern extension of the
MRGL risk due to considerably less deep layer moisture advecting
inland at this juncture for the D3 time frame. FFG indices are
still pretty high all the way towards the windward side of the
Cascades, so the prospects for FFG exceedance were pretty low and
generally fall below the 5% threshold criteria. The second change
was to cut out the higher elevations in the Olympic Peninsula due
to the primary ptype trending towards snow with more emphasis on
rainfall closer to the coast and below 4000ft MSL. The risk area
still encompasses the lower elevations surrounding the Olympics and
channels into the Olympic National Forest on the southern flank of
the Peninsula. Anticipate totals of 2-4" with locally as high as 5"
in the risk area extending from coastal southwest OR up through the
Olympic coast of WA.

Kleebauer
$$
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FOUS30 KWBC 261951
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 PM EST Thu Dec 26 2024

Day 1
Valid 16Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EAST TEXAS
THROUGH THE ARKLATEX AND ADJACENT AREAS OF THE LOWER MISSISSIPPI VALLEY...

...16z update...
Recent observational trends, 12z Hi-Res CAMs and HREF
probabilities further support initial Day 1 reasoning and placement
of Slight Risk area across E TX into the Arklatex and Lower MS
Valley later today. As such, only slight westward adjustments were
needed with this update, to account for slightly earlier convective
initiation observed in the observational trends.

Additionally, southward adjustment with Marginal Risk in central
CA also appears on solid track with the arrival of the next AR wave
in the later 6-12 hours of the Day 1 period.

Gallina

~~~Prior Discussion~~~

..East Texas...ArkLaTex...Lower Mississippi Valley..

GOES satellite imagery shows a strong shortwave trough digging
southeastward across the southern Rockies which will be ejecting
and amplifying out across the southern Plains this afternoon and
the Lower MS Valley tonight. The mid to upper-level trough is
expected to become negatively tilted this evening as it crosses
through the ArkLaTex and moves downstream, and there will be a
substantial amount of upper-level jet energy/forcing in place via
the arrival of a 90+ kt jet streak and associated jet couplet which
the guidance has been quite consistent in adverting. Coinciding
with this will be surface cyclogenesis and the northward advance
of a warm front into the Lower MS Valley which will foster the
northward advance of moisture and instability. The 00Z HREF
consensus favors a convergent low-level jet increasing to the
order of 50 to 70 kts this evening across areas of far eastern TX
through western and northern LA and much of southern AR and western
MS. Strong convection will be developing over areas of central and
especially eastern TX this afternoon and impacting much of the
broader ArkLaTex region and the Lower MS Valley this evening and
overnight. Multicell and supercell thunderstorm activity is likely
given the available moisture and instability transport and the
enhanced shear profiles. The convection should tend to become more
progressive in time with a QLCS evolution expected tonight, but
concerns with cell-merger activity and some periodic cell-training
will support rainfall rates reaching 1 to 2 inches/hour with some 2
to 4+ inch storm totals possible. Scattered areas of flash
flooding, especially around the more sensitive urban locations,
will be possible. This will also be aided by the fact that at least
locally there has been recent rainfall that has moistened the soil
conditions and increased the streamflows. Therefore, the Slight
Risk area has been maintained and locally expanded to account for
the heavy rainfall threat and antecedent conditions.

...Pacific Northwest through Northern California...

The arrival of a new Pacific shortwave trough and associated
atmospheric river surge will bring a new round of heavier rainfall
to the coastal ranges of the Pacific Northwest on down through
northern CA later today through Friday morning. The shortwave
energy will be quite vigorous and will be amplifying which will
drive strong cyclogenesis just offshore of western WA/OR tonight.
Enhanced IVT magnitudes around the southern flank of the low center
are expected to impact especially southwest OR and northwest CA
with values increasing to as much as 750 to 1000 kg/m/s. This
strong moisture transport and related warm air advection will
facilitate rainfall rates increasing to as much as 0.50" to
0.75"/hour at least briefly as the core of the stronger IVT arrives
and with aid from orographic ascent over the coastal ranges.
Additional rainfall amounts by Friday morning with this next
atmospheric river surge are expected to be on the order of 3 to 6
inches, with the heaviest totals likely focused over southwest OR
and northwest CA. Given the level of additional rains and
wet/saturated soil conditions, there will be increasing concerns
for more flooding impacts. The Marginal Risk area will be
maintained across the region, except it has been expanded somewhat
farther south in northern CA to include the San Francisco Bay
metropolitan area given expectations for heavy rain arriving here
by early Friday morning along with some urban flooding concerns.

Orrison


Day 2
Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF MISSISSIPPI, ALABAMA AND
TENNESSEE...

...1930 UTC Update...
Minor tweaks made to both Marginal Risk areas, based on the latest guidance/trends (now within the high-res CAM windows). Still
expecting the risk areas to remain sub-Slight, based on the degree
and duration of favorable moisture transport, also supported by the
latest CSU ERO first guess fields.

Hurley

...Previous Discussion...

...Southeast...

A Marginal risk was maintained across portions of MS, AL and TN with
this update. Convection will likely be ongoing at 12z Friday along
this corridor as a front pushes eastward. The front slows and starts
to wash out during the day Friday as large scale forcing weakens.
This weakening of the front and forcing decreases confidence in
convective evolution, as these factors could result in decreased
convective coverage/organization and little to no flash flood risk.
However a corridor of modest convergence will likely still exist
near the fading front, and some chance outflow from earlier
convection could locally enhance this convergence. There will be a
supply of instability upstream, and so if convection is able to
persist then there is a potential for a few training segments to
evolve within the Marginal risk area. Given the dry antecedent soil
conditions and streamflows, and only a conditional threat of a few
training segments, this is generally considered a lower end Marginal
risk. However given this risk was only recently introduced, and the
continued non-zero risk of localized training resulting in 3"+
amounts, we will hold on to the Marginal for now and continue to
monitor trends. The risk area was shifted a bit off to the southeast
to better match where the latest models depict the best chance of
these localized higher rainfall amounts.

...Northwest...

After what should be a relative lull in rainfall to start the
period, another round of stronger IVT should move into the Pacific
Northwest Friday night. 24 hour rainfall totals through 12z Saturday
will generally not be too significant. Most areas are looking at 1-
2", with peak values around 3" possible from far northwest CA into
far southwest OR. These amounts themselves would typically not pose
much of a flood concern. However it has been a wet recent stretch
over this area, and thus soul saturation and streamflows are running
well above average. Given this increased sensitivity, the uptick in
rainfall coverage and intensity Friday night may continue to pose
some flood risk.

Chenard
$$
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FOUS30 KWBC 270800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Fri Dec 27 2024

Day 1 Valid 12Z Fri Dec 27 2024 - 12Z Sat Dec 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PACIFIC NORTHWEST AND PORTIONS OF THE MID-SOUTH...

...Northwest...

After what should be a relative lull in rainfall for most of the
day, there will be another round of stronger IVT/atmospheric river
activity arriving this evening and continuing overnight across the
Pacific Northwest with an emphasis on southwest OR and northwest
CA. This will be driven by the arrival of another Pacific shortwave
trough and attendant surface low that will be yielding an eastward
advance of strong low to mid-level flow. The latest GFS/ECMWF
solutions suggest IVT values here surging upwards of 750+ kg/m/s in
the 00Z to 12Z time frame tonight across especially southwest OR
and far northwest CA. A combination of strong warm air advection,
enhanced moisture transport and orographic ascent over the coastal
ranges will support 0.25" to 0.50"/hour rainfall rates with these
rates likely exceeding a 0.50"/hour at least occasionally across
areas of Coos, Curry and Del Norte Counties. More broadly across
the region, the additional rainfall amounts should reach 1 to 2
inches by early Saturday morning, but the latest HREF guidance
suggests some of the favored windward slopes seeing as much as 2 to
4 inches where the rainfall rates will be highest in conjunction
with the stronger IVT values. Given the continuation of the very
wet pattern across the region and with streamflows already running
high, these additional rainfall totals may pose concerns for
increasing runoff problems and flooding. Thus, a Marginal Risk for
excessive rainfall remains in place for this period.

...Mid-South...

A Marginal Risk remains depicted across portions of MS, AL, TN and
has been tweaked to include a small part of northwest GA for this
update. A few broken bands of convection will be crossing through
central and northern MS this morning and gradually advancing
downstream across areas of central and northern AL, middle TN and
far northwest GA going through the afternoon and early evening
hours. The activity will be associated with the northeast ejection
of a strong mid-level shortwave trough out of the Lower MS Valley
early this morning which will cross the Mid-South and lift into the
OH Valley. Convection should remain focused in close proximity to
a trailing front, but instability along the boundary should be
quite modest and this coupled with the stronger forcing lifting
away off to the northeast should favor the activity being quite
disorganized overall with fairly modest rainfall rates. A low-
end, very modest threat for some runoff problems will exist if the
broken pockets of convection near the front can train over the same area.

Orrison

Day 2 Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

...Northwest...

A stalling front and IVT plume will allow for a prolonged period of
moderate to locally heavy rain Saturday into Saturday night across
portions of northwest CA into southwest OR. The magnitude of the
event should peak Saturday night as IVT values likely exceed 750
J/kg/m^2 and large scale forcing increases ahead of an approaching
mid level trough. Not expecting there to be any instability to work
with, but the increasing IVT and forcing should allow for rainfall
rates around a half inch per hour, and the stalling of the front/IVT
axis will allow these higher rates to persist. There are some model
solutions that even suggest 5-7" of rain is a possibility.

Even though this area is used to seeing rainfall of this magnitude
without significant impacts, in this case this rain will be falling
after what has already been a prolonged wet period. So saturated
soil conditions and elevated stream and river levels will likely
increase the susceptibility to flooding. For this reason think we
will see a notable increase in the flood risk over this area by
Saturday night, justifying the Slight risk. The inland extent of the
heavier rain could also end up greater than normal, with 1-3"
possible even in inland areas of southwest and central Oregon.

Given that setup, in coordination with MFR/Medford, OR forecast
office, the inherited Slight Risk was expanded northeastward to
include much of Coos, Douglas, and Jackson Counties in Oregon. An
internal higher end Slight was introduced to include Coos & Curry
Counties in OR and Del Norte County in CA. The lion's share of the
rainfall expected in this Day 2 period will be from the evening
through much of the overnight. Since the AR will remain relatively
stationary through this period, expect widespread 4-6 inch rainfall
amounts. This will be added to the rainfall expected today into
tonight and well-above-average river levels already established in this area.

...Lower Mississippi Valley into the Southeast...

A swath of heavy rain is likely across portions of the Southeast
into the TN valley on Saturday into Saturday night. A strong
shortwave at the base of the longwave trough will eject eastward
into the area, accompanied by a strong upper jet. These two features
support robust divergence aloft and support a widespread convective
threat. The system as a whole should remain progressive, however as
low level moisture transport increases there does appear to be a
window for some training/backbuilding of convection. Overall this
looks to be a widespread 1-2" event, however localized swaths of
heavier amounts are likely. Given the magnitude of mid/upper forcing
in place, combined with PWs increasing over the 90th percentile for
late December, and sufficient upstream instability forecast...it
seems plausible that we could see localized swaths of 3-4".

As yesterday, it appears with some instability and potential for
training convection that there remains a reasonable likelihood that
somewhere in the Marginal Risk area that Slight Risk impacts will
materialize. Unfortunately, guidance remains all over the map as to
where that potential will be realized, from the mountains of the
western Carolinas to central Louisiana. Given last evening's
rainfall and several Flash Flood Warnings, it seemed prudent to
expand the inherited Marginal south and west to cover much of
Louisiana. While the heaviest totals remain most likely over
northern MS/AL and TN, the dry antecedent soil conditions should
preclude more than isolated Slight Risk impacts...so the large
Marginal Risk remains for this update.

Wegman/Chenard

Day 3 Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

...Northwest...

The A.R. moving into the Northwest will weaken through Sunday
morning. Thus, rainfall rates should diminish with time. Since the
bulk of the Day 2/Saturday A.R. will occur overnight Saturday night,
much of the impacts of the A.R. will be felt with the diminishing
rainfall on Sunday morning. Thus, for consistency both in messaging
and the timing of the worst impacts, a small Slight Risk area was
introduced with this update for the southwest corner of Oregon and
the northwest corner of California. This area will be the hardest
hit area through Saturday night, so the continued rainfall into
Sunday morning will have the greatest impacts in this area.
Otherwise, expect continued rainfall all up and down the Pacific
Northwest, with high elevation snow.

...Mid-Atlantic...

A mature low over the Midwest will track north up the Mitten of
Michigan and into Ontario by Monday morning. Ahead of the low's cold
front, a plume of subtropical moisture will stretch up the entire
Eastern Seaboard. The combination of the moisture and much warmer
temperatures will support widespread light to moderate rain over
much of the Mid-Atlantic. While much of the Mid-Atlantic has been
dry, especially south of the Mason Dixon line, some upslope may help
wring out the moisture along the I-81 corridor through New York and
the I-95 corridor from Richmond through NYC. Any localized training
may cause widely scattered instances of flash flooding.

Wegman
$$
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FOUS30 KWBC 280815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sat Dec 28 2024

Day 1
Valid 12Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA ALONG WITH PORTIONS OF EASTERN
TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND MID-SOUTH...

...Northwest...

A front will be in the process of gradually stalling out in a west
to east fashion this morning across areas of southwest OR and
northwest CA as the deeper layer Pacific flow becomes parallel to
the boundary. This will maintain atmospheric river conditions today
across the coastal ranges which will be rather modest at least for
this morning. However, the last of a series of offshore shortwaves
embedded within the deeper layer Pacific flow should approach
later today and cross the region tonight which will yield a strong
resurgence of IVT magnitudes that should approach and locally
exceed 750 kg/m/s. This will be driven by shortwave amplification
and the development of a new area of surface low pressure which
will move into the Pacific Northwest by early Sunday morning.
Rainfall rates are expected to increase once again as the strong
warm air advection pattern ahead of the shortwave combine with the
stronger low to mid-level moisture transport into the terrain. The
00Z HREF guidance supports rainfall rates rising to locally over a
0.50"/hour, but they will tend to peak along and just ahead of the
arrival of the offshore cold late tonight. Some stronger convective elements/heavy showers are suggested in the HREF guidance with
passage of the cold front closer to 12Z/Sunday and this may foster
some spotty rainfall rates peaking into the 0.75" to 1.0"/hour range.

This latest atmospheric river will then weaken as the front moves
inland, but additional rainfall amounts of as much as 3 to 6+
inches are expected, with the heaviest amounts likely tending to be
in vicinity of Coos, Curry and Del Norte Counties involving the
favored coastal ranges of southwest OR and northwest CA. Lesser
amounts are expected to advance farther south down the coast, with
some locally heavy rains likely making it as far south as the north
side of the Bay Area Sunday morning along with the northern Sierra
Nevada. Overall, given the wet antecedent conditions and high
streamflows, these additional rains are likely to increase the
flooding threat on area streams and tributaries and thus the Slight
Risk area is maintained for the coastal ranges. The Marginal Risk
area for this update was tweaked and adjusted a bit farther south
to account for some heavy rainfall getting closer to the
aforementioned Bay Area.

