FOUS11 KWBC 170810
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 AM EST Fri Jan 17 2025

Valid 12Z Fri Jan 17 2025 - 12Z Mon Jan 20 2025

...Great Lakes... Days 2-3...

By Friday night, a deepening area of low pressure will escort a
strong Arctic front south that delivers a frigid air-mass that is
the coldest and most dangerous of the season this weekend and into
next week (see Key Messages linked below). The Arctic front will
turn on the LES machine over Lake Superior by Friday night while
periods of snow develop along the northern and western coasts of
Michigan's Mitten by Saturday morning. Following an initial WAA
thump of snow Saturday afternoon, LES bands look to form over
parts of central and western New York Saturday night that likely
persist through the remainder of the weekend. Through 12Z Monday,
WPC probabilities show moderate-to-high chances (50-70%) for
snowfall >4" and low chances (~10%) for snowfall >8" in the Tug
Hill Plateau. The Michigan U.P. sports moderate-to-high chance
probabilities (50-70%) for >8" of snow in the Porcupine Mountains
and in parts of the eastern U.P.. Expect these probabilities and
snowfall totals to rise as LES bands increase in coverage an
intensity throughout the region as winds accelerate and the heart
of the bitterly cold temperatures aloft race over the Great Lakes next week.

...Rockies and High Plains... Days 1-2...

The combination of both upper level diffluence ahead of a deep-
layer trough and the arrival of a bitterly cold Arctic air-mass
that is destined to be the coldest air-mass of the season to
infiltrate the Lower 48 will bring about Snow squalls and mountain
snow to parts of these regions through Saturday. This exceptional
cold front (a "blue norther") out ahead of the upper trough will
push south this morning while at the same time, 500mb PVA ahead of
an approaching upper trough occurs over the northern Rockies/High
Plains. Snow showers will breakout across Montana this morning and
make its way south throughout the day into Wyoming, then into
northern Colorado by Friday afternoon. Southern Montana and into
Wyoming, in particular, sport notably higher snow squall parameters
given the stronger surface-based heating that will cause steeper
low-level lapse rates. Snow squalls are also possible today along
the cold front as it tracks east across the Dakotas Snow squalls
can cause rapid reductions in visibility due to a combination of
heavy snow rates and whipping wind gusts, as well as rapid
accumulations on roads as temperatures plummet well below freezing.
Motorists in these areas should keep a close eye on potential snow
squalls as it can cause dangerous travel conditions in a matter of seconds.

As the front plunges south Friday evening, the snow potential
shifts south into Colorado with periods of snow beginning near the
start of the evening rush hour in the Denver/Boulder metro area.
While the snow squall potential may not be as high by this time,
the easterly upslope-enhancement of snowfall rates will make for
occasional periods of heavy snow Friday evening. Snowfall rates
will weaken as the front works its way south of the Palmer Divide
Friday night and down the spine of the Sangre De Cristo early
Saturday morning with some snow along the Front Range lingering
into the day on Saturday as a dome of ~1050mb high pressure builds
in from the Canadian Prairies. WPC probabilities are keying in on
the Front Range as the focus for the heaviest snowfall with
moderate-to-high chances (50-70+%) for snowfall >4" from the
Laramie Range on south to parts of the Sangre De Cristo. The I-25
corridor from Fort Collins on south to the Colorado Springs area
Friday have similar probabilities for >4" of snowfall with even
low chances (10-30%) for greater than 6" between Friday evening
and Saturday morning in spots. Some of the higher peaks (>10,000ft)
have moderate chances (40-60%) for snowfall >8" through Saturday
morning. The WSSI is currently depicting Minor Impacts for just
about all of these referenced areas, suggesting the potential for
winter weather conditions that would require enhanced caution while
driving in these affected areas. The one notable exception is the
Denver metro area as Moderate Impacts are now depicted, indicating
there are likely to be disruptions ot daily life with potential
closures and hazardous driving conditions.

...Mid Atlantic and Northeast... Day 3...

