FOUS30 KWBC 280109 CCA
QPFERD
Excessive Rainfall Discussion...CORRECTED
NWS Weather Prediction Center College Park MD
809 PM EST Fri Dec 27 2024
Day 1
Valid 01Z Sat Dec 28 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...
Corrected for removal of extra header
...Northwest...
GOES West water vapor imagery showed a shortwave rounding the top
of a ridge axis 400-500 miles west of the WA coast with an
elongated W-E axis of mid-upper level moisture to its south, moving
across OR. A longwave upper trough axis was noted back to the west
near 160W with Blended TPW imagery showing the next round of
moisture poised to reach the West Coast later tonight, containing
PWAT values of 1.0 to 1.4 inches a couple hundred miles west of OR.
As the shortwave and longwave troughs track eastward/downstream, a
surface cyclone and accompanying warm front will allow for a surge
in IVT values from northern CA into southern OR (peaking near 800
kg/m/s through ~05Z). With this initial surge, rainfall rates are
likely to peak near 0.5 in/hr from the central/southern OR coast
into the northern CA coast. Beyond 05Z, IVT values are forecast by
the latest model consensus to come down a bit but remain moderate
in intensity (~500 kg/m/s) as the moisture transport axis shifts
southward, primarily into northern CA. Low level flow oriented
nearly orthogonal to the coastline will still maintain the threat
for higher rain rates near 0.5 in/hr. Peak 12 hour rainfall=20
totals, ending at 12Z Saturday, are expected to range from 2 to 4
inches (locally higher) from the favored west-facing terrain of
southern OR into northern CA. Additional rainfall atop wet=20
antecedent conditions and high water levels in area streams/rivers=20
may result in localized flooding.
...Mississippi, Alabama and far western Georgia...
As a negatively tilted shortwave trough axis over the=20
MS Valley continues to lift north and upper level jet forcing=20
begins to weaken, the threat for flash flooding across the central=20
Gulf Coast into inland areas is expected to decrease in coverage=20
and become more localized, likely focused near the greatest=20
instability (500 to 1000+ J/kg MLCAPE) near the coast. PWATs near=20
or just over 1.5 inches and potential for SW to NE training will=20
continue to support the potential for 1-3 in/hr rainfall rates=20
across southern MS/AL into far western FL. Farther north into=20
interior portions of AL and far western GA, while MLCAPE reduces to
near zero, weak elevated instability will maintain a lower end=20
flash flood threat through the night.
Otto
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...
20Z Update...
Forecast remains largely unchanged from overnight. Over the
Southeast, 12Z models still show enough spatial spread to preclude
a focused Slight Risk outline, and the higher QPF may lie over
areas with higher FFG (even after today's rain).
Fracasso
...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...
20Z Update... Very little change to the forecast from overnight.
Rain rates into southwestern OR may still exceed 0.5/hr across 12Z
thus supporting the Slight Risk. Trimmed a bit from the
northeastern side of the Marginal outline into the Northeast and
expanded a bit on the southwest side to account for just a tad
slower motion of the system in the east, but otherwise left a
somewhat broader Marginal in place that may focus more to the
northwest of I-95 per current trends.
Fracasso
...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
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7QE-m-ghyFnltON9ArMcMvomumlGJpjY8KUQc5zlwtho= y4yv4As39waqKg2POrU3heQkrwc4pUeTk5w6l0M0LkTQPQg$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7QE-m-ghyFnltON9ArMcMvomumlGJpjY8KUQc5zlwtho= y4yv4As39waqKg2POrU3heQkrwc4pUeTk5w6l0M0PbY5L_Y$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7QE-m-ghyFnltON9ArMcMvomumlGJpjY8KUQc5zlwtho= y4yv4As39waqKg2POrU3heQkrwc4pUeTk5w6l0M0-gbDL44$=20
=3D =3D =3D
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