...Eastern Texas into the Lower Mississippi Valley/Mid-South and
Ohio Valley...

A well-defined and organized outbreak of strong convection is
expected to begin this morning across portions of central and
especially eastern TX as a strong shortwave trough amplifies
across the southern Plains and drives cyclogenesis from west to
east while interacting with an increasingly strong southerly low-
level jet. A substantial pool of warm-sector instability with
MLCAPE values of 1500 to 2500 J/kg has already pooled across south-
central to southeast TX and stretching over into southern LA. This
will be lifting north through midday as a warm front lifts north
ahead of the approaching surface low. Strong convection with
widespread concerns for supercell thunderstorms will evolve by
midday and through this afternoon across eastern TX with impacts
overspreading the Arklatex and broader Lower MS Valley by this
evening. The latest HREF model consensus supports the low-level jet
increasing to as much as 40 to 60+ kts across far eastern TX, LA,
southern AR and into western and northern MS by 00Z. As this
unfolds, the aforementioned warm front attempting to lift north
ahead of the surface low should become a focus for widespread
convection with potentially multiple rounds of very heavy rainfall
impacting the same area. Given the level of moisture and
instability transport coupled with strong shear, the stronger and
more organized convective cells will be capable of producing
rainfall rates of 1.5 to 2.5 inches/hour. Concerns for cell-mergers
and cell-training will exist and by this evening, some regional
swaths of 2 to 4 inches of rain with isolated 5+ inch totals will be possible.

Going into the overnight hours, the strong shortwave energy should
take on a bit of a negative tilt as it approaches the Mid-South and
aims for the OH Valley. This will drive the strong low-level jet
up through large areas of MS, AL, TN and nosing up toward the OH
River by early Sunday morning. Strong bands of heavy showers and
thunderstorms with a large degree organization should impact the
region, with a QLCS evolution expected to generally unfold across
much of the Mid-South going through the overnight hours given
enhanced deep layer forcing/shear and robust moisture and
instability transport. Localized swaths of 2 to 4 inches of rain
will also be possible across the Mid-South where any cell-training
can take place.

Given the wet antecedent conditions and elevated streamflows
across much of eastern TX and the Lower MS Valley, scattered to
locally numerous areas of flash flooding are expected to evolve,
and some of the flash flooding may be locally significant. The
flash flood threat will be a bit more conditional downstream across
the Mid-South and especially areas farther north into the OH
Valley, but given the heavy rainfall potential, the threat of flash
flooding will certainly exist. Given the wetter trend in the 00Z
hires model CAMS and broader convective outbreak that is expected,
a much more expansive Marginal Risk has been accommodated along
with a large Slight Risk area involving eastern TX, the Lower MS
Valley and basically all of the Mid-South. It cannot be ruled out
that a need for a targeted Moderate Risk upgrade will exist with
the next cycle update across some portions of the Slight Risk area.

Orrison

Day 2
Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

...Northwest...

The A.R. moving into the Northwest will weaken through Sunday
morning. Thus, rainfall rates should diminish with time. Since the
bulk of the Day 1/Saturday A.R. will occur overnight Saturday night,
much of the impacts of the A.R. will be felt with the diminishing
rainfall on Sunday morning. Thus, for consistency both in messaging
and the timing of the worst impacts, the Slight Risk area was
continued with this update with few changes for the southwest corner
of Oregon and the northwest corner of California. Otherwise, expect
continued rainfall all up and down the Pacific Northwest, with high
elevation snow.

...Mid-Atlantic...

A mature low over the Midwest will track north up the Mitten of
Michigan and into Ontario by Monday morning. Ahead of the low's cold
front, a plume of subtropical moisture will stretch up the entire
Eastern Seaboard. The combination of the moisture and much warmer
temperatures will support widespread light to moderate rain over
much of the Mid-Atlantic. While much of the Mid-Atlantic has been
dry, especially south of the Mason Dixon line, some upslope may help
wring out the moisture along the I-81 corridor through New York and
the I-95 corridor from Richmond through NYC. Any localized training
may cause isolated instances of flash flooding, especially should
any training of heavier rain occur over flood-sensitive and urban areas.

Wegman

Day 3
Valid 12Z Mon Dec 30 2024 - 12Z Tue Dec 31 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 281950
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Sat Dec 28 2024

Day 1 Valid 16Z Sat Dec 28 2024 - 12Z Sun Dec 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA ALONG WITH PORTIONS OF EASTERN
TEXAS THROUGH THE LOWER MISSISSIPPI VALLEY AND MID-SOUTH...

...16Z update...

...Eastern Texas into the Lower Mississippi Valley/Mid-South and
Ohio Valley...

Complex and messy evolution of convection is underway across
northeastern TX into the Lower MS Valley this morning. Increasing
low level winds ahead of an approaching potent mid-level shortwave
trough will allow for increasing instability from the TX/LA border
into and across the Lower MS Valley during the day. The
orientation of a warm front from northeastern TX into northern MS
(SW to NE) matches mean steering flow from the SW which will
promote areas of training heavy rain. Forcing for ascent will
continue to increase across these same areas ahead of the shortwave
trough and with increasing upper level jet divergence/diffluence.
Storm scale evolution remains a bit uncertain with the latest 12Z
HREF in disagreement with the exact placement of a stripe of 3 to
6+ inches of rain from northeastern TX into northern LA and
northern MS, which precludes an upgrade to Moderate but within that
zone of expected training, Moderate-type impacts could be
observed, especially given overlap with potentially sensitive
grounds due to recent rainfall and/or urban environments.

...Northwest...
No significant changes were made to the Marginal and Slight Risks
in place across the West Coast with similar thinking in the
previous discussion still holding true. Ongoing steady rain into
western OR and northwestern CA will continue during the day with an
expected increase in precipitation intensity near 00Z ahead of an
approaching Pacific cold front. The main area of concern is across
the northern CA/southern OR coast where heavy rain has been
observed over the past week or so and adding an additional 3 to 5
inches through 12Z Sunday (locally higher possible), where what is
essentially a higher end Slight Risk is in place.

Otto

...previous discussion follows...

...Northwest...

A front will be in the process of gradually stalling out in a west
to east fashion this morning across areas of southwest OR and
northwest CA as the deeper layer Pacific flow becomes parallel to
the boundary. This will maintain atmospheric river conditions today
across the coastal ranges which will be rather modest at least for
this morning. However, the last of a series of offshore shortwaves
embedded within the deeper layer Pacific flow should approach
later today and cross the region tonight which will yield a strong
resurgence of IVT magnitudes that should approach and locally
exceed 750 kg/m/s. This will be driven by shortwave amplification
and the development of a new area of surface low pressure which
will move into the Pacific Northwest by early Sunday morning.
Rainfall rates are expected to increase once again as the strong
warm air advection pattern ahead of the shortwave combine with the
stronger low to mid-level moisture transport into the terrain. The
00Z HREF guidance supports rainfall rates rising to locally over a
0.50"/hour, but they will tend to peak along and just ahead of the
arrival of the offshore cold late tonight. Some stronger convective elements/heavy showers are suggested in the HREF guidance with
passage of the cold front closer to 12Z/Sunday and this may foster
some spotty rainfall rates peaking into the 0.75" to 1.0"/hour range.

This latest atmospheric river will then weaken as the front moves
inland, but additional rainfall amounts of as much as 3 to 6+
inches are expected, with the heaviest amounts likely tending to be
in vicinity of Coos, Curry and Del Norte Counties involving the
favored coastal ranges of southwest OR and northwest CA. Lesser
amounts are expected to advance farther south down the coast, with
some locally heavy rains likely making it as far south as the north
side of the Bay Area Sunday morning along with the northern Sierra
Nevada. Overall, given the wet antecedent conditions and high
streamflows, these additional rains are likely to increase the
flooding threat on area streams and tributaries and thus the Slight
Risk area is maintained for the coastal ranges. The Marginal Risk
area for this update was tweaked and adjusted a bit farther south
to account for some heavy rainfall getting closer to the
aforementioned Bay Area.

...Eastern Texas into the Lower Mississippi Valley/Mid-South and
Ohio Valley...

A well-defined and organized outbreak of strong convection is
expected to begin this morning across portions of central and
especially eastern TX as a strong shortwave trough amplifies
across the southern Plains and drives cyclogenesis from west to
east while interacting with an increasingly strong southerly low-
level jet. A substantial pool of warm-sector instability with
MLCAPE values of 1500 to 2500 J/kg has already pooled across south-
central to southeast TX and stretching over into southern LA. This
will be lifting north through midday as a warm front lifts north
ahead of the approaching surface low. Strong convection with
widespread concerns for supercell thunderstorms will evolve by
midday and through this afternoon across eastern TX with impacts
overspreading the Arklatex and broader Lower MS Valley by this
evening. The latest HREF model consensus supports the low-level jet
increasing to as much as 40 to 60+ kts across far eastern TX, LA,
southern AR and into western and northern MS by 00Z. As this
unfolds, the aforementioned warm front attempting to lift north
ahead of the surface low should become a focus for widespread
convection with potentially multiple rounds of very heavy rainfall
impacting the same area. Given the level of moisture and
instability transport coupled with strong shear, the stronger and
more organized convective cells will be capable of producing
rainfall rates of 1.5 to 2.5 inches/hour. Concerns for cell-mergers
and cell-training will exist and by this evening, some regional
swaths of 2 to 4 inches of rain with isolated 5+ inch totals will be possible.

Going into the overnight hours, the strong shortwave energy should
take on a bit of a negative tilt as it approaches the Mid-South and
aims for the OH Valley. This will drive the strong low-level jet
up through large areas of MS, AL, TN and nosing up toward the OH
River by early Sunday morning. Strong bands of heavy showers and
thunderstorms with a large degree organization should impact the
region, with a QLCS evolution expected to generally unfold across
much of the Mid-South going through the overnight hours given
enhanced deep layer forcing/shear and robust moisture and
instability transport. Localized swaths of 2 to 4 inches of rain
will also be possible across the Mid-South where any cell-training
can take place.

Given the wet antecedent conditions and elevated streamflows
across much of eastern TX and the Lower MS Valley, scattered to
locally numerous areas of flash flooding are expected to evolve,
and some of the flash flooding may be locally significant. The
flash flood threat will be a bit more conditional downstream across
the Mid-South and especially areas farther north into the OH
Valley, but given the heavy rainfall potential, the threat of flash
flooding will certainly exist. Given the wetter trend in the 00Z
hires model CAMS and broader convective outbreak that is expected,
a much more expansive Marginal Risk has been accommodated along
with a large Slight Risk area involving eastern TX, the Lower MS
Valley and basically all of the Mid-South. It cannot be ruled out
that a need for a targeted Moderate Risk upgrade will exist with
the next cycle update across some portions of the Slight Risk area.

Orrison
$$
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FOUS30 KWBC 290802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Sun Dec 29 2024

Day 1
Valid 12Z Sun Dec 29 2024 - 12Z Mon Dec 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL
RANGES OF SOUTHWEST OREGON AND NORTHWEST CALIFORNIA ALONG WITH THE
FOOTHILLS OF THE NORTHERN SIERRA NEVADA...

...Western Oregon and Northern California...

The tail end of a fairly strong atmospheric river will be impacting
especially southwest OR and northern CA this morning as a shortwave
trough and associated wave of low pressure advances into the
Pacific Northwest with a trailing cold front extending well to the
south and gradually moving inland across northern CA. IVT
magnitudes along and just ahead of the cold front will be on the
order of 750 to 1000 kg/m/s across the northwest CA coastal ranges
including Del Norte and Humboldt Counties. Lesser magnitudes will
be advancing inland with the front into the Shasta/Siskiyou ranges
and the northern Sierra Nevada. The IVT values should come down by
18Z, but sufficient levels of onshore flow, convergence along the
trailing front, and orographic ascent over the terrain should favor
areas of heavy rain lingering well into the 18Z to 00Z time frame
across the foothills of the northern Sierra Nevada. The 00Z HREF
guidance favors 0.50"+/hour rainfall rates at least occasionally
impacting these areas, and the threat of heavy rain will also
envelope the Bay Area where the coastal ranges here may also see
some occasionally enhanced rainfall at least briefly with rates
exceeding a 0.50"/hour before the front clears the area. For the
period, some additional 2 to 3 inch rainfall amounts will be
possible locally across the coastal ranges of southwest OR and
northwest CA, and also separately in the foothills of the northern
Sierra Nevada by tonight. A Slight Risk of excessive rainfall has
depicted across these areas, and is an introduction in the case of
the northern Sierra Nevada foothills. Wet antecedent conditions
along with high streamflow will encourage there being at least a
small stream and urban flooding threat, and there may be a low-end
flash flood threat involving the more sensitive burn scar locations
where these heavier rains persist. Please consult MPD #1204 for
more details concerning the near-term heavy rainfall/flooding
concerns across the region.

...Central/Southern Appalachians into the Southeast...

An early-morning QLCS will be crossing through the eastern Gulf
Coast region and the broader interior of the Southeast as a strong
southerly low-level jet of 40 to 60 kts drives enhanced moisture
transport along with a nose of favorable instability just ahead of
an advancing cold front. This will be occurring as a surface low
and associated mid to upper-level trough rides northeastward up
into the Great Lakes region. The QLCS activity should at least clip
portions of the southern Appalachians and then advance across the
Piedmont of the Southeast and eventually the coastal plain. Some
of the more urbanized locations that see this QLCS passage may see
a quick 1 to 1.5 inches of rain in less than an hour, and this may
result in some urban runoff problems with a highly isolated threat
of flash flooding. In general, some of the moist flow into the central
and southern Appalachians this morning should favor at least some
locally heavy rainfall totals that may reach upwards of a couple
inches before the cold front clears the region. A Marginal Risk of
excessive rainfall has been depicted across all of these areas.

...South Florida and the Keys...

The latest GOES-E IR satellite imagery shows a cluster of very
heavy showers and thunderstorms over the southeast Gulf of Mexico
in association with a low-level warm front that is attempting to
advance northeastward toward the southern FL Peninsula. The
convection has been focusing in close proximity to a well-defined
instability gradient and there has been some well-organized
convective cells with some mesocyclone characteristics evolving
northwest of the lower FL Keys over the last few hours as seen in
the Key West radar. Deep moisture and elevated instability has
already been favoring very high rainfall rates, but the challenge
for today will be whether or not any of this organized convection
can advance bodily inland across South FL. The 00Z HREF model
consensus, 00Z NSSL MPAS guidance, and 06Z HRRR suggests Collier
and Monroe Counties will see heavy rainfall this morning as this
warm front pushes eastward at least toward the coast while also
lifting up through the FL Keys. It is possible that Miami-Dade
County could get into at least some peripheral bands of showers and thunderstorms as well, some of which will be capable of producing
heavy rainfall rates of 1 to 2 inches/hour. This would certainly
introduce concerns for some urban flash flooding. Given the
mesoscale nature of this setup, there is a concern for locally very
heavy rainfall totals (possibly 5+ inches over parts of Collier
and Monroe Counties and portions of the Keys). Given the level of
uncertainty with the coverage and the amounts, a Marginal Risk will
be depicted for the time being, but it would not be a stretch that
a targeted upgrade to a Slight Risk will be needed this morning to
address the threat for at least some areal flooding and
potentially urban flash flooding.