...Confidence is increasing in a disruptive winter storm from the
Mid-Atlantic to the Northeast with locally heavy snowfall possible
along the I-95 corridor...

Model diagnostics: Over the past 24-36 hours, guidance has come
into better agreement on a deepening 500mb trough over the Ohio
Valley and higher height rises in the Atlantic south of Bermuda.
AI-based guidance (ECMWF-AIFS and GFS GraphCast) have been showing
this increasing potential over this time span while ensembles
(ECMWF-EPS, GEFS, GEPS) have all gradually trended snowier as
well. While this is the case, deterministic guidance still shows
the lingering uncertainties with the 500mb trough's amplitude and
position 12Z Sunday. The 00Z GFS is slightly deeper vs the 00Z
ECMWF. The 00Z CMC is similar to the ECMWF 500mb disturbance's
strength, but with less confluence over the Northeast, allowing
for a more storm track on the northern side of the track
distribution. The UKMET also has a similar look to the Canadian,
albeit with a colder boundary layer setup given stronger confluence
over the Northeast. Overall, the AI-based guidance and the trends
in the ensembles provide an increased confidence in a synoptic-
scale setup that is boosting confidence in a winter storm from the
Mid-Atlantic to the Northeast, but more specific details on storm
track still need to be ironed out over the next 24-36 hours.

Synoptic & mesoscale analysis: The approaching upper-level trough
Saturday night will amplify over the Ohio Valley and place a
strengthening 250-500mb layer averaged jet streak over the Mid-
Atlantic by Sunday morning. NAEFS shows 200mb and 500mb winds over
the Southeast U.S. are above the 90th climatological percentile
and foster healthy upper level divergence aloft on Sunday. At
lower levels, a cold front Saturday night will stall out along the
Mid-Atlantic coast and into southern Virginia. The front, residing
beneath the synoptically-favorable jet stream setup, will spawn an
area of low pressure Sunday morning in the Carolinas and gradually
strengthen as it heads for the Mid-Atlantic coast Sunday
afternoon. The heaviest banding of snow is likely to reside north
of 850mb frontogenesis, which will become more and more pronounced
as the 850mb low forms somewhere along the Jersey Coast and tracks
northeast towards Long Island and the MA Capes Sunday evening. The
storm is a progressive one, but given the sufficient synoptic and
mesoscale processes, the deformation zone of heavy snow would be
more than capable of generating ~1"/hr snowfall rates, especially
for areas from the Delaware Valley on north and east through the
Tri-State area and into southern New England where they are more
likely to have better mesoscale forcing as the 850mb low deepens Sunday night.

Timing & snowfall/impact probabilities: As mentioned before, this
is a quick hitting system that is all but gone by midday Monday as
it races toward the Canadian Maritimes by Monday afternoon. Snow
is forecast to be at its heaviest Sunday morning and afternoon for
the DC/Baltimore metros, the Philly/NYC metros Sunday afternoon
and evening, then across southern New England Sunday night into
early Monday morning. Latest WPC probabilities show moderate-to-
high chances (50-70%) for snowfall totals >4" from the Delaware
Valley on north through the Lower Hudson Valley and across southern
New England. Low-to-moderate chances (30-50%) for >4" extend as
far north and east as Downeast Maine. WPC probabilities even show
some spotty low chance probabilities (10-20%) for snowfall totals

8" in parts of southern New England. The DC/Baltimore metro areas

sport low chance probabilities (10-30%) for >4" of snowfall at the
moment, while the Philly and NYC metros are in the moderate range
(40-60%) for >4" of snow. While exact amounts remain unclear, the
WSSI-P Minor probabilities are >60% for the full length of I-95
from DC to Boston, indicating the heightened potential for hazardous
travel conditions Sunday into Monday. WPC has initiated Key
Messages for this event and can be viewed at the bottom of this discussion.

