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FOUS11 KWBC 030755
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EST Wed Dec 3 2025

Valid 12Z Wed Dec 03 2025 - 12Z Sat Dec 06 2025


...Southern/Central Rockies...
Days 1-2...

Shortwave which emerged from British Columbia Monday will continue
to track southeast across the Four Corners today while amplifying.
The guidance has trended a bit farther south and more amplified
with the evolution today, which should result in heavier snowfall
especially across the San Juans and Sangre de Cristos on D1.
Forcing for ascent downstream of this trough (in response to height
falls and modest divergence) will be enhanced by a strengthening
jet streak, especially tonight into Thursday morning, and periods
of impressive upslope flow as cold N/NE winds develop in advance of
an Canadian high pressure dropping into the Plains. Moisture within
the region is progged to be around to slightly below normal as
reflected by NAEFS ensembles, but the impressive overlapping ascent
will still maximize favorable to support heavy snow, especially
above 4000-5000 ft, with snow levels falling in conjunction with
waning snowfall intensity. WPC probabilities, especially across the
San Juans and Sangre de Cristos, have increased tonight, and now
feature a high risk (>70%) for more than 12 inches in the higher
terrain of these ranges. Into the lower terrain, WPC probabilities
indicate a moderate threat (30-50%) for at least 4" along the I-25
urban corridor, with locally more than 6" in the higher terrain of
the Raton Mesa and southern Palmer Divide.


...Great Lakes & Northeast...
Days 1-3...

Progressive flow across the Northeast will manifest in response to
a large gyre centered near Hudson Bay, which will shed periodic
shortwaves from the Great Lakes through the Northeast leading to
occasional enhanced ascent and lowered heights throughout. In
response to this evolution, the period will remain active through
LES and arctic fronts, and multiple rounds of snowfall, sometimes
heavy, are expected.

The first event will be associated with an arctic cold front
dragging southeast beneath a shortwave aloft. This front will
likely move rapidly across the Great Lakes today, and then across
the Northeast Thursday. Along and ahead of this front, the
environment continues to suggest an increasing threat for snow
squalls, especially across PA/NY and New England on Thursday. The
NAM SnSq parameter continues to be more aggressive than the GFS,
but the antecedent low-level environment ahead of the front appears
favorable for squall development. With 0-2km fgen maximizing along
the front into elevated RH and SBCAPE as high as 300 J/kg,
widespread linear squalls could result. Upstream into MI the threat
is not as high on D1, and although isolated squalls could occur,
the greater risk will be D2 in the Northeast. While total snowfall
will be minimal in any squall, rapidly changing conditions due to
heavy snow and gusty winds will create periods of dangerous travel
Thursday aftn. These squalls could become widespread, and after
coordination with the northeast WFOs, it is possible that with
future model cycles some enhanced messaging may be needed to
highlight the threat.

Behind this front, as well as behind a subsequent shortwave on
Friday, increased CAA will spread across the Great Lakes leading to
periods of lake effect snow (LES). The progressive flow will
support shortwave ridging between the two impulses, so the duration
of any LES with each shortwave/front should be somewhat limited.
However, 850mb temps falling to -15C to -20C Wednesday
night/Thursday, and then -10C to -15C late Friday, will create
steep lapse rates and deepening inversion depths to support bands
of heavy LES. The heaviest LES snowfall accumulations within the
LES bands are expected in the typical NW snow belts across the U.P.
of MI, the NW L.P. near Traverse City, and then along the
Chautauqua Ridge and into the Tug Hill Plateau. In these belts,
3-day snowfall of 6-12 inches is possible, highest across the U.P.


...Mid-South to the Mid-Atlantic...
Day 3...

Uncertainty abounds D3 with respect to a strung-out shortwave
exiting the Central Plains and becoming embedded within the more
zonal flow to the east. Although this shortwave is expected to be
of modest amplitude and fast moving, the accompanying upper jet is
progged to be powerful at over 150kts across the Mid-Atlantic,
leading to surface low pressure development across the Southeast.
This low will then race E/NE, producing a swath of at least light
wintry precipitation from the Tennessee Valley to the Mid-Atlantic
states.

There has been at least a modest northward shift/trend in the MSLP
fields from the various global ensembles the past few runs, and
while the ECMWF operational remains the most aggressive/north, that
solution cannot be ruled out. In fact, the trends in the 250mb jet
streak and from evaluation of the clusters (for which the primary
variance is the amplitude of the trough across the east) could
suggest a more subtle northward track of this low which would shift
more wintry precipitation into the area. It is still too early to
iron out these details, but at least a low-end risk for a few
inches of snow is possible as reflected by WPC probabilities which
reach 10-30% for 1+ inches of snow across the Central Appalachians
and into eastern VA/MD. This is notably lower than the recent NBM
probabilities, and the trends continue to increase, so it is worth
watching the evolution of this event during the next few days.

South of the potential moderate snow, an axis of light freezing
rain is also possible where a p-type transition occurs. While the
location of the heaviest icing is also uncertain, current WPC
probabilities indicate a 10-30% chance for at least 0.1" of ice
across northern NC and into southern VA.


...Northern to Central Rockies...
Days 2-3...

The next chance for heavy snowfall totals in the northern Rockies
will develop as a low amplitude shortwave slides off of the top of
the Pacific ridge into the region late Thursday into Friday,
coincident with the LFQ of an impressive upper jet streak swinging
across the Northern Pacific. This will result in favorable mid to
upper level forcing along with a notable influx of Pacific
moisture, sufficient for some heavy totals , especially above
around 5000 ft as snow levels rise coincident with this moisture
plume moving into the area. Heavy snowfall accumulations are likely
D2 from the WA Cascades through the terrain of NW WY near
Yellowstone NP as reflected by WPC probabilities that suggest a
high risk (>70%) for 4+ inches, with impacts to many of the area
passes becoming likely.

Probabilities become even more impressive D3 across the higher
terrain from Glacier NP through Yellowstone NP including portions
of the Sawtooth/Salmon River ranges, Absarokas, terrain of NW WY,
and into the Park Range of CO. Here, WPC probabilities D3 reach
above 70% for 8+ inches, and D3 snowfall could be impressive above
generally 6000 ft where locally 1-2 feet is possible.

Weiss



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