Orrison


Day 2
Valid 12Z Mon Dec 30 2024 - 12Z Tue Dec 31 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Tue Dec 31 2024 - 12Z Wed Jan 01 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 310802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 31 2024

Day 1
Valid 12Z Tue Dec 31 2024 - 12Z Wed Jan 01 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman


Day 3
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA...

A strong front associated with a compact low will approach and move
into the Pacific coast of Oregon and California late Thursday
night. IVT values from the GEFS peak at about 600 kg/ms around 12Z
Friday. While river levels have come down a bit from the recent
rainfall and atmospheric river event, expected rainfall on Day
2/Wednesday should bring levels back up and resaturate the soils.
Thus, with this next round Thursday night, the expected 2-4 inches
of rain could still cause isolated instances of flash flooding.
The front does not have a true connection to the tropics, so this
round of rain is expected to be of lighter intensity and lesser
duration than the previous A.R. event.

Wegman
$$
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FOUS30 KWBC 010752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Wed Jan 1 2025

Day 1
Valid 12Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST
OREGON AND NORTHWEST CALIFORNIA...

The 00Z HREF guidance has come in with a somewhat wetter footprint
for today across areas of southwest Oregon and northwest California
as a new round of offshore shortwave energy and proximity of a
front helps to bring in a new round of moderate to heavy rainfall.
The GFS and ECMWF solutions suggest IVT values increasing to near
500 kg/m/s along the immediate coast from Curry County, Oregon down
through Del Norte and Humboldt Counties in northwest California.
Offshore CMORPH2/microwave data shows heavier rainfall rates
associated with this next modest surge in atmospheric river
activity that will be arriving, and the HREF guidance suggests
rainfall rates will generally peak in the 0.25" to 0.50"/hour range
with perhaps an occasional instance or two of 0.50"+/hour rates.
Going through 12Z/Thursday, the HREF consensus suggests some
24-hour rainfall amounts of 2 to 4 inches with a couple of spotty
5+ inch amounts possible, and especially for northwest California
where some of the higher IVT values will persist a bit longer. The
antecedent conditions are wet across the region, and these rains
may cause some isolated runoff concerns. As a result, a Marginal
Risk of excessive rainfall has been depicted for this area.

Orrison


Day 2
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

Multiple impulses of upper level energy racing along a 150 kt WSW
jet will move into the Oregon and California coast near the state
line during this Day 2/Thursday period. The first is a front that
will have already moved into the area tonight, and will diminish in
intensity and slowly shift north through the early morning. A second
impulse moves in Thursday afternoon, resulting in a much more
concentrated area of rain about 50 miles or so to either direction
of the state line. Finally, a third and the strongest impulse will
move in late Thursday night and Friday morning. Each of these 3
impulses of energy will cause rainfall rates to pick up on the
Oregon and far northern California coast. None of them are expected
to advect in much instability, so upslope will likely be the primary
forcing mechanism for rainfall. With nearly continuous rainfall,
albeit to varying intensity with each impulse over already very
saturated ground, runoff and small stream flooding will be
possible. Since the heavier/steadier rainfall will be ongoing at
the start of the period over northern California, and guidance has
been creeping up as far as total rainfall amounts go, the Marginal
Risk extends south into Humboldt County to account for continuing
rainfall and isolated runoff problems that may be ongoing from the
Day 1/Wednesday period.

Wegman


Day 3
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A strong shortwave and associated surface low will plow into the
coast of California and Oregon on Friday. Locally heavy rain with
rates around 1/2 inch per hour will be ongoing Friday morning along
the coast. The shortwave and low will move into the coast Friday
evening and through the overnight. This will shift the heaviest
rains both southward and inland, bringing northern portions of the
Sacramento Valley, which has been largely missed with the previous
days' rains into the potential for heavy rain and higher elevation
snow. The Marginal Risk inherited was left unchanged with this
issuance. The heaviest rains are likely to be Friday morning as the aforementioned shortwave and surface low approach. Once they move
into the coast Friday evening and overnight, rainfall rates should
diminish quite a bit as the associated moisture moves inland. Any
flooding from the Day 2/Thursday period will likely continue into
Friday morning along the Oregon and California coasts...with the
flooding for the Sacramento Valley more likely Friday afternoon.

Wegman
$$
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FOUS30 KWBC 020705
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
205 AM EST Thu Jan 2 2025

Day 1
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

A relatively weak, long duration atmospheric river will continue
to impact portions of the Pacific Northwest coast today, shifting
focus from northwest California into far southwest Oregon. While
there is a distinct lack of instability to fuel rainfall rates
beyond 0.25"/hr, tropospheric moisture is still rather elevated
with precipitable water values of 1.0-1.2" (2.0-2.5+ standard
deviations, near the 95th percentile). Two distinct rounds of
precipitation (one from late this morning through mid afternoon,
and another round from late evening through the overnight) are
expected to bring additional rainfall of 3-5" (primarily for Del
Norte county, CA and Curry county, OR). While rapid onset flooding
is generally not expected (given the muted rainfall rates), these
areas (and surroundings) have already received 1-2" over the past
24 hours with USGS streamflow data indicating widespread above
normal to much above normal streamflows (with some even at record
territory for the date), suggesting additional rainfall may lead to
localized flooding impacts.

Churchill


Day 2
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A strong shortwave and associated surface low will plow into the
coast of California and Oregon on Friday. Locally heavy rain with
rates around 1/2 inch per hour will be ongoing Friday morning along
the coast. The shortwave and low will move into the coast Friday
evening and through the overnight. This will shift the heaviest
rains both southward and inland, bringing northern portions of the
Sacramento Valley, which has been largely missed with the previous
days' rains, into the potential for heavy rain and higher
elevation snow. The Marginal Risk inherited was left unchanged with
this issuance. The heaviest rains are likely to be Friday morning
as the aforementioned shortwave and surface low approach. Once they
move into the coast Friday evening and overnight, rainfall rates
should diminish quite a bit as the associated moisture moves
inland. Any flooding from the Day 2/Thursday period will likely
continue into Friday morning along the Oregon and California
coasts...with any minor flooding for the Sacramento Valley more
likely Friday afternoon.

Wegman


Day 3
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Wegman
$$
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FOUS30 KWBC 030830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Fri Jan 3 2025

Day 1
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A long duration atmospheric river has resulted in 2-4" rainfall
totals over portions of far northwestern CA over the past several
days, with 1-3" occurring over the past 48 hours (extending into
portions of far southwestern OR as well). A final push of low-level
moisture transport is beginning to reach the coastline early this
morning, as a potent shortwave and associated surface cyclone
brings the most significant source of uplift yet (as well as
elevated instability with the HREF indicating MU CAPE of 250-500
J/kg in association with height falls aloft). The shortwave and
low will move into the coast this evening and through the
overnight shifting, shifting the heaviest rainfall both southward
and inland with time (bringing northern portions of the Sacramento
Valley, which has been largely missed with the previous days'
rainfall, into the Marginal risk for excessive rainfall). Areas
most at-risk for rainfall rates exceeding 0.25"/hr (which can
become problematic for particularly sensitive terrain, such as
burn scars) are along the upslope portions of the Coast Range,
Klamath Mountains, Cascade Range, into the northern Sierra Nevada
(per HREF Ensemble Agreement Scale, 10-100 km, neighborhood
probabilities of 60-90% for 1-hr, 0.25" exceedance). Additional
24-hr totals of 1-3" are expected (with a slight majority of the
rainfall occurring prior to 00z).

Churchill


Day 2
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 3
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone is anticipated to develop over
the central US into Day 3 (originating from the aforementioned
shortwave impacting the Pacific Northwest on Day 1), as strong DPVA
via a digging shortwave (cutting off into an upper low) interacts
with the right-entrance region of a broad phased jet structure
(~130 kt jet streak @ 250 mb over the Mid-Atlantic). A substantial
low-level jet (850 mb of 40-50 kts) is expected to form as a
result, ushering in highly anomalous (for this time of year)
precipitable water values nearing 1.5" (near the 90th percentile to
max moving average, per JAN/BNA/BMX sounding climatology). While
downscaled global guidance (GFS/ECMWF) suggests only localized 1-2"
totals at this stage, the impressive dynamics of this system
(along with the anomalous moisture) suggest the potential for this
guidance to be exceeded (as hi-res CAMs will shed a light on as
the event gets closer in range). That said, both the progressive
nature of the system and a distinct lack of instability should be
limiting factors for organized heavy rainfall. Maintained an
inherited Marginal risk area for much of the Southeast (with some
expansion towards the central Gulf Coast, where instability will be
more abundant) encompassing the expected warm sector of the system
(while excluding areas farther north where more impressive broad
QPF exists, as this precipitation will mostly occur in the form of
ice and snow).

Churchill

$$
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FOUS30 KWBC 040815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sat Jan 4 2025

Day 1
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone will develop over the central US
into Day 2, originating from a potent shortwave trough currently
digging into the Four Corners region. The trough is expected to
cut-off into an upper-low as it emerges into the central/southern
Plains with strong DPVA interacts with the right-entrance region
of a broad phased jet structure (~130 kt jet streak @ 250 mb over
the Mid-Atlantic, and a smaller ~100 kt jet streak near the Ozarks).
A substantial low-level jet (850 mb of 40-60 kts) is expected to
form as a result of deep cyclogenesis, ushering in highly
anomalous tropospheric moisture (for this time of year) with
precipitable water values nearing 1.5" (near the 90th percentile to
max moving average, per JAN/BNA/BMX sounding climatology). While
downscaled global guidance (GFS/ECMWF) continues to suggest only
localized 1-2" totals, the impressive dynamics of this system
(along with the anomalous moisture) suggest the potential for this
guidance to locally be exceeded (as the 00z FV3 suggests, but
additional CAMs should better capture the even with the upcoming
12z runs). That said, both the progressive nature of the system and
the limited window of time in the afternoon to take advantage of destabilization (with SB CAPE from east TX and southern LA to the
Mid-South reaches 500-2000 J/kg) are expected limit localized
totals to the 2-3" range (with as much as 1-2" in an hour with the
main line of convection). Have maintained the inherited Marginal
risk area for much of the Southeast (with some additional expansion
towards the southwest, where instability will be most abundant)
generally encompassing the expected warm sector of the system
(while excluding areas farther north where more impressive broad
QPF exists, as this precipitation will mostly occur in the form of
ice and snow).

Churchill


Day 3
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill

$$
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FOUS30 KWBC 050815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Sun Jan 5 2025

Day 1
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone is taking shape over the central
US early this morning, originating from a potent shortwave trough
digging through the Central Rockies over the past couple of days.
The trough is in the process of cutting off into an upper-low as
it emerges from the Rockies into the central Plains. Strong DPVA
in association with the trough will interact with the right-
entrance region of a broad phased jet structure (~140 kt jet
streak @ 250 mb over the Mid-Atlantic, and a smaller ~100 kt jet
streak near the Ozarks). A substantial low-level jet (850 mb of
40-60 kts) is expected to form as a result of deep cyclogenesis,
stretching from the Central Gulf Coast to the Mid-South, ushering
in highly anomalous tropospheric moisture (for this time of year)
with precipitable water values nearing 1.5" (between the 90th
percentile and max moving average, per JAN/BNA/BMX sounding
climatology). Most hi-res CAMs continue to suggest localized 1-2"
totals in association with combined totals from scattered
convection in the open warm sector and the following primary squall
line in association with the approaching cold front, though a
couple of solutions (FV3 and CMCreg) suggest localized streaks of
2-3" totals (from northeast LA through north/central MS into
northwest AL). While much of these totals may occur over a
relatively short period (with any flash flooding conditions more
likely due to storm mode and sub-hourly rates and totals of 1-2"),
00z HREF 40-km neighborhood probabilities for 2" exceedance over 3
hours barely peak at 20-30%. Therefore the progressiveness of this
system (and the limited window of time in the afternoon to take
advantage of destabilization, with SB CAPE from east TX and
southern LA to the Mid-South reaching 500-2000 J/kg) should largely
preclude any organized flash flood threat. Have maintained the
inherited Marginal risk area once again, generally encompassing the
expected warm sector of the system (while excluding areas farther
north where more impressive broad QPF exists, as this precipitation
will mostly occur in the form of ice and snow).

Churchill


Day 2
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 3
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill

$$
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FOUS30 KWBC 052013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
313 PM EST Sun Jan 5 2025

Day 1
Valid 16Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

...16z update...
Small adjustments were made to the broad Marginal Risk area based
on guidance trends and 12z Hi-Res/HREF probability output. 12z HREF
continues to show a solid high probability axis with the convective
line for 1"/hr or 1"/3hr (over 90%). However, the forward speed of
cells continues to quickly diminish the potential for high overall
totals, as 2"/3hr values only peak at 30% in west-central MS,
eventually expanding into northern MS and mainly in the 18-00z time frames.

Based on the evolution from the Hi-Res CAMs, the highest potential
is across this axis, with some lower signals further south and west
into central LA and far SE TX though higher FFG values in that
region would have lower potential of exceedance anyway.
Observational trends and rapid refresh guidance from the HRRR/RAP
denote the core of surface to 850mb moisture and flux is generally
parallel to the approaching front and about 25-50 miles downstream.
Increasing insolation and steepening lapse rates will allow for the
instability axis to align with the moisture, that pre-cursory
convection is likely to develop in the 15-18z time frame across SE
TX into central LA, ahead of the developing convection noted in W
AR/NE TX attm. Eventually, the gap between closes and
streams/ascent merge for the potential for some very short-term
enhancement of totals to 1-2" ranges. This axis does align with an
area of recent dryness compared to locations further west over the
last 7-10 days per AHPS...further diminishing the need for a Slight
Risk area; though central/northeast LA into northern MS will be
the area of greatest potential for any incidents of localized flash
flooding concerns.

Elsewhere, the warm advection across the mid-MS Valley has trended
a bit northward and some of these cells may reach SE MO/W KY where
grounds may have some ice in place. As such, the Marginal Risk was
lifted a bit northward with this update. Equally, trends with
convection across central LA into S MS and slightly slower cold
front pressing eastward after 06z; has warranted a bit further
trimming of the eastern side of the Marginal Risk in AL.

Gallina

---Prior Discussion---

An impressive deep layer cyclone is taking shape over the central
US early this morning, originating from a potent shortwave trough
digging through the Central Rockies over the past couple of days.