Mullinax

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

$$
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FOUS11 KWBC 180834
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
334 AM EST Sat Jan 18 2025

Valid 12Z Sat Jan 18 2025 - 12Z Tue Jan 21 2025

...Great Lakes... Days 1-3...

Strong low pressure tracking through southeast Canada will escort
a strong Arctic front through the Great Lakes today that will
revitalize the LES machine throughout the region today. Strong
NWrly flow with exceptional CAA will trigger LES bands from the
U.P. of Michigan on south along the western-most portions of
Michigan's Mitten. Farther east, a slug of moisture aloft will run
parallel to the Arctic front as it races east today with the
Chautauqua Ridge and the Tug Hill the more favored locations for
locally heavy snow beginning this morning and concluding by
Saturday evening. WPC probabilities through 12Z Sunday show
moderate-to-high chances (50-70%) for snowfall totals >4" in the
Tug Hill while there are low-to-moderate chances (30-50%) for >4"
snowfall amounts from northeast Ohio to along the Chautauqua
Ridge. Portions of the eastern Michigan U.P. and the Porcupines
also sport moderate chances (40-60%) for snowfall amount >4". As
winds accelerate over the Great Lakes on Sunday and inverted
troughing ensue over Lake Superior, LES bands will increase in
both coverage and intensity in the usual snow belts of the
Michigan U.P. and the northern most portions of Michigan's Mitten.
Including Sunday and Monday, WPC probabilities depicted high
chances (>70%) for snowfall > than 8" along the eastern most areas
of Michigan's U.P. and just north of the Traverse City area.
Single-banded LES look to emerge off of Lakes Erie and Ontario
Monday evening and generate heavy snowfall rates of 1-2"/hr through
the day on Tuesday.

...Colorado Rockies and High Plains... Day 1...

Mountain snow will continue over the eastern slopes and Front Range of
the Colorado and New Mexico Rockies, including the adjacent high
plains along the I-25 corridor today. The shortwave trough tracking
south through the Intermountain West this morning will generate
synoptic-scale lift over Colorado and northern New Mexico which
will also have low-level, post-frontal upslope NErly flow with a
1050mb dome of high pressure moving over Montana on Saturday.
Snowfall rates around 1"/hr are possible along the Raton Mesa early
Saturday and will work their way south along the spine of the
Sangre De Cristo Saturday afternoon. Saturday's snow probs for >4"
are high (60-80%) over the Front Range from Pikes Peak on south to
the Raton Mesa, while areas along I-25 have low-to-moderate
chances (20-40%) for >4" of snowfall.

...Central Appalachians, Mid-Atlantic and Northeast... Days 1-3...

...Significant winter storm to produce a swath of heavy snow from
the Central Appalachians and northern Mid-Atlantic to the Northeast
Sunday and Monday...

As model guidance continues to come into better agreement on the
overall synoptic-scale setup, it is becoming clearer that a
significant winter storm is likely to produce heavy snow snow from
as far south and west as western Kentucky to as far north as
northern Maine. As the Arctic front approaches Saturday night, the
front will be draped over the Mid-Atlantic while at the same time,
strong upper-level divergence ensues aloft courtesy of a ~150kt
250mb jet streak's diffluent left-exit region is in place. As
850-700mb WAA increases over the Mid-Atlantic, it will clash with
the approaching Arctic air-mass to the north, resulting in a band
of heavy snow that initially starts out over the Laurel Highlands
and Allegheny Highlands early Sunday morning, then spreads over
northern MD and southern PA later on that morning. As the 850-700mb
FGEN increases throughout the day, snowfall rates will increase
across eastern PA, northern NJ, and the Lower Hudson Valley through
Sunday evening. 00Z HREF does suggest there are low-to-moderate
chances (20-50%) for 1"/hr snowfall rates during the day on Sunday.
By 00Z Monday, the 850mb low will track over Long Island and head
for the New England coast with the axis of heavy snow residing
along the western flank of the 850mb circulation. Heavy snow will
unfold across portions of coastal New England Sunday night where
snowfall rates >1"/hr are possible. The storm races northeast past
Nova Scotia by Monday morning with lingering periods of snow
throughout parts of Maine gradually tapering off by midday Monday.

Latest WPC guidance depicts a swatch of moderate-to-high chances
(50-70%) for snowfall >4" from central WV on north and east through
central PA. From the Poconos on north and east through the Lower
Hudson Valley, the Berkshires, and into both the Worcester Hills
and White Mountains, there are moderate-to-high odds (50-70%) for
snowfall >6", as well as moderate chances (4-60%) for snowfall >8".
Note that WPC probabilities for >12" are generally <10%,
indicating most areas are unlikely to receive a foot of snowfall,
although localized amounts in the Allegheny Highlands and parts of
interior New England cannot fully be ruled out. Latest WSSI shows a
healthy swath of Moderate Impacts from northern MD through the
Lower Susquehanna Valley, the Delaware Valley, the Tri-State area,
and into portions of southern New England. Hazardous driving
conditions are anticipated in these areas on Sunday. With bitterly
cold temperatures spilling in for the upcoming work-week, snow and
instances of re-freezing/black ice are likely to stick around for
much of the week in wake of this winter storm.

...Southern Plains & Gulf Coast...

...Disruptive winter storm growing in confidence across the
Southern U.S. next week...

Day 3...
For a winter storm to unfold from as south as the San Antonio area
on east to the central Gulf Coast, the first ingredient is for an
exceptionally cold air-mass to be locked in place. With the arrival
of the Arctic air-mass this weekend, bone-chilling temperatures
will be anchored in place by a dome of high pressure that NAEFS
shows is above the 97.5 climatological percentile as of 12Z Monday
over the Lower MS Valley. By 00Z Tuesday, 850mb temperatures remain
below the 2.5 climatological percentile and subfreezing 850mb temps
protruding as far south as the Gulf Coast. Meanwhile, a positively
tilted 500mb trough tracking through the Southwest will work in
tandem with the subtropical jet to increase upper-level divergence
over Texas Monday afternoon. In addition, SWrly 850mb flow will
deliver low-level moisture northward while at the same time, the
850mb WAA over South Texas will overrun the Arctic air-mass in
place over Texas. This will result in the beginning of an icy
wintry mix over south-central Texas and eventually spread east
towards the Upper Texas coast and Lower Mississippi Valley by early
Tuesday morning.

WPC probabilities show moderate chances (40-60%) for >0.01" of ice accumulations in south-central Texas through 12Z Tuesday, which
would result in slick and dangerous road conditions for the Tuesday
morning commute. Farther north, WPC probabilities show low-chance
odds (10-30%) for snowfall >2" in parts of eastern Texas and
western Louisiana. It is worth noting that the scope of this
discussion goes through 12Z Tuesday, so expect snowfall probabilities
to increase over the next 24 hours. Please be sure to visit WPC's
Medium Range discussion and our Key Messages for more details on
the potential impacts throughout the South later this week.

Mullinax

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

$$
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FOUS11 KWBC 270856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Mon Jan 27 2025

Valid 12Z Mon Jan 27 2025 - 12Z Thu Jan 30 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1-3...

An active period of winter weather is expected through mid-week as
progressive flow drives multiple impulses with rounds of snow from
the Great Lakes through the Northeast.

The period begins with a lead shortwave exiting New England into
the Atlantic, leaving sharp NW CAA in its wake. THe most impressive
CAA is likely to linger primarily across Lake Ontario and into
Upstate NY, at least early in the period before a subtle surge of
WAA occurs ahead of the next front. This will result in a band of
heavy LES waning right at the start of the period, with minimal
additional LES accumulation.