The trough is in the process of cutting off into an upper-low as
it emerges from the Rockies into the central Plains. Strong DPVA
in association with the trough will interact with the right-
entrance region of a broad phased jet structure (~140 kt jet
streak @ 250 mb over the Mid-Atlantic, and a smaller ~100 kt jet
streak near the Ozarks). A substantial low-level jet (850 mb of
40-60 kts) is expected to form as a result of deep cyclogenesis,
stretching from the Central Gulf Coast to the Mid-South, ushering
in highly anomalous tropospheric moisture (for this time of year)
with precipitable water values nearing 1.5" (between the 90th
percentile and max moving average, per JAN/BNA/BMX sounding
climatology). Most hi-res CAMs continue to suggest localized 1-2"
totals in association with combined totals from scattered
convection in the open warm sector and the following primary squall
line in association with the approaching cold front, though a
couple of solutions (FV3 and CMCreg) suggest localized streaks of
2-3" totals (from northeast LA through north/central MS into
northwest AL). While much of these totals may occur over a
relatively short period (with any flash flooding conditions more
likely due to storm mode and sub-hourly rates and totals of 1-2"),
00z HREF 40-km neighborhood probabilities for 2" exceedance over 3
hours barely peak at 20-30%. Therefore the progressiveness of this
system (and the limited window of time in the afternoon to take
advantage of destabilization, with SB CAPE from east TX and
southern LA to the Mid-South reaching 500-2000 J/kg) should largely
preclude any organized flash flood threat. Have maintained the
inherited Marginal risk area once again, generally encompassing the
expected warm sector of the system (while excluding areas farther
north where more impressive broad QPF exists, as this precipitation
will mostly occur in the form of ice and snow).

Churchill


Day 2
Valid 12Z Mon Jan 06 2025 - 12Z Tue Jan 07 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 070732
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
232 AM EST Tue Jan 7 2025

Day 1
Valid 12Z Tue Jan 07 2025 - 12Z Wed Jan 08 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

Day 2
Valid 12Z Wed Jan 08 2025 - 12Z Thu Jan 09 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 3
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

Highly anomalous upper trough pattern across the Western CONUS will
help generate a distinct shortwave ejection evolution that will
create a litany of issues across TX into the Lower Mississippi
Valley. Upper low expected to be centered over Sonora the prior
period will open as it advances eastward with a second shortwave
digging around the backside of the mean trough creating a fairly
dynamic setup across the Southern Plains. A strong diffluent axis
of ascent ahead of the ejecting trough will create a blossoming of precipitation over TX, spreading eastward as the trough migrates
through the Lone Star State. Surface low pressure will spawn in-of
the western Gulf with the low center generally close to the TX
coastal plain, opening the door for a modest instability gradient
to align within the immediate TX coast up to southwest LA as the
pattern progresses. Convective regime will be most prominent along
the coast where forecast theta_E's will slowly climb just above
seasonal norms allowing for a focused area of surface instability
along with the already favorable mid and upper level dynamics.

A cluster of thunderstorms will likely form over the western Gulf
within the confines of the surface reflection lending a better
potential for convection to move inland with a heavy rain footprint
tied to any storm moving onshore. There will be a sharp northern
extent of how long any convective regime can last as much colder to
the north of the coast will limit surface based instability and
thwart the opportunity for convection to march inland. As of now,
the forecast MRGL risk inherited from D4 was relatively maintained,
although the northern periphery was trimmed back given the
lackluster instability fields and relatively progressive storm
motion on the northern fringe of the best instability coupled with
ascent. The most favorable locations for rates that could generate
enough support for flash flooding potential is still relegated to
the immediate Gulf coast with a small buffer further inland if any
cells can motion past the immediate coastal plain and affect some
of the urban areas extending from CRP up through HOU and LCH.

Kleebauer

$$
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FOUS30 KWBC 080803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Wed Jan 8 2025

Day 1
Valid 12Z Wed Jan 08 2025 - 12Z Thu Jan 09 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer


Day 2
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Large scale ascent ahead of an open, yet vigorous upper trough
will help initiate surface cyclogenesis over the western Gulf with
a modest push of warm unstable air in-of the TX coast, eventually
into southwestern LA. Current guidance is coming into agreement on
a skinny axis of heavier rainfall near the middle and upper TX
coasts propagating east-northeast into the Lower Sabine by the end
of the period with local amounts of 2-4" plausible within the
heavier rain footprint. The driving component of the potential is
still very much the increasing frictional convergence pattern on
the western flank of the surface low as easterly winds bisect the
TX coast up into LA with the core of better theta_E's remaining at
the immediate coast and over the Gulf waters. The key is the
onshore flow mechanism taking some of the convective cores offshore
and moving them inland causing some heavier rainfall to protrude
inland before running into a wall of cooler, more stable air thanks
to an Arctic push that occurred prior to the evolving surface pattern.

Recent NBM prob fields are >50% for locally 2+" of rainfall within
some of the urban footprint in coastal TX which allows the
opportunity for local flood concerns within those urbanized zones
thanks to higher runoff potential. The saving grace from this being
a higher risk is the agreement on minimal surface based
instability with the mean SBCAPE in relevant ensembles hovering
between 50-100 J/kg with some deterministic maybe exceeding that
at the immediate coast. With the cells likely to struggle being
rooted at the surface, the prospects of more widespread flash
flooding are lower than normal creating a threat that is more
localized and within the lower to mid bounds of the MRGL risk
threshold. The previous forecast was generally maintained outside
some adjustment further on the northern periphery of the risk area,
aligning with the probability and mean instability fields.

Kleebauer


Day 3
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

Our favorable longwave trough and accompanying surface pattern will
move eastward with the surface low over the Gulf moving onshore
within LA by the first half of the period. Increasing surface
convergence within the periphery of the surface low will generate
an axis of heavier precipitation aligned within the modest
frontogenetic setup within the northern periphery of the surface
circulation. There's growing support for a heavy rain footprint
over south-central LA, moving eastward into southeastern LA where
the urban corridors centered around New Orleans come into play.

Like the prior period, the instability pattern is meager,
especially at the surface with most of the ascent driven within the
top of the boundary layer and above where the area lies in the
right-entrance region of the upper jet focused to the north. There
is enough consensus on the location of heaviest rainfall within the
that frontogenetic evolution, coinciding with a strong IVT pulse
(~1000 kg/ms) being depicted within the NAEFS, a solid 4+ standard
deviation interval for the time of year. This should help mitigate
the negligible instability pattern in place and maintain a solid
heavy rain footprint in that focal area mentioned above. Another
consideration is the much colder grounds occurring prior to the
arrival of the rainfall as Arctic air will navigate southward and
allow for a period of near and sub-freezing air to harden the top
soil layer creating a slightly higher potential for runoff.

The MRGL risk from D4 was maintained and expanded to include much
of south-central LA through southeast LA and coastal MS. There is a
potential for a targeted SLGT risk in future updates, mainly within
the corridor extending along and south of I-20 and just north of
the coastal Parishes as the best flood threat will exist within the
more urban zones.

Kleebauer

$$
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FOUS30 KWBC 090751
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Thu Jan 9 2025

Day 1
Valid 12Z Thu Jan 09 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
MIDDLE AND UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Increased mid and upper level forcing is ongoing over portions of
TX thanks to the slow ejection of a potent ULL situated over
Sonora. A digging shortwave trough analyzed over the Central
Rockies will continue plunging southward with an eventual partial
phase with the primary shortwave disturbance moving east into West
TX. At the surface, a maturing surface low over the western Gulf
will slowly wander northward towards the middle TX coast with
expected moisture advection regime to be a significant player in
the prospects of locally heavy rainfall in-of the immediate TX
coast. As of this time, there is a general consensus on the
heaviest precip being focused up the coast near Port Aransas up
through the remainder of the middle and upper TX coasts, pinning a
bullseye closer to the Galveston area and points northeast. This is where
a frictional convergence regime with persistent easterly flow on
the north side of the main surface low will help initiate a period
of convection just off the immediate coast within the primary axis
of higher theta_E's located on the western flank of the surface
reflection. Convection will hug the coast with some of the heavier
precip cores moving ashore creating opportunity for rainfall rates
to reach 1-1.5"/hr at peak intensity and producing totals between
2-3" with locally as high as 4" along that immediate stretch of
coast. The longwave evolution will generate ample ascent within the
diffluent axis downstream of the potent mean trough, pulling
moisture inland and generation pockets of heavy rain just inland,
but to less of a degree of the immediate coast thanks to the local
instability maximum likely hugging the coast with near 0 CAPE just
inland.

Recent probabilities from the 00z HREF are indicative of the
locally heavy rain threat with the neighborhood probs of >3" up
between 40-70% for the stretch of the TX coast beginning from Port
Aransas up the Lower Sabine where TX/LA border along the Gulf
coast. There's non-zero 5+" probs, but a much lower factor of
probabilities comparatively (<20%) and mainly confined to a small
area near Galveston. This has been the zone of heaviest precip
potential and likely strongest low-level convergence due to the
forecast proxy of the surface low and greatest theta_E advection
regime. Considering all the above variables, there was little need
to deviate from the previous forecast. The previous MRGL risk was
maintained given the steady forecast continuity and recent
favorable probability fields.

Kleebauer


Day 2
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

Our western Gulf disturbance will migrate east-northeastward with a
focus of heavier precip across southern LA through the urban zones
of southeast LA during the morning Friday. The system will be
fairly progressive and the trends have brought down storm totals
due to the progressive nature and limited instability factoring in
the local maxima relegated <2". The expectation is for there to be
a thin line of heavier rates along a developing cold rain band,
something not too common at these latitudes. There is a textbook
surface convergence pattern right within the north and western
flank of the low center as it cross through the southeastern
Parishes of LA and this aligns within a narrow tongue of elevated
theta_E's that get advected just ahead of the surface reflection.
This will lend of a period of potentially significant rainfall
rates bordering 1"/hr at peak intensity, a factor that could be
enough to generate some localized flooding within the urban zones
centered around New Orleans and points north, west, and east.

This is very much of a lower end MRGL risk considering the trends
of the forward propagation and such a skinny axis of the heavier
precip anticipated. This also aligns with more of a morning and
early afternoon risk with a quick decay behind the low as light
stratiform takes over, then dries out as drier air advects in
behind the departing low to the east. The previous MRGL risk was
maintained in full with an emphasis on flooding potential along
and south of I-10/12 with the best prospects located around urban
zones surrounding Lake Pontchartrain.

Kleebauer


Day 3
Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Kleebauer

$$
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FOUS30 KWBC 100038
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
738 PM EST Thu Jan 9 2025

Day 1
Valid 01Z Fri Jan 10 2025 - 12Z Fri Jan 10 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
UPPER TEXAS COAST AND WESTERN LOUISIANA GULF COAST...

Increased mid and upper level forcing is ongoing over portions of
TX caused by a potent ULL moving towards the Gulf Coast, with a
surface low deepening near the Middle and Upper TX Coasts at the
time of this discussion's writing. Warm air/moisture advection is
occurring near a coastal front to the northeast of the cyclone
which is allowing hourly rain totals to range between 0.5-1" as of
late. A smidgen of mixed layer and most unstable CAPE is present
near and ahead of the low, which should increase a little more
tonight. When combined with increasing frontogenesis along the
coastal front and a cooler air mass, there is concern for
occasional organized convection as low-level inflow/effective bulk
shear is sufficient for such should enough instability be present.
Convective elements should try to build further along and near the
coastal plain/swamp/bayous in this region. While hourly rain totals
to 1.5" and local amounts to 4" remain in the cards, recent
dryness suggests that urban areas would be most impacted. IVT
values with this system top 1,000 kg/m/s presently, which is well
above the maximum observed IVT in the CFSR database for this time
of year and implies some upward potential for rainfall efficiency
despite the expectation of some forward progress with time. Some
eastward extension to the Marginal Risk area was made to align with
10%+ chance of 5"+ noted in the 18z HREF probabilities, and the
back edge carved back due to system progression.

Roth/Mullinax/Kleebauer


Day 2
Valid 12Z Fri Jan 10 2025 - 12Z Sat Jan 11 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

...20Z Update...

The inherited Marginal Risk remains in good shape as the region
will struggle to see a dearth of instability aloft and any
storms/showers will be progressive in their easterly movement.
That said, NAEFS shows that around 12Z Friday, IVT values aimed at
the central Gulf Coast will be as high as 1250 kg/m/s, which would
be above all observed IVT values in the CFSR database for this
time of year. Similar to the Upper Texas Coast on the Day 1 ERO,
soundings are highly saturated and warm cloud layers are around
12,000ft deep. This should result in efficient warm rainfall that
may support highly localized flash flooding potential, especially
in urbanized areas that drain poorly along the I-10 corridor. But
the progressive storm motions and lack of modest instability should
keep the areal extent and severity of the flash flood threat to a
limited number of at-risk urbanized areas through the early
afternoon hours on Thursday.

Mullinax

--Previous Discussion--

Our western Gulf disturbance will migrate east-northeastward with a
focus of heavier precip across southern LA through the urban zones
of southeast LA during the morning Friday. The system will be
fairly progressive and the trends have brought down storm totals
due to the progressive nature and limited instability factoring in
the local maxima relegated <2". The expectation is for there to be
a thin line of heavier rates along a developing cold rain band,
something not too common at these latitudes. There is a textbook
surface convergence pattern right within the north and western
flank of the low center as it cross through the southeastern
Parishes of LA and this aligns within a narrow tongue of elevated
theta_E's that get advected just ahead of the surface reflection.
This will lend of a period of potentially significant rainfall
rates bordering 1"/hr at peak intensity, a factor that could be
enough to generate some localized flooding within the urban zones
centered around New Orleans and points north, west, and east.

This is very much of a lower end MRGL risk considering the trends
of the forward propagation and such a skinny axis of the heavier
precip anticipated. This also aligns with more of a morning and
early afternoon risk with a quick decay behind the low as light
stratiform takes over, then dries out as drier air advects in
behind the departing low to the east. The previous MRGL risk was
maintained in full with an emphasis on flooding potential along
and south of I-10/12 with the best prospects located around urban
zones surrounding Lake Pontchartrain.

Kleebauer

Day 3
Valid 12Z Sat Jan 11 2025 - 12Z Sun Jan 12 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Mullinax

$$
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FOUS30 KWBC 120829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sun Jan 12 2025

Day 1
Valid 12Z Sun Jan 12 2025 - 12Z Mon Jan 13 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

...Louisiana Coast...
A frontal wave spawned by an upper level disturbance moving between
a Southwest U.S. upper level low and a subtropical ridge near Cuba
is expected to bring precipitable water values up to 1.5" over
portions of the central Gulf Coast as early as late this afternoon
in response to low level flow becoming southwesterly at 20-35 kts.
Despite the moisture transport into the area...the overall risk of
excessive rainfall appears to be held in check by meager
instability and the progressive nature of the system. The 12/00Z
HREF guidance maintained continuity with the 11/12Z run that show
the probability of 0.5"+ totals don't persist anywhere along or
near the Louisiana coast for more than an hour or two. There was a
slight southward shift and a subtle decrease in rainfall amounts in
the guidance which resulted in decreasing neighborhood
probabilities for 2+ inch rainfall amounts compared with the
previous runs.