However, the respite will be short as a potent shortwave,
basically demarcating the edge of the larger cyclonic gyre digging
south from Hudson Bay will drop south Monday night. This will
drive a potent cold front southward, first into the U.P and L.P. of
MI late Monday evening, and then continue to race east through the
interior Northeast and New England Monday night into Tuesday
morning. Impressive CAA along the front will drive an intense
baroclinic boundary, leading to a stripe of impressive low-level
fgen, especially in the 925-850mb layer. This will drive strong
mesoscale lift into a saturating column reflected by high low-level
RH, and as lapse rates steepen, some weak to modest instability
will develop. The high-res guidance has become more aggressive
showing a band/corridor of convective snow showers and snow
squalls, with HREF probabilities showing a 20-30% chance for 1"hr
snowfall rates within these elements. The accompanying steep lapse
rates will help mix down strong winds, encouraging the development
of low visibility during heavy snow showers. The limiting factor to
true snow squalls may be a relative dearth of 0-2km CAPE, and it
is beneficial that the most significant impacts will occur at
night, but still, convective snow showers and isolated snow squalls
are expected Monday night into Tuesday morning from the U.P. of
Michigan through much of the interior Northeast and New England,
and potentially as far southeast as the Mid-Atlantic.
Accumulations during this time will generally be minor, but some
LES and some upslope flow with the front across the Adirondacks has
resulted in WPC probabilities reaching 70-90% for 4 inches on D1,

Some modest LES will follow in the wake of this front, but more
significant and widespread snow will occur D2 into D3, as yet
another fast moving impulse dives through the broad cyclonic flow
across the east. This third impulse will track along the residual
baroclinic gradient left by the potent cold front, and will be
overlapped by a strengthening jet streak downstream of the trough
axis to produce more robust deep layer ascent. This will support
modest cyclogenesis as a clipper-type low swings southeast from
Minnesota Tuesday aftn, to Upstate New York Wednesday morning, and
then across New England, exiting into Atlantic Canada by Wednesday
evening. Brief but impressive WAA downstream of this low will
result in expanding precipitation falling as moderate to heavy snow
across many areas.

Then, in the wake of this system, additional LES will develop as
NW flow causes additional accumulations in the favored NW snow
belts, and with strong winds, some of this could push as far SE as
the coastal Mid-Atlantic states by the end of the forecast period.
WPC probabilities D2 are high (70-90%) for 4+ inches across the
U.P., the NW L.P. near Traverse City, and east of Lakes Erie and
Ontario, with a swath of moderate probabilities for 2+ inches
surrounding those areas for the more synoptically forced snowfall.
For D3, the greatest risk for 4+ inches of snow shifts into the
Adirondacks and Greens where WPC probabilities feature a high risk

70%) while additional modest LES occurs east of Lakes Erie and

Ontario. Locally 1-2 feet is possible in some areas through the period.

Finally, a second round of convective snow showers or snow squalls
is becoming more likely on Wednesday, this time a little south of
the event Tuesday morning. These convective snow showers will occur
during a more favorable time of day to support better CAPE behind
the secondary front, and the SnSq parameter is suggesting a risk
across areas from eastern OH through southern Upstate NY and PA.
This secondary set of squalls, if they occur, will again be
accompanied by strong winds and heavy snow rates leading to the
potential for significant travel impacts despite modest snowfall
accumulations.

The two rounds of potential snow squalls have prompted the issuance
of Key Messages linked below.

...Southwest through the Four Corners... Days 1-3...

The anomalous upper low pivoting slowly across CA will begin to
slip east this week, bringing snowfall across the Southwest US
through Thursday. This cutoff begins the period quite amplified,
with 500-700mb heights as low as -2 sigma over CA according to the
NAEFS ensemble tables. Two distinct spokes of vorticity will dance
around the core of this upper low, helping to elongate it with
time as it becomes stretched NE to SW into the Four Corners by
Tuesday night. The interaction of these vorticity impulses
rotating around the closed center will help keep the low amplified,
but also very slow moving, reaching NM/CO by the end of the
forecast period.

During this synoptic evolution, the upper pattern will alter
considerably as well. Initially, a strong 250mb jet streak will be
arced downstream of the upper low, lifting northeast into the
Southern Plains. However, this feature will weaken D1 as the upper
low elongates, leaving less substantial diffluent-caused ascent
into D2. However, a secondary surge of jet level energy will occur
Wednesday into Thursday as the jet streak re-amplifies and arcs
meridionally from the Gulf of California into the Southern Plains,
placing intense LFQ diffluent ascent over the Four Corners. This
jet evolution, combined with the mid-level closed low, and at least
subtle low-level baroclinicity in the vicinity of a weakening front
will cause waves of low pressure to move eastward from CA to the
Four Corners D1-D2. This will cause periods of snowfall from the
Transverse and Peninsular Ranges of CA eastward through the
Mogollon Rim and Kaibab Plateau where WPC probabilities are
moderate (30-50%) D1 for 4 inches of snow, and high (>70%, but only
in higher elevations of AZ) on D2.

However, the most significant snowfall is likely to begin D3 as a
low pressure develops near the TX Big Bend and moisture advection
surges on isentropic ascent across TX. The accompanying theta-e
ridge is progged to lift cyclonically into a TROWAL, with the most
intense theta-e advection being directed into the DGZ over NM/CO.
There is still quite a bit of uncertainty regarding the placement
and evolution of this system, but snowfall chances are increasing,
especially in the higher terrain above 5000 ft in the Sangre de
Cristos and San Juans. Here, WPC probabilities D3 are as high as
70% for 6+ inches, and impactful snow is becoming more likely for
parts of this area Wednesday into Thursday.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 280709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
209 AM EST Tue Jan 28 2025

Valid 12Z Tue Jan 28 2025 - 12Z Fri Jan 31 2025

...Great Lakes, Northeast, Mid-Atlantic... Days 1-2...

Progressive pattern continues across the east as a cyclonic gyre
centered near the Hudson Bay drifts east, but vorticity lobes
shedding around it will maintain/re-amplify the upper trough into Thursday.

The first of these shortwaves will be racing east across Upstate NY
and northern New England this morning, pushing an arctic front
southeast in tandem. Scattered convective snow showers and isolated
snow squalls will remain possible along this front through the
morning hours, with briefly heavy snow rates above 1"/hr and gusty
winds combining to produce scattered impacts as far east as I-95 to
NYC and Boston. However, the intensity of these features is likely
to be much less substantial than occurring overnight into Tuesday morning.

Behind this front, a brief respite in snowfall will occur before a
second impulse digs rapidly from Alberta, Canada towards the Great
Lakes and Northeast. The interaction of this shortwave trough with
the leftover baroclinic boundary behind the cold front will yield
surface cyclogenesis in the form of an "Alberta Clipper" which is
progged to track quickly from the Arrowhead of MN this afternoon
through the Upper Great Lakes tonight, and then across New England
on Wednesday. Weak secondary development is possible east of Massachusetts/Maine Wednesday as well, but should have marginal
impact on the winter impacts. Ahead of this clipper, intensifying
SW flow will drive enhanced WAA and moist isentropic ascent,
especially along 285-290K surfaces, to produce an expanding swath
of moderate to heavy snow. The DGZ becomes exceptionally deep
during this time as reflected by SREF probabilities of 90% for
100+mb of depth during the period of most impressive WAA. This will
create a near iso-thermal layer within the -12C to -18C
temperatures, explaining this deep DGZ. PWs are progged to exceed
the 97th percentile over MN/MI according to NAEFS, which will help
additionally enhance snowfall, and despite PWs falling to the east
with time, a period of heavy snow is likely within this downstream WAA.

Then, in the wake of this system and behind a subsequent cold front
Wednesday morning, lake effect snow (LES) will develop as NW flow
causes additional accumulations in the favored NW snow belts. With
strong winds progged in forecast soundings, some of this could
push as far SE as the coastal Mid-Atlantic states by Wednesday
night. WPC probabilities D1 and D2 for areas that receive both the
WAA snow and then the subsequent LES are moderate to high (50-90%)
for 4+ inches from the western U.P. southeast through Traverse City
area, east of Lakes Ontario and Erie, and into the western
Adirondacks, Greens of VT, and Whites of NH. Storm total snow of
12-18" is possible across the U.P. and the Tug Hill, with 6-12" in
lollipops elsewhere across this swath, especially in the higher terrain.