Bann


Day 2
Valid 12Z Mon Jan 13 2025 - 12Z Tue Jan 14 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 3
Valid 12Z Tue Jan 14 2025 - 12Z Wed Jan 15 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann

$$
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FOUS30 KWBC 160757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Thu Jan 16 2025

Day 1
Valid 12Z Thu Jan 16 2025 - 12Z Fri Jan 17 2025


The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 2
Valid 12Z Fri Jan 17 2025 - 12Z Sat Jan 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Oravec


Day 3
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

PW values expected to rise to 2 to 2.5 standard deviations above
the mean Saturday into early Sunday along and ahead of the arctic
frontal boundary pushing across the northeast Gulf into the
Southeast and North Florida. The axis of strong low level west
south westerly flow along and ahead of the arctic front will slow
for a period late Saturday into early Sunday as mid level height
falls dive into the base of the broad vortex across eastern North
America. This will support potential for training of precip areas
along and just to the south of the arctic front across far southern
Georgia into North Florida. The non NCEP models...EC..CMC...CMC
GEM and UKMET...are showing a more organized axis of heavy rainfall
potential than the GFS and NAM. WPC qpf is leaning more toward the
non-NCEP guidance given the anomalous PW values and potential for
a period of training. The expected axis of heavy rains is forecast
across regions that have seen increases in relative soil moisture
over the past week from recent heavy rains that have stretched from
the central Gulf Coast into the FL Panhandle and North Florida.
Still, FFG values remain high, with the forecast additional
rainfall amounts of 1-2" likely only resulting in isolated runoff
issues.


Oravec
$$
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FOUS30 KWBC 170813
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Fri Jan 17 2025

Day 1
Valid 12Z Fri Jan 17 2025 - 12Z Sat Jan 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

The latest CAM guidance suite has some signals for heavy rainfall
in the vicinity of the Florida Keys along a quasi-stationary
frontal boundary during the day Friday, and the model consensus is
for most of this convection to remain over the Florida Straits and
the western Bahamas. Although some of these cells may affect
coastal areas of South Florida, the coverage and rainfall rates are
low enough to preclude any risk areas at this time.

Hamrick


Day 2
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

A corridor of moisture convergence in the warm sector of a
developing Southeast U.S. surface low, in the presence of anomalous
PWs for this time of year, will fuel the development of multiple
rounds of showers and thunderstorms. Some of these cells may
train over the same areas across portions of northern Florida and
into far southern Georgia, with some rainfall rates perhaps
reaching an inch per hour. The overall trend has been for slightly
lower overall QPF with this event in the latest 00Z model guidance
suite, so the existing Marginal Risk area from the prior Day 3 has
been reduced in size some. Although the UKMET was one of the
wettest solutions for its 12Z run, it is less expansive with the
area affected, more in line with the other guidance for its 00Z
run. The NAM remains one of the less impactful solutions. It is
possible that the Marginal Risk area could be removed if model
trends continue lower in future updates.

Hamrick

Day 3
Valid 12Z Sun Jan 19 2025 - 12Z Mon Jan 20 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick
$$
d
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FOUS30 KWBC 180722
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
222 AM EST Sat Jan 18 2025

Day 1
Valid 12Z Sat Jan 18 2025 - 12Z Sun Jan 19 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR
SOUTHERN GEORGIA INTO NORTH FLORIDA...

A corridor of moisture convergence in the warm sector of a
developing Southeast U.S. surface low, in the presence of PWs on
the order of 1.5 to 1.75 inches, will fuel the development of
multiple rounds of showers and thunderstorms across northern
Florida and across the southern tier of counties in Georgia. The
majority of the heavier rainfall should happen between 00Z and 12Z
Sunday when mixed layer CAPE rises into the 250-750 J/kg range with southwesterly flow from the eastern Gulf. This could result in
rainfall rates in excess of an inch per hour with the most
persistent cells. Some of the CAM guidance indicates swaths of 2-3
inch QPF during this period, and this may lead to some minor
instances of flooding in poor drainage areas. The axis of heavy
rainfall is expected to become more progressive by 12Z Sunday and
beyond across the central Florida Peninsula.

Hamrick

Day 2
Valid 12Z Sun Jan 19 2025 - 12Z Mon Jan 20 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 3
Valid 12Z Mon Jan 20 2025 - 12Z Tue Jan 21 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick
$$
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FOUS30 KWBC 190724
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
224 AM EST Sun Jan 19 2025

Day 1
Valid 12Z Sun Jan 19 2025 - 12Z Mon Jan 20 2025

The broken line of convection crossing the northern Florida
Peninsula early Sunday morning is expected to become more
progressive and weaken after 12Z Sunday, and thus the potential
drops off enough to have no risk areas for the Day 1 time period.
Therefore, the probability of rainfall exceeding flash flood
guidance is less than 5 percent.

Hamrick

Day 2
Valid 12Z Mon Jan 20 2025 - 12Z Tue Jan 21 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 3
Valid 12Z Tue Jan 21 2025 - 12Z Wed Jan 22 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick

$$
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FOUS30 KWBC 230737
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Thu Jan 23 2025

Day 1
Valid 12Z Thu Jan 23 2025 - 12Z Fri Jan 24 2025


The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 2
Valid 12Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hamrick


Day 3
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

A developing closed low over central California late Saturday is
expected to advect enough moisture from the Pacific to generate
scattered to numerous showers that should start around 00Z Sunday,
with most guidance indicating QPF on the order of 0.25 to 0.50 of
an inch, with most of this within the last 6 hours of Day 3.

After collaboration with WFO Los Angeles, a Marginal Risk area has
been introduced for the Transverse Ranges and the surrounding
valleys. Even though the next round of rain appears to be mainly
moderate in intensity for the 00Z-12Z Sunday time period, the new
burn scar areas are extremely sensitive to run-off, and thus a
normally beneficial and modest rainfall could become problematic
and easily lead to flooding, with potentially serious impacts if
rainfall is heavier than expected and concentrated over a burn scar
near populated areas. There is still uncertainty regarding this
event, so please monitor future forecasts and outlooks as details
become better refined.

Hamrick

$$
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FOUS30 KWBC 240825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Fri Jan 24 2025

Day 1
Valid 12Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

A developing closed low over central California by late Saturday
will foster an area of surface low pressure across the Central
Valley which will then refocus offshore of the coastal ranges
Saturday night and Sunday morning. This will help set the stage
for weak and relatively moist onshore flow to aim into the
Transverse Ranges (and eventually into the Peninsular Ranges as
well). This onshore flow coupled with DPVA and steepening mid-
level lapse rates will allow for the development and expansion of
moderate to locally heavy shower activity, and especially by Sunday
morning when some pockets of locally stronger convection may be
possible. This will include areas from Santa Barbara County
southeast through San Diego County, and also some western portions
of San Bernadino and Riverside Counties. A somewhat stronger and
wetter solution continues to be advertised by much of the latest
model guidance, and generally as much as a 0.5" to 0.75" of rain is
forecast by Sunday morning (with highly localized stronger
orographic ascent/upslope flow facilitating isolated convective
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains Saturday night and Sunday morning may be heavy enough
to result in some localized debris flow and related flash flooding
activity. Therefore, the Marginal Risk has been maintained over
portions of the Transverse Ranges and the Los Angeles Basin. Some
local adjustments were made based on the addition of high-res
guidance (primarily the NAM-nest) focusing on upslope regions most
at risk for highly localized 1"+ totals.

Churchill/Orrison


Day 3
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas). An additional 0.75"
to 1.50" is generally forecast for much of the same areas as Day 2,
and concerns are greatest for isolated convection with 0.25"+/hr
rates interacting with recent burn scar areas resulting in the
potential for localized flash flooding and debris flows. While the
bulk of the rainfall should be beneficial for the region, interests
in the burn scars (and more sensitive flash flooding areas) should
stay tuned to the forecast as more high resolution model data
becomes available in the next 24-48 hours to better hone in on the
threat and specific areas.


...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast resulting in
areas of moderate to heavy rainfall by Sunday morning. While areal
average QPF is generally expected to be capped at 1"/6-hr period,
24-hr totals could add up to be locally as high as 3-4" (per
ensemble bias corrected guidance and GEFS/ECENS probabilities for
3" exceedance of 1-5%). While instability will be rather limited,
MU CAPE up to 500 J/kg could support localized embedded convection
(with high-res guidance providing more insight in future cycles).
For now, the inherited Marginal Risk area was maintained and
adjusted based on the latest guidance (and is in line with the
GEFS-driven machine learning first guess guidance, depicting a
Marginal Risk area).

Churchill

$$
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FOUS30 KWBC 241526
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1026 AM EST Fri Jan 24 2025

Day 1
Valid 16Z Fri Jan 24 2025 - 12Z Sat Jan 25 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Orrison


Day 2
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

...Southern California...

A developing closed low over central California by late Saturday
will foster an area of surface low pressure across the Central
Valley which will then refocus offshore of the coastal ranges
Saturday night and Sunday morning. This will help set the stage
for weak and relatively moist onshore flow to aim into the
Transverse Ranges (and eventually into the Peninsular Ranges as
well). This onshore flow coupled with DPVA and steepening mid-
level lapse rates will allow for the development and expansion of
moderate to locally heavy shower activity, and especially by Sunday
morning when some pockets of locally stronger convection may be
possible. This will include areas from Santa Barbara County
southeast through San Diego County, and also some western portions
of San Bernadino and Riverside Counties. A somewhat stronger and
wetter solution continues to be advertised by much of the latest
model guidance, and generally as much as a 0.5" to 0.75" of rain is
forecast by Sunday morning (with highly localized stronger
orographic ascent/upslope flow facilitating isolated convective
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains Saturday night and Sunday morning may be heavy enough
to result in some localized debris flow and related flash flooding
activity. Therefore, the Marginal Risk has been maintained over
portions of the Transverse Ranges and the Los Angeles Basin. Some
local adjustments were made based on the addition of high-res
guidance (primarily the NAM-nest) focusing on upslope regions most
at risk for highly localized 1"+ totals.

Churchill/Orrison


Day 3
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas). An additional 0.75"
to 1.50" is generally forecast for much of the same areas as Day 2,
and concerns are greatest for isolated convection with 0.25"+/hr
rates interacting with recent burn scar areas resulting in the
potential for localized flash flooding and debris flows. While the
bulk of the rainfall should be beneficial for the region, interests
in the burn scars (and more sensitive flash flooding areas) should
stay tuned to the forecast as more high resolution model data
becomes available in the next 24-48 hours to better hone in on the
threat and specific areas.


...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast resulting in
areas of moderate to heavy rainfall by Sunday morning. While areal
average QPF is generally expected to be capped at 1"/6-hr period,
24-hr totals could add up to be locally as high as 3-4" (per
ensemble bias corrected guidance and GEFS/ECENS probabilities for
3" exceedance of 1-5%). While instability will be rather limited,
MU CAPE up to 500 J/kg could support localized embedded convection
(with high-res guidance providing more insight in future cycles).
For now, the inherited Marginal Risk area was maintained and
adjusted based on the latest guidance (and is in line with the
GEFS-driven machine learning first guess guidance, depicting a
Marginal Risk area).

Churchill

$$
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FOUS30 KWBC 250800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Sat Jan 25 2025

Day 1
Valid 12Z Sat Jan 25 2025 - 12Z Sun Jan 26 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA...

...Southern California...

A developing closed low over central California will foster an
area of surface low pressure across the Central Valley which will
then refocus offshore of the coastal ranges tonight into Sunday
morning. This will help set the stage for weak and relatively moist
onshore flow to aim into the Transverse Ranges (and eventually
into the Peninsular Ranges as well). This onshore flow coupled with
DPVA and steepening mid-level lapse rates will allow for the
development and expansion of moderate to locally heavy shower
activity, and especially by Sunday morning when some pockets of
locally stronger convection may be possible. This will include
areas from Santa Barbara County southeast through San Diego County,
and also some western portions of San Bernadino and Riverside
Counties. Generally as much as a 0.5" to 0.75" of rain is forecast
by Sunday morning (with highly localized stronger orographic
ascent/upslope flow possibly facilitating isolated convective
activity capable of 0.25"+/hr rainfall rates and totals of 1"+).

Given the ongoing wildfire/burn scar sensitivities over the region,
these rains may be heavy enough to result in some localized debris
flow and related flash flooding activity. Therefore, the Marginal
Risk has been maintained over portions of the Transverse Ranges and
the Los Angeles Basin. While these conditions are unlikely to
manifest (as the rainfall will largely be beneficial for fire
fighting activities), sub-hourly rates up to 0.25" would be the
primary driver of the threat.

Churchill/Orrison


Day 2
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES AND LOS ANGELES BASIN OF
SOUTHERN CALIFORNIA, AS WELL AS FOR PORTIONS OF THE TEXAS GULF
COAST INTO THE LOWER MISSISSIPPI VALLEY...

...Southern California...

Localized areas of moderate to heavy shower activity look to
continue into Sunday and Sunday night for upslope portions of the
Transverse and Peninsular Ranges (and to a lesser extent the
adjacent Los Angeles Basin and coastal areas), though snow levels
will be falling and that should help to mitigate runoff issues at
higher elevations. An additional 0.75" to 1.50" is generally
forecast for much of the same areas as Day 1, and concerns are
greatest for isolated convection with 0.25"+/hr rates interacting
with recent burn scar areas resulting in the potential for
localized flash flooding and potential debris flows/mudslides
(driven primarily by sub-hourly rates of 0.25", if able to
manifest with very limited instability). While the bulk of the
rainfall should be beneficial for the region, interests in the burn
scars (and more sensitive flash flooding areas) should stay tuned
to the forecast.

...Texas Gulf Coast into Lower Mississippi Valley...

As a shortwave trough departs eastward by Saturday, low-level
return flow will return to the western Gulf Coast along with a wave
of low pressure, resulting in areas of moderate to heavy rainfall
by Sunday morning. While areal average QPF is generally expected to
be capped near 1" over a 3-6 hr period, 24-hr totals could add up
to be locally as high as 3-4" (mainly now indicated by the ECMWF
and ECENS suite). While instability will be rather limited, MU
CAPE up to 500 J/kg could support localized embedded convection
(with high-res guidance providing more insight in future cycles).
For now, uncertainty remains high given differences in the ensemble
systems and how the deterministic solutions handle the mass fields
(with the more aggressive EC suite suggesting the potential for a
future targeted upgrade for the Houston metro region, should hi-res
guidance support this with future updates. The inherited Marginal
Risk area was maintained with no adjustments necessary, as the
guidance remains fairly consistent spatially with depiction of QPF.

Churchill


Day 3
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill

$$
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FOUS30 KWBC 260830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Sun Jan 26 2025

Day 1
Valid 12Z Sun Jan 26 2025 - 12Z Mon Jan 27 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND THE
PALISADES, EATON, AND HURST BURN SCARS, GENERALLY JUST NORTH AND
WEST OF THE CENTER OF LOS ANGELES, CALIFORNIA...

...Southern California...