Finally, a second round of convective snow showers or snow squalls
continues to look likely Wednesday. These convective snow showers
will occur during a more favorable time of day to support increased
CAPE forecast to reach 100-200 J/kg in the 0-2km layer, overlapping
some impressive low-level fgen from central New England, Upstate
NY, and into the northern Mid-Atlantic states. This secondary set
of squalls will again be accompanied by strong winds and heavy
snow rates leading to the potential for significant travel impacts
despite modest snowfall accumulations.

After this second front pushes east and snow squalls wind down
Wednesday evening/night, much of D3 will be quiet across the
region. A larger scale storm system is likely to approach late D3
from the south with increasing moisture and some light mixed
precipitation, but at this time any impacts from that event are
most likely into D4, just beyond this forecast period, and WPC
probabilities for 0.01" of ice peak around 50% across northern PA
before the end of the forecast period.

The two rounds of potential snow squalls continue the issuance of
Key Messages linked below.


...Southwest through the Four Corners... Days 1-3...

Modestly anomalous upper cutoff low (NAEFS 700-500mb heights
falling to below the 10th percentile of the CFSR climatology) will
roll slowly eastward from southern CA this morning. This feature
will move slowly (such is the nature of cutoffs) across the Desert
Southwest and Four Corners Wednesday before pinwheeling into the
Central Plains on Thursday. The exact track and placement of this
upper low remains uncertain due to vorticity lobes which will
periodically dance around the central gyre, tugging it in subtly
different directions at different times, but the large scale
pattern is well agreed upon at this time by the various global models.

This evolution will result in pronounced synoptic forcing across
the region from west to east, primarily due to height falls,
divergence, and periods of PVA. However, other forcing at play
will enhance deep layer ascent. This includes increasing LFQ
diffluence as a subtropical jet streak intensifies downstream of
the closed low, especially Wednesday into Thursday across NM/CO,
and periods of upslope flow embedded within isentropic ascent.
There has also been an increase this morning in post-system
deformation as a surge of wrap-around precipitation develops over
eastern CO Thursday morning and then pivots southward. These
parameters together should produce sufficient lift in an area of
increasing moisture to produce warning-level snow above generally
4000-5000 ft in the Sangre de Cristos, San Juans, and Jemez
Mountains. However, there remains some uncertainty as to the
movement of an accompanying dry slot, which could lower
accumulations in some areas.

WPC probabilities for 4+ inches on D1 are minimal, but by D2 expand considerably and amplify into the San Juans and Sangre de Cristos,
where they reach as high as 70-90% for 6+ inches, with locally over
1 foot possible in the highest terrain. Additionally WPC
probabilities are high (70-90%) for 4+ inches across the Raton Mesa
and higher elevations of I-25 near the NM/CO border. By D3 the
event begins to ramp down, but WPC probabilities for 4+ inches
above 30% continue in the Sangre de Cristos, and expand into parts
of the Palmer Divide as well.


...Pacific Northwest... Day 3...

A trough moving eastward across the Northern Pacific will generate
an intensifying jet streak downstream, pivoting ascent and moisture
into the region after 00Z Friday. This will manifest as a corridor
of enhanced IVT as high as 500 kg/m/s (above the 90th CFSR
percentile according to NAEFS) reaching as far inland as Idaho
before the end of the period. This will drive an expanding area of precipitation spreading eastward, but with the accompanying WAA
driving snow levels to as high as 5500 ft, much of this will fall
as rain. However, in the Olympics and Cascades, a heavy wet snow is
likely, which could accumulate to more than 4 inches before 12Z
Friday as reflected by WPC probabilities reaching 30-50% in these areas.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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