Shower coverage is becoming more numerous to widespread early this
morning across much of the Southern California coastline and
adjacent inland areas, as a deep layer closed low churns near the
central California coast. As the low gradually slides southward
along the coastline today, localized moderate to heavy downpours
will become more likely along upslope portions of the Transverse
and Peninsular Ranges (and to a lesser extent the adjacent Los
Angeles Basin and coastal areas). While falling snow levels should
help to mitigate runoff issues at higher elevations, there are
increasing concerns for potentially significant localized impacts
for more sensitive localities, chiefly for recent burn scars where
runoff concerns are highest. While an additional 0.5" to 1.0 of
areal average QPF is expected for the region, some hi-res CAM
solutions (including the HRRR) indicate highly localized totals of
1"+. Most concerningly, these localized totals are possible in and
around the Palisades, Eaton, and Hurst burn scars. The CAMs also
indicate the potential for sub-hourly (15-min) rainfall rates of
0.25"+, which would drive an elevated threat for mudslides and
debris flows in and around burn scars. While the bulk of the
rainfall should be largely beneficial for the region, the
heightened risk for significant flash flood impacts in association
with these burn scars has necessitated an upgrade to Slight risk
for a target region (generally encompassing the aforementioned burn scars).


...Texas Gulf Coast into Lower Mississippi Valley...

Moist southerly low-level flow from the western Gulf has returned
to much of the western Gulf Coast, spreading northeastward into
portions of the Lower Mississippi Valley. This return flow will
combine with a wave of low pressure and associated surface frontal
zone which will likely result in embedded moderate to heavy
showers by late morning to midday, gradually driven southeastward
towards the coastline with the progression of the front. While
hourly rates will generally be capped near 1", 24-hr totals could
add up to be locally as high as 3-4" (per 00z HREF 40-km
neighborhood probabilities for 3" exceedance of 20-30%). While
instability will be somewhat limited, MU CAPE up to 1000 J/kg
(maximized primarily in and between Corpus Christi to Houston)
could support a period of localized 1-2"/hr rainfall rates (which
adds a bit to the threat, especially within urbanized terrain).
While models are in relatively good agreement concerning the
magnitude of QPF (localized 3-4" totals), there is still a good bit
of uncertainty with regard to the placement of these higher totals
(with the ECMWF the most consistent in indicating these amounts in
and around Houston, while the latest HRRR-EXT run indicates totals
nearly this high as far southwest as Corpus Christi). Overall, the
CAM consensus is clustered farther southwest than much of the
coarser global guidance has been over the past several days, and
convection tends to verify farther southwest than expected (giving
credence to the HRRR solution, which resulted in an expansion of
the Marginal risk area farther southwest). A targeted upgrade to a
Slight risk remains possible with subsequent updates, which would
be most likely across portions of TX (where instability and the
potential for 1-2"/hr rainfall rates is expected to be maximized).

Churchill


Day 2
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES, LOS ANGELES BASIN, AND INLAND
EMPIRE OF SOUTHERN CALIFORNIA...

Shower activity in association with the deep closed low looks to
continue into Monday morning over portions of Southern California,
before the low pivots inland and brings an end to the potential for
sub-hourly (15-min) rainfall rates of 0.25"+. This will continue
the Marginal risk for excessive rainfall into Day 2 for both
coastal and more inland low elevation areas (with particular
concern for any lingering shower activity over aforementioned burn
scars). The threat is expected to last for only the first half of
Day 2 (prior to 00z Tuesday).

Churchill


Day 3
Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill
$$
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FOUS30 KWBC 270750
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 AM EST Mon Jan 27 2025

Day 1
Valid 12Z Mon Jan 27 2025 - 12Z Tue Jan 28 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE TRANSVERSE/PENINSULAR RANGES OF SOUTHERN CALIFORNIA...

Light to moderate shower activity in association with a deep
closed low looks to continue for a bit longer early this morning
over portions of Southern California, gradually diminishing by
midday as the low weakens and pivots inland and brings an end to
the (now very low) potential for sub-hourly (15-min) rainfall
rates of 0.25"+. Maintained the inherited Marginal risk for
excessive rainfall for this very low-end potential, primarily due
to elevated concerns for any lingering shower activity over the
recent burn scars. This activity should largely come to an end by
18z, and the Marginal risk will likely be discontinued with an
update later this morning or afternoon.

Churchill


Day 2
Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill/Wegman


Day 3
Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHERN OKLAHOMA...

An upper level cutoff low ejecting out of the Southwest into the
Plains will provide the forcing for ascent for a developing low
over North Texas Wednesday and Wednesday night. Return flow of Gulf
moisture ahead of the low will advect north into Texas and
Oklahoma along with increasing instability. The typical diurnal
increase of the LLJ Wednesday night will greatly increase the
coverage and intensity of storms along the developing low's cold
front, while the warm front locally increases lift into Oklahoma.
Since the upper level low will be slow-moving (though increasing in
forward speed with time), this will allow the storms that form to
train over the same areas. Soils in the area have been drier than
normal, meaning most of the rain will be beneficial, however where
storms train over more flood-sensitive areas and urban areas,
localized flash flooding is possible. Ensemble guidance have
increased in the areal coverage of at least 1 inch of rain, though
the pattern is much more favorable to narrow streaks of much
heavier totals where the storms track. Thus, confidence has
increased enough to introduce a Marginal Risk area.

The more significant flash flooding risk will be largely north and
east of the Day 3 Marginal Risk area going into D4/Thursday.

Wegman
$$
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FOUS30 KWBC 281509
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1009 AM EST Tue Jan 28 2025

Day 1
Valid 12Z Tue Jan 28 2025 - 12Z Wed Jan 29 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina


Day 2
Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHERN OKLAHOMA...

An upper level cutoff low ejecting out of the Southwest into the
Plains will provide forcing for ascent for a developing low over
the Texas and Oklahoma Panhandles Wednesday and Wednesday night.
Return flow of Gulf moisture, warmth, and instability ahead of the
low will advect north across Texas and into Oklahoma and Arkansas
Wednesday and especially Wednesday night. The typical diurnal
strengthening of the LLJ Wednesday night will greatly increase the
coverage and intensity of storms along the developing low's cold
front across Texas, while the warm front locally increases lift
into Oklahoma. The surface low will track north into Colorado,
contributing to a winter storm there. Meanwhile much of the
convection will be associated with the cold/occluded front as it
pushes east towards the Mississippi Valley. Since the upper level
low will be slow-moving (though increasing in forward speed with
time), this will allow the storms that form to train over the same
areas. Training convection will be of greatest concern as the
storms are first forming over Texas. Soils in the area have been
drier than normal, meaning most of the rain will be beneficial,
however where storms train over more flood-sensitive and urban
areas, localized flash flooding remains possible.

Wegman


Day 3
Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID-SOUTH AND ARKLATEX REGION...

Thunderstorms will likely be ongoing on the northern and western
sides of the Slight and Marginal Risks at the start of the period
Thursday morning. A vigorous upper level cutoff low will begin to
rejoin the jet stream by Friday morning. Before it does so however,
it will provide the forcing for numerous showers and thunderstorms
across much of the Mid-South and Arklatex region. A 50-60 kt LLJ
will pump plentiful Gulf moisture across the Arklatex region
Thursday morning. The trailing cold front will push east across
Oklahoma and Texas, providing ample forcing for the showers and
storms. The primary limiting factor for the storms will be
instability. The greatest instability (around 500 J/kg) will be
across southern and central Texas, while the greatest forcing will
be further north from the Arklatex north and east into the
Mississippi Valley. This disconnect may help to diminish the flash
flooding potential a bit. However, this will be partially offset
by upsloping into the Ozarks across Oklahoma and Arkansas.

It appears that many of the storms will be associated with a line
that moves east across the area. However, storms embedded within
the line may move somewhat parallel to the line towards the
northeast, this will allow for the potential for training. Since
there will be plenty of Gulf moisture (PWATs above 1.5 inches),
the showers and storms will have the potential to cause localized
flash flooding issues, especially in the terrain of the Ozarks
where upsloping may contribute to locally higher rainfall totals.
By Thursday night, the storms will push across the Mississippi
River. By then the eastward forward speed of the storms should
increase, thus diminishing the flooding potential with time.

Wegman

$$
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FOUS30 KWBC 290824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Wed Jan 29 2025

Day 1
Valid 12Z Wed Jan 29 2025 - 12Z Thu Jan 30 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH TEXAS AND SOUTHEASTERN OKLAHOMA...

An upper level cutoff low ejecting out of the Southwest into the
Plains will provide ample forcing for ascent (along with the left
exit region of a ~120 kt jet streak) for a developing low over the
Texas and Oklahoma Panhandles into tonight. Return flow of Gulf
moisture, warmth, and instability ahead of the low will advect
north across Texas and into Oklahoma and Arkansas today (and
especially into tonight). The typical diurnal strengthening of the
LLJ overnight will greatly increase the coverage and intensity of
storms along the developing low's cold front across Texas
(particularly after 06z), while the warm front locally increases
lift into Oklahoma. The surface low will track north into Colorado, contributing to a winter storm there. Meanwhile much of the
convection will be associated with the cold/occluded front as it
pushes east towards the Mississippi Valley. Since the upper level
low will be slow-moving (though increasing in forward speed with
time), this will allow the storms that do form to have a high
likelihood of training over the same areas. Training convection
will be of greatest concern as the storms are first forming over
North TX and into southeast OK (where a targeted upgrade to Slight
risk was made, given 00z HREF 40-km neighborhood probabilities for
3" exceedance of 50-70%, and primarily over a relatively short
period from 06z-12z Thurs). Soils in the area have been drier than
normal, meaning most rainfall should (in theory) be beneficial,
however the rapid development of rainfall rates of 1-2"/hr over the
dry soils may locally exacerbate the flash flood threat (due to
hardened and compacted soil resulting in reduced infiltration and
increased runoff).

Churchill/Wegman


Day 2
Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
ARK-LA-TEX, OZARKS, AND MID-SOUTH REGIONS...

Thunderstorms will likely be ongoing on the northern and western
sides of the Slight and Marginal Risks at the start of the period
Thursday morning. A vigorous upper level cutoff low will begin to
rejoin the polar jet by Friday morning. Before it does so,
however, it will provide the forcing for numerous showers and
thunderstorms across much of the Mid-South and Ark-La-Tex region.
A 50-60 kt LLJ will pump plentiful anomalously high Gulf moisture
across the Ark-La-Tex region Thursday morning, as the trailing
cold front provides ample forcing for showers and storms across
Oklahoma and Texas (and depending on how much rainfall occurs in
the 6-12 hours prior to the start of Day 2, some ongoing flash
flooding for prior training storms may be locally significant in
portions of North TX into southeast OK, where probabilities for
excessive rainfall are towards the higher-end of the Slight risk
spectrum, being 25%+). The main limiting factor will likely be
decreasing instability (to around 500 J/kg of MU CAPE into Thursday
morning), which looks to be maximized across southern and central
Texas. The greatest forcing looks to be displaced farther north
(from the Ark-La-Tex north and east into the Mississippi Valley).
This disconnect may help to diminish the flash flooding potential a
bit (though concerns are greatest where the best instability and
forcing may meet over aforementioned areas of North TX into southeast OK).

It appears that many of the storms will be associated with a line
that moves east across the area. However, storms embedded within
the line may move somewhat parallel to the line towards the
northeast, this will allow for the potential for training farther
upstream into portions of the Middle MS Valley and OH Valley.
Since there will be plenty of Gulf moisture (PWATs above 1.5
inches, above the max moving average using BNA as a proxy
sounding), the showers and storms will have the potential to cause
isolated to scattered flash flooding. By Thursday night the flash
flood threat should diminish as the last storms cross the MS River
with eastward forward speed of the storms increasing along with
continued decreasing instability, thus diminishing the flooding
potential with time.

Churchill/Wegman


Day 3
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST COASTAL RANGES AND FOOTHILLS...

A wetter pattern will take shape across the Pacific Northwest
region beginning Thursday night, as a series of storm system look
to impact the area (originating from a closed low from the North
Pacific interacting with another closed low/digging trough moving
south along the west coast of British Columbia). After an initial
round of light to moderate precipitation late on Day 2 in the
coastal ranges and foothills, rainfall will become moderate to
heavy at times throughout much of Day 3. While forecast maximum
IVT values are relatively weak overall (500-700 kg/ms), a longer
duration event could lead to a more 'moderate' atmospheric river
event. 24 hour rainfall totals are forecast to generally range from
1-3" (though may locally exceed 3" in favored upslope terrain).

Churchill

$$
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FOUS30 KWBC 301034
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
534 AM EST Thu Jan 30 2025

Day 1
Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST
TEXAS INTO ARKANSAS AND PORTIONS OF KENTUCKY AND TENNESSEE...

Not much change to the inherited Slight risk stretching from
northeast TX, across central AR and into portions of TN and KY.
Both PWs and IVT are approaching climatological maximum values for
late January across this corridor, so a very dynamic system with
plenty of moisture to work with. Instability will be a limiting
factor, keeping rainfall rates and the overall flash flood
potential lower than it otherwise could be. The highest rates will
probably be early in the period over the southwestern portion of
the Slight risk (northeast TX into southwest AR), where hourly rain
could approach 1.5". Further downstream over AR into KY and TN
rainfall rates off the 06z HRRR and 00z HREF peak closer to 0.5-0.75"/hr.

Much of the Slight risk area is expected to see at least 2" of
areal averaged rainfall today into this evening, with the areas
from northeast AR into far western TN/KY having the highest
probabilities of 3"+ amounts. There is good overlap between the
higher 3" EAS probabilities from the HREF and the 06z HRRR max QPF axis...adding confidence in the potential for a 3-5" rainfall
swath from northeast AR into far western KY/TN. Rainfall rates here
will not be all that high to start out, with these higher totals
driven more by a persistent moderate rain. However by 00z guidance
indicates these areas could move into the warm sector and see more
robust convection with higher rate potential. If this occurs then a
greater coverage of flash flooding could evolve.

Stronger convection should develop over portions of southern AR
into MS by this evening. However this activity will be progressive
enough to limit the flash flood risk.

A Marginal risk was maintained in the developing comma head from
eastern KS into northern MO, southern IA and west central IL. Low
topped convection could locally increase rainfall rates, with a
broad 1-3" of rain forecast (highest probs of approaching 3" are in
far northeast KS into northwest MO).

Chenard


Day 2
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA...

...West Virginia...
A Slight risk was issued for portions of central and eastern
WV...primarily for areas where significant snowpack still exists.
The latest NOHRSC analysis indicates upwards of 2-4" of snow water
equivalent over the terrain of eastern WV and the latest model
consensus is for 1-3" of rainfall over this area. With
temperatures and dewpoints forecast to at least get into the 40s
(maybe 50s in spots) the rain on snow should trigger a fairly
efficient melting of the snowpack. This is likely to result in
numerous instances of areal flooding and at least some river flooding.

Rainfall rates are generally forecast to stay in the 0.25" to 0.5"
an hour range, which is probably not enough to generate flooding
of a more flash variety. However there are indications in both the
06z HRRR and 00z HREF that embedded convective elements Friday
could locally push hourly rainfall a bit over 0.5". Given the
amounts of runoff that will be occurring from the combined snow
melt and rain, any of these higher rates could locally result in a
flash flood threat as well.

...Northern Illinois and Indiana...
A Marginal risk was maintained across this area as comma head
rainfall drops up to 1.5" of rain in a 6hr period Friday. Typically
this degree of rain would not result in impacts, however frozen
ground will likely enhance runoff potential. Given the extra
runoff this will generate, amounts around 1.5" in 6 hrs could be
enough to trigger at least some flood concerns across this corridor.

...West Coast...
An atmospheric river will bring heavy rainfall from western WA
southward into northern CA Friday into Friday night. The IVT plume
is generally progressive and weakening through the period, so not
expecting major rainfall totals or impacts. Generally forecasting a
1-3" rainfall (locally higher in the favored terrain), which should
mainly drive an isolated minor flood threat.

The IVT axis does start to stall as it shifts into northern CA, and
so could be looking at totals more in the 2-4" range across the
terrain here. Will need to monitor for a potential Slight risk
upgrade across portions of the northern CA coast, but for now will
stick with the Marginal. Leaning towards this day 2 rain mainly
helping saturate conditions leading towards a greater flood risk
on later days. However should things trend quicker and/or wetter
an upgrade may need to be considered.

Chenard


Day 3
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA...

A pretty significant model change has been noted over the past
couple cycles across the eastern Pacific into the west coast. The
earlier model consensus was for a stalling of a moderate to strong
AR over northern CA and southwest OR resulting in significant
rainfall amounts. However recent model runs have unanimously
shifted further south with this AR, now stalling it more over
central to north central CA. There has also been a weakening trend
in the AR, so QPF values are not as high as earlier forecast.
However, with the southward shift the max rainfall axis is now over
areas that tend to be more sensitive to heavy rainfall and related
flood impacts. Thus even with the drop in QPF, think a southward
shift of the Slight risk is warranted. The Slight risk now
stretches from Mendocino county south into the San Francisco area
and as far south as Santa Cruz county. It also extends inland
across the Sacramento Valley and into the Sierra Nevada.

Rainfall along the coast will start on Friday and continue into
the day Saturday. A gradual saturating of soils are expected to
lead to potential greater impacts on Saturday, warranting the
Slight risk. Two day rainfall totals are currently forecast in the
2-4" range. Two day rainfall amounts of 1-3" are expected within
Sacramento valley locations, with upwards of 4-6" within the
Sierra Nevada (most of which will be rain given high snow levels).

Chenard
$$
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FOUS30 KWBC 310815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Fri Jan 31 2025

Day 1
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA AND NORTHERN ILLINOIS...

...West Virginia...
Only minimal changes to the Slight risk area over portions of WV. The
latest NOHRSC analysis indicates upwards of 2-4" of snow water
equivalent over the terrain of eastern WV and the latest model
consensus continues to depict 1-3" of rainfall over this area.
With temperatures and dewpoints forecast to at least get into the
40s (50s in spots) the rain on snow should trigger a fairly
efficient melting of the snowpack. This is likely to result in
numerous instances of areal flooding and at least some river flooding.

Rainfall rates are generally forecast to stay in the 0.25" to 0.5"
an hour range, which is probably not enough to generate flooding
of a more flashy variety. However there are indications in the 00z
HREF that embedded convective elements could locally push hourly
rainfall a bit over 0.5". Given the amount of runoff that will be
occurring from the combined snow melt and rain, any of these higher
rates could locally result in a flash flood threat as well.

...Northern Illinois and Indiana...
Only minor changes to the inherited Slight risk area within the
comma head rainfall axis over northern IL. Despite little to no
instability, strong frontogenesis within this axis is resulting in
moderate HREF probabilities of rainfall exceeding 0.5" in an hour.
Rainfall amounts upwards of 1.5" in 6 hrs should be enough to at
least trigger some minor flood impacts given the extra runoff
generated by the mostly frozen ground.

...West Coast...
An atmospheric river will bring heavy rainfall from western WA
southward into northern CA today into night. The IVT plume is
generally progressive and weakening through the period, so not
expecting major rainfall totals or impacts. Generally forecasting
1-3" of rainfall (locally higher in the favored terrain), which
should mainly drive an isolated minor flood threat.

The IVT axis does start to stall as it shifts into northern CA, and
so could be looking at totals locally in the 2-4" range across the
terrain here. Given the generally dry antecedent conditions still
think this initial batch of rain will mainly just set the stage
for a greater flood threat in the following days, so will keep the
risk at Marginal. However we may see at least some uptick in flood
related impacts over central to northern CA by tonight.

Chenard


Day 2
Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

The atmospheric river is forecast to stall over central/northern
CA on Saturday. Most model solutions indicate an increase in IVT
moving into the CA coast near San Francisco just after 12z
Saturday, which should bring an uptick in rainfall rates onshore.
PWs will be quite impressive for the time of year, with values
forecast to exceed the 99th percentile. A lack of strong dynamics
and frontal convergence, combined with only moderate IVT values,
will cap the magnitude of rainfall rates. Nonetheless, expecting to
see enough of an uptick in rainfall Saturday with the stalling AR
to support an increase in flood impacts, warranting the Slight risk.

While IVT values along coastal areas will be moderate, what this
AR will have going for it is an impressive inland extent of IVT magnitudes...with values exceeding the 99th percentile (and around climatological max values) within the Sierra. This supports the
inland extension of the Slight risk across the Sacramento Valley
and into the mountains. Snow levels will be high, so the Slight
risk extends pretty far into the terrain. We also made sure to
extend the Marginal risk over the crest and into the eastern slopes
of the Sierra, as given the IVT, we should see enough rain make it
over the crest to support at least a minor flood risk.

Two day rainfall totals are currently forecast upwards of 2-4"
along coastal areas, 1-3" within Sacramento valley locations, and
upwards of 4-7" within the Sierra Nevada (most of which will be
rain given high snow levels).

Chenard


Day 3
Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

The slow moving atmospheric river will continue to bring a heavy
rainfall and flood threat to portions of northern CA Sunday into
Sunday night. The IVT core and max rainfall axis should shift a bit
north compared to day 2 (Saturday). PWs drop off compared to
earlier in the event, however the persistent IVT should support a
prolonged moderate to locally heavy rain. Snow levels are quite
high, and thus the Slight risk was expanded into the terrain, and
even a bit to the east of the crest given the inland IVT
anomalies. Rainfall rates within the higher terrain of the Sierra
and just downwind of the crest will more be driven by the
persistent IVT over the entire period, and thus flood impacts will
be possible through Sunday and into the overnight hours.

Along coastal and valley areas, rainfall rates should increase by
later Sunday night as convergence increases along a front moving
into the CA coast. Given what should be saturated conditions by
this time, this uptick in rainfall rates along coastal areas into
portions of the northern Sacramento Valley may result in an uptick
in flood impacts by later Sunday night.

Chenard

$$
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FOUS30 KWBC 010757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Sat Feb 1 2025

Day 1
Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA...

A nearly stationary positively tilted longwave trough will be
anchored across central British Columbia today. It will direct a
zonal but powerful 150+ kt jet streak into the Pacific Northwest.
The jet streak will in turn guide an atmospheric river (AR) moving
eastward after soaking Hawaii into northern California today. The
support of the zonal jet will allow the AR's moisture to extend well
inland from the coast, including into Nevada, Utah, Idaho, and
Wyoming. The areas getting the greatest rainfall will be the
northern Sierra Nevada mountains and secondarily the coastal ranges
around and north of the Bay Area. For just this Day 1 period, expect
4-6 inches of rain from west of Lake Tahoe north and west to about
Paradise, CA. 1-3 inches are expected for the northern Coastal
Ranges from the Bay Area through southern Trinity County.

IVT values for this atmospheric river event peak at around 850 kg/ms
around 18Z/10am PST in the Bay Area this morning according to both
GFS and EC ensemble output. While this alone is notable, it's far
from extreme. It will be the duration of this event that will make
it stand out from most ARs. While IVT values will drop closer to 500
kg/ms in the following days, they will remain elevated near that
level for 3 consecutive days after today. Using IVT as a proxy for
heavy rain, this would suggest the heaviest rain of the event will
be this afternoon and evening, but smaller disturbances responsible
for brief upticks in precipitation rates become increasingly
difficult to predict at extended time ranges, so it's very possible
additional heavy rainfall will occur in the subsequent days.

PWATs advecting into the coast with the AR will get as high as 1.25
inches today. NAEFS mean PWAT probabilities put this at the 98th
percentile as compared with a 30 year average. Additional flooding
concerns are also forecast around the 2024 Park Fire east of Red
Bluff and north of Chico. Here, hydrophobic soils and a lack of
vegetation will convert most, if not all of the rainfall to runoff,
resulting in locally worse flooding as compared with surrounding areas.

Changes from the previous forecast were minimal, with expansions of
the Marginal Risk drawn in around Lassen N.P. The highest elevations
will see snow, but with such high snow levels (6,000-7,500 ft), the
large majority of the expected precipitation in northern California
will be in the form of rain. NASA Sport soil moisture imagery shows
much of Northern California's soils are below to well below normal,
so much of the rain, especially in the populated valleys should
start out as beneficial...whereas the flooding threat will be the
most impactful in the foothills of the Sierras.

Wegman


Day 2
Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA...

The slow moving atmospheric river will continue to bring a heavy
rainfall and flood threat to portions of northern CA Sunday into
Sunday night. The IVT core and max rainfall axis should shift a bit
north compared to day 1 (Saturday). PW anomalies will gradually
drop, however the persistent IVT should support a prolonged
moderate to locally heavy rain. Snow levels are quite high, and
thus the Slight risk area goes into the northern Sierra, and even
a bit to the east of the crest given the strong inland IVT anomalies.

The highest rainfall rates will likely be across portions of the
northern Sierra where more anomalous IVT will focus into the
terrain. 48hr rainfall amounts of 8-12" are forecast over portions
of the northern Sierra (generally from Placer up to Butte and
Plumas counties) through 12z Monday. This portion of the Sierra
has some of the highest 48hr ARI thresholds in the country, so
this region is clearly used to heavy rainfall amounts. With that
said, these forecast totals are still exceeding the 2 to 5yr
thresholds. Seems like it'll be a cold rain so not sure how much
SWE loss we'll get in the higher elevations where there is
snowpack (rain might mostly get absorbed into the snowpack). Given
these totals and periodic rates around 0.5" per hour, at least some
flood impacts appear likely, with higher end Slight risk
probabilities justified. The lack of convective rainfall rates,
combined with the fact this area can generally take quite a bit of
rain (as seen by the high ARI thresholds), suggests these higher
end Slight probabilities should suffice for now.

A prolonged steady rain along coastal to valley areas will result
in a widespread 1-3" of additional rainfall over the Slight risk
area, brining 48hr totals into the 3-6" range. A lack of strong
frontal convergence should keep rainfall rates lower here, but the
prolonged nature of the rain should still allow for an uptick in
flood impacts as conditions continue to saturate.

Chenard


Day 3
Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA...

Not much change to the IVT axis is forecast into Monday, meaning
the prolonged moderate to heavy rainfall event will continue across
northern CA. Snow levels should begin to drop by this time
resulting in less impacts over the higher terrain of the Sierra,
but a continued uptick in flood impacts over coastal areas into the
foothills where conditions will continue to saturate. By Monday
afternoon model guidance indicates we should see a strengthening of
frontal convergence across northern CA, which should help drive an
uptick in rainfall rates along the coast.

Model spread begins to increase by day 3 with the 00z NAM and GFS
a bit further north with the heavier rainfall axis than the 00z
ECMWF, UKMET and GEM. Interestingly the 18z ECMWF AIFS is closer
to the GFS/NAM camp. Big picture wise the differences do not end
up having a major impact on QPF or the ERO (differences get bigger
on day 4), but if the further north GFS/NAM end up being right the
Slight risk area could end up shifting a bit north with future
updates. There is some potential for higher end Slight risk, or
maybe even MDT risk, probabilities along coastal areas given the
expected uptick in rainfall rates over what should be fairly
saturated ground conditions by this point. However a lack of
stronger dynamics and lack of instability interacting with the IVT
plume may prevent rates from getting high enough to cause more
widespread significant impacts. So will just maintain the Slight
risk for now, but either way anticipate at least some continued
flood impacts Monday and will continue to monitor trends.

Chenard
$$
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FOUS30 KWBC 020803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Sun Feb 2 2025

Day 1
Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN CALIFORNIA...

A prolonged atmospheric river (AR) event is underway across almost
all of northern California this morning. A large area of high
pressure off southern California is teaming up with a pair of
lows...one well north of Hawaii and the second near the northern tip
of Vancouver Island, BC. These features will interact to form a long
corridor where abundant tropical moisture can continue riding
northeastward from the tropics and into northern California.
Meanwhile, the lows are limiting the northward extent of the
moisture, especially the one off Canada, which is supplying colder
air into Oregon and Washington State.

In the upper levels, a stationary upper level low will guide upper
level disturbances around its southern periphery and into northern
California throughout this period, perhaps even retrograding a bit
west by tonight which will only further lengthen the fetch over the
Pacific. The strong high in place near southern California will form
a tight gradient between the two features, resulting in a
hyperactive zonal jet which will support continuous forcing for lift
supporting the rainfall within the AR.

Nasa Sport Soil Moisture imagery shows much of the soils of northern
California have become nearly saturated as a result of the 1-4
inches of rain that fell yesterday through the time of this writing.
Thus, expect much of the rainfall going forward associated with this
AR to largely convert to runoff.

A pair of upper level disturbances are moving along the atmospheric
river. The first will clear California and move into the interior
this morning. Heavy rain ongoing now over far northern California
will gradually drift south through the day in response to this first disturbnace. This will give coastal regions a brief break in the
rainfall behind the disturbance from the start of this period
through late morning/midday. Then the second disturbance will renew
the heavy rainfall beginning midday/early afternoon and continue
right through tonight. The rainfall plume with this second round of
rain will be very slow to move, only crawling northward with time.
Most areas of far northern California will see several hours of
continuous, steady rain. Rates don't look to be quite as high with
this second disturbance, likely averaging between 1/4 and 1/2 inch
per hour. Thus, the primary hazards with it will be associated with
the long-duration of the rain.

Potential impacts from this AR today and tonight include possible
mudslides and rock slides in the foothills and mountains below snow
level (~6,500 ft), as well as the potential for localized flooding
in flood sensitive and low-lying areas with poor drainage.

Wegman


Day 2
Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA...

Not much change to the IVT axis is forecast into Monday, meaning
the prolonged moderate to heavy rainfall event will continue across
northern CA. Snow levels should begin to drop by this time
resulting in less impacts over the higher terrain of the Sierra,
but a continued uptick in flood impacts over coastal areas into the
foothills where conditions will continue to saturate. Model
guidance continues to show a strengthening of the frontal
convergence by this time over northern CA to go along with an
uptick in IVT values moving into the coast. This should result in
an increase in rainfall rates by late morning into the early
afternoon, with this axis of higher rates shifting south through
the afternoon and evening.

There is some potential for higher end Slight risk, or maybe even
MDT risk, probabilities along coastal areas given the expected
uptick in rainfall rates over what should be fairly saturated
ground conditions by this point. However a lack of stronger
dynamics, and lack of instability interacting with the IVT plume,
should put a cap on upward vertical motion and rainfall rates.
Thus while some flooding impacts appear likely, more widespread
significant impacts may be harder to come by, and thus not
planning on any upgrade at this time.

Chenard

Day 3
Valid 12Z Tue Feb 04 2025 - 12Z Wed Feb 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND NORTHERN CALIFORNIA...

The atmospheric river will continue dropping southward on Tuesday,
bringing another day of moderate to heavy rainfall and some
flooding risk. Still some uncertainty with the latitude of the AR
by this time, although models do appear to be converging on a
solution covered by the Slight risk area. Some trimming on the
northern end of the Slight risk may eventually be warranted, but
with some lingering uncertainty and the saturated conditions over
that area, we were a bit conservative with the northern extent of
the risk area.

Model solutions indicate the approach of a mid level shortwave
and a stronger upper jet after 18z Tuesday. These features will
help drive an increase in IVT, and may also help briefly stall the
southward progression of the front. Thus it appears likely that we
will see an uptick in rainfall rates after 18z Tuesday from
approximately San Francisco south into Monterey county. Still some
uncertainty on the exact location of this enhanced rainfall rate
activity, as well as exactly how intense the rain will be...but
current model solutions support upwards of 1.5" in a 3 hour
period, and 2-4" in total. This rainfall will likely drive an
increase in the flood threat over these areas. Luckily most of the
heavier rainfall on the preceding days will have been north of this
corridor, so not expecting conditions to be too saturated leading
into the uptick in rain. This fact, combined with the overall
progressiveness of the system, keeps the flood risk at a Slight
level. Although will need to continue to monitor, as if rainfall
rates trend higher can not rule out a localized corridor of MDT
risk level impacts.

The Marginal risk extends further down the coast into Santa
Barbara, Ventura and Los Angeles counties. The front should be
quick moving by the time it makes it this far south, but a
quick shot of heavy rainfall is possible. The 00z GFS and NAM do
not bring the better moisture transport this far south, however the
00z ECMWF, UKMET and GEM do. Even these latter models only get
rainfall totals into the 1-2" range given the fast frontal motion.
So generally not expecting widespread or significant impacts at
this time, although rainfall rates could be briefly high enough to
result in some localized minor flood concerns. We will keep a close
eye on the sensitive burn scars over Los Angeles county, but at
this time even the more aggressive ECMWF would suggest rainfall
rates will be on the decline by the time they get that far south.
Will need to continue to monitor, but think a Marginal risk will
suffice for now.

Chenard

$$
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FOUS30 KWBC 030816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Mon Feb 3 2025

Day 1
Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN CALIFORNIA...

The atmospheric river event across much of northern California will
continue through the entire Day 1/Monday period. A narrow (100-200
mile) wide plume of rainfall rounding the northwestern periphery of
a broad subtropical high is also being steered around a positively
tilted upper level trough centered over British Columbia near the
northern tip of Vancouver Island. The jet stream is rounding the
upper level trough from the northeastern side of the trough around
to its west and south then ejecting around the southeastern side of
the trough. The trough will retrograde southwestward, allowing a
cutoff upper level low to form as the jet stream eventually cuts off
from the upper level low and maintains a trough over much of eastern
Canada.

The upper low will drift southwestward through 12Z Tuesday, which
will work to reorient the atmospheric river from its current west-to-
east to more of a southwest-to-northeast orientation. This will have
several ramifications: First, it will maintain the plume of rain
into northern California as the reorienting atmospheric river pivots
over this area. Rates are expected to remain largely steady based on
the latest HiRes guidance. Second, the "pivot" will be associated
with a subtle surface low tracking along the atmospheric river. This
will locally increase rainfall rates from midday through this
afternoon area-wide...though localized convergence in the northern
Sacramento Valley will keep rainfall rates elevated there throughout
the period. Third, the southwesterly flow may support the ongoing
low level jet in the Sacramento Valley better since the broader flow
will align better with the low-level jet. The result will be
continued heavy rain for the northern and eastern Sacramento Valley
and the foothills of the northern Sierras with a widespread 3-5
inches of rain expected.

The southwesterly reorienting will then begin to push the
atmospheric river southward from this afternoon through tonight,
reaching the Bay Area this evening. Upslope flow into the Sierras
will keep heavy rain ongoing into the foothills of the Sierras all
the way up the Sacramento Valley, however, the rain will finally
taper off and give the northern coastal ranges a much needed break
by tonight as the core of the AR moves south of the Bay Area and
weakens with time. The faster southerly movement of the AR from late
this afternoon through the overnight should significantly lower the
flooding threat with decreasing latitude, so the southern end of the
Slight risk is largely the same, stopping just north of the Bay
Area, as amounts from the Bay Area south should only necessitate a
Marginal Risk since they've been relatively dry in recent days.

With this expected evolution, a few minor changes were made to the
Day 1 ERO from continuity, namely to begin expanding the Marginal
Risk southward to include the Santa Cruz mountains through Monterey
Bay. Otherwise, only minor tweaks were made to better match both
Marginal and Slight risk areas with the local higher elevations that
will see mostly snow, while including some of the valleys in between.

Wegman


Day 2
Valid 12Z Tue Feb 04 2025 - 12Z Wed Feb 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND NORTHERN CALIFORNIA...

The atmospheric river event will continue into Tuesday across
central and northern CA. Model solutions continue to indicate the
approach of a mid level shortwave and a stronger upper jet after
18z Tuesday. The approach of these features should help slow/stall
and increase the magnitude of the IVT axis during the day. The
increase in forcing/IVT and a strengthening of the low level
convergence along the front, will both support an uptick in
rainfall rates from near San Francisco into the Sacramento Valley
and foothills of the Sierra. Hourly rainfall could approach and
locally exceed 0.5" along this front, which should drive an
increase in the flood threat. The good news is that most of these
areas in line for the heaviest rainfall Tuesday have been south of
the heaviest rain the past few days. This combined with the fact
that the front/IVT axis (while briefly stalling), will start to
make quicker southward progress by 00z, should limit the threat of
more widespread significant flood impacts. Nonetheless, do
anticipate some flooding and landslide potential to accompany this
rainfall Tuesday.

The Slight risk extends south into coastal Monterey county, where
stronger orographic effects will help drive heaver rainfall totals
in the 4-7" range. As the front drops into this area there will
also be an uptick in hourly rainfall, with high probabilities of
0.5" in an hour and localized totals approaching 0.75" in an hour
probable. Heavy rainfall is common along this corridor, and
antecedent conditions this far south are quite dry. Both these
factors should help put a cap on the magnitude of flood impacts
from this rain, however would still expect to see at least some
flood and landslide potential.

The Marginal risk extends further down the coast into Santa
Barbara, Ventura and far western Los Angeles counties. The front
should be quick moving by this time, and the IVT axis should be
weakening. The GFS is the weakest model, but most other models have
come around to at least a short period of locally heavy rainfall
getting into the favored upslope terrain areas. Even these wetter
models (and the WPC forecast) only get rainfall totals into the
1-2" range given the fast frontal motion. So generally not
expecting widespread or significant impacts at this time, although
rainfall rates could be briefly high enough to result in some
localized minor flood concerns.

Chenard


Day 3
Valid 12Z Wed Feb 05 2025 - 12Z Thu Feb 06 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO VALLEY...

Strong southwesterly flow will advect abundant moisture northward
into the OH Valley on Wed into Wed night, with PWs forecast to
increase over the 90th percentile for early February. Increasing
WAA, weak mid level shortwaves moving east within the quick zonal
flow, and upper level divergence in the right entrance region of a
jet to the north and left exit region of a strong jet over the MS
Valley, will broadly support an expansion of showers across the
region through the period. Model guidance also indicates decent
instability near and south of a northward moving warm front, thus
expect we will see embedded thunderstorms as well. Given the
forcing, moisture and instability forecast, do anticipate we will
see some locally heavy rainfall rates materialize, especially by
Wed evening into the overnight near the lifting warm front.

Still some uncertainty on the location of this front, with the
ECMWF the furthest north solution and the GFS on the southern side
of the model spread. Precipitation north of the front will likely
be either sleet or freezing rain, and thus the position of this
feature will be important to delineate the northern edge of the
flood risk area. At this time generally favor something in the
middle, which would favor an axis from KY into southern IN/OH and
into WV for the greatest rain rate potential, and the Marginal risk
encompasses these areas. However, if a warmer/further north
solution becomes more likely, then the Marginal may need to be
expanded further north, with the opposite being true if a
colder/south scenario comes to fruition. Portions of
central/eastern KY into WV have above average soil saturation and
streamflows, and so this area is likely most susceptible to some
flooding impacts if higher rainfall rates do materialize.

Chenard

$$
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FOUS30 KWBC 040827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 AM EST Tue Feb 4 2025

Day 1
Valid 12Z Tue Feb 04 2025 - 12Z Wed Feb 05 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND NORTHERN CALIFORNIA...

A large and energetic cutoff low that will remain largely stationary
off the coast of Vancouver Island will be the upper level animus for
the active weather ongoing across California today. An atmospheric
river event that has been ongoing for the past several days will
come to an end (or at least take a significant break) during this
period. However, to get to the break, one last potent area of low
pressure moving along the atmospheric river still has to move across California, resulting in another period of heavy rain. The cold air
following behind its cold front will push the AR far enough south
but also far enough away from the forcing to cause it to mostly
dissipate tonight.

The atmospheric river currently is located along the I-80 corridor
from San Francisco through Lake Tahoe. It is expected to shift south
to about a Monterey to Modesto line, leaving the I-80 corridor in
periodic shower activity. It will not move much along the foothills
of the Sierras, where upslope flow will keep the steady rain going
well north of where the core of the atmospheric river is. From the
Bay Area north along the coast, expect a break from the rainfall
through until about 15Z/7am PST. That all changes beyond 15Z, when
the aforementioned low begins to move ashore, resulting in steady
rain for the almost the entire northern half of California. As in
previous days, localized convective enhancement and convergence
bands are expected to develop within the broader rainfall shield.
This will be supported by a strengthening LLJ through the Sacramento
Valley, which will intensify due to the approach of the southerly
flow ahead of the low. Colder and drier air following behind the low
will begin to clear the precipitation out from northwest to
southeast starting around 22Z/2pm PST. Until the back edge clears,
locally heady rain will continue, which will be enhanced by the
passage of the surface cold front itself, bringing a brief period of
heavy rain for most of northern California during the late afternoon
and evening hours, potentially impacting this evening's commute.

After about sunset or so, the entire atmospheric river will weaken,
though the prevailing southwesterly upslope flow may continue much
lighter rain into the Transverse Ranges, the southern Sierras, and
eventually the Los Angeles Metro area. Despite the recent fires in
that area, the rain should be light enough east of Santa Barbara
County that there should not be enough to cause flooding concerns,
so the light rain from the showers should be beneficial, if not
nearly enough to dent the ongoing drought conditions in southern California.

All of the guidance has shifted significantly northward with the
track of the incoming low for the day today. This resulted in a
northward shift in the ERO risk areas, with the Slight Risk being
expanded well up the coast to Mendocino County and all of the
northern Coast Ranges and trimmed out of the southern Central
Valley. An internal higher-end Slight was added to most of the
foothills of the northern Sierras since this area stands to pick up
an additional 3-5 inches of rain on top of the foot+ they've seen
the past few days.

The ERO trimming was due to rain shadowing from the southern Coast
Ranges. Unlike previous days, the southern Coast Ranges from the
Santa Cruz Mountains south through the western Transverse Ranges
should all get a healthy soaking today from the atmospheric river,
though since those areas have seen significantly less rainfall in
recent days compared to areas further north, have left most of these
areas in the Marginal.

Wegman


Day 2
Valid 12Z Wed Feb 05 2025 - 12Z Thu Feb 06 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY...

Strong southwesterly flow will advect abundant moisture northward
into the OH Valley on Wed into Wed night, with PWs forecast to
increase over the 90th percentile for early February. Increasing
WAA, a mid level shortwave moving east within the quick zonal
flow, and upper level divergence in the right entrance region of a
jet to the north and left exit region of a strong jet over the MS
Valley, will broadly support an expansion of showers across the
region. Model guidance continues to indicate upwards of 500-1000
j/kg near and south of a northward moving warm front, thus expect
we will see embedded thunderstorms as well. Given the forcing,
moisture and instability forecast, do anticipate we will see some
locally heavy rainfall rates materialize, especially by Wed evening
into the overnight near the lifting warm front.

Still some uncertainty on the location of this front, although
models are in much better agreement tonight compared to last
night. Precipitation north of the front will likely be either sleet
or freezing rain, and thus the position of this feature will be
important to delineate the northern edge of the flood risk area.
The current consensus for the axis of heaviest rainfall is across
portions of KY, southern IN/OH into much of WV. Some of these areas
may start as sleet/freezing rain, but will transition to locally
heavy rain as the warm front lifts north. Areal averaged rainfall
is currently forecast in the 0.5"-1" range, although given the
forecast instability, locally higher totals ~2" appear possible.
Portions of central/eastern KY into WV have above average soil
saturation and streamflows, and so this area is likely most
susceptible to some flooding impacts if higher rainfall rates do materialize.

Chenard


Day 3
Valid 12Z Thu Feb 06 2025 - 12Z Fri Feb 07 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO VALLEY AND CALIFORNIA...

...OH Valley...
Another round of locally heavy rain is expected Thursday across
portions of KT/TN and WV along and ahead of an approaching cold
front. Locally heavy rainfall should be ongoing at 12z Thursday
across portions of WV, before a relative lull in activity by late
morning into the early afternoon. However by later in the afternoon
into the overnight hours another round of showers and embedded
thunderstorms should develop near the cold front over KY and
spread eastward into WV. Seeing enough instability forecast to
support locally heavy rainfall rates. Some of this corridor already
has above average soil saturation and streamflows, and there is
also some overlap between this day 3 rainfall and the day 2 rains.
Two day rainfall totals of 1-2" are forecast, but given the
instability both days, some areas could locally exceed 3". This
rainfall should be enough to result in at least some flooding impacts.

...California...
After a one day break another system is forecast to move into CA
on Thursday. This is expected to be a pretty dynamic system, with a
strong mid level shortwave and upper level jet. These dynamics
should support a short duration period of enhanced rainfall
rates...although the quick forward motion of the system will limit
overall rainfall magnitudes. Not seeing much in the way of
instability currently forecast, and IVT and PWs are not overly
anomalous with this system either. These limitations will be
somewhat made up for by the stronger dynamics, however still not
expecting this to be a high end event. Rainfall totals around 1"
are forecast along coastal areas with 1-2" in the foothills of the
Sierra (snow levels will be lower with this system limiting
impacts). Given the saturated conditions over the region, this
quick shot of locally heavy rainfall may still be enough to
generate localized flood impacts, supporting the Marginal risk.

The Marginal risk extends southward into Santa Barbara, Ventura and
Los Angeles counties as well. Less rainfall forecast here, but
there is an uptick in IVT into the terrain, and so would not be
surprised if QPF trends up a bit as the event nears. Still not
expecting significant rainfall, but possibly enough for localized impacts.

Chenard
$$
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