Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Tue Dec 9 2025

Valid 12Z Tue Dec 09 2025 - 12Z Fri Dec 12 2025


...Pacific Northwest through the Northern High Plains..
Days 1-3...

Prolonged moisture spilling east from a strong atmospheric river
(AR) with multiple embedded waves will spread heavy snow into the
Intermountain West through late-week.

The strong AR will surge onshore the Pacific Northwest this
evening as a shortwave lifts into British Columbia with
impressively confluent and zonal flow pushing into WA/OR. IVT
probabilities for 750 kg/m/s exceed 90% during this time near the
WA/OR border, aided by a powerful Pacific jet streak of over 150
kts moving into British Columbia. This will be the beginning of a
long duration and impressive plume of IVT surging from the Pacific
Northwest and then spilling across the terrain and diving into the
High Plains through late week. While there will be multiple surges
of elevated IVT (and so not a continuous period of heavy
precipitation), in general the entire forecast period will be
unsettled, and IVT exceeding the CFSR climatology is progged via
NAEFS for multiple periods Wednesday and Thursday - indicating the
impressive available moisture associated with this system.

As with most ARs, the forcing and moisture will be attributed to
potent WAA, and this will drive snow levels to well above normal
levels for early December, likely reaching 8000-9000 ft in many
areas west of the Continental Divide and south of a wavering
stationary front/baroclinic boundary that will not only be a
demarcation for colder temperatures, but also provide a surface for
a subtle increase in isentropic ascent and fgen across the region.
Since snow levels south of this front will be so high, any heavy
snow should be confined to only the higher peaks of the northern WA
Cascades, far Northern Rockies, and into NW WY D1-D2 where WPC
probabilities are continuously high (above 70%) for 6+ inches both
days, especially in the northern WA Cascades and higher elevations
surrounding Yellowstone NP.

Then during D3, more significant snowfall becomes probable east of
the Northern Rockies in response to Arctic high pressure sinking
south out of Saskatchewan, driving a cold front to bank into the
terrain from the NE. This will sharpen the baroclinic gradient,
intensifying the resultant fgen, and increase isentropic ascent as
IVT continues to surge eastward beneath the potent Pacific jet
streak arcing to the southeast from British Columbia. With latest
WSO and WSSI-P guidance now showing >50% probabilities for snowfall
totals surpassing warning criteria and Moderate Impacts, confidence
is increasing in an axis of heavy snow spread NW to SE beneath
this jet streak and just north of the front across MT through D3,
and
into the western Dakotas by D4. Current WPC probabilities for more
than 8 inches of snow are as high as 70% in the Northern Rockies
near Glacier NP above 7000 ft, with a larger footprint of
moderate-to-high chances (50-80%) across the heart of Big Sky
Country that includes mountain ranges such as the Little Belt, Big
Snowy, and Absaroka.


...Northern Plains through the Great Lakes and Northeast...
Days 1-3...

Two distinct fast-moving shortwaves will race across the region
through late-week bringing several rounds of wintry precipitation.

The first of these will stem from a potent vorticity maxima and
associated shortwave trough which will dig out of Manitoba this
evening, reaching Michigan this morning, and then crossing New
England to exit into Canada by Wednesday morning. This feature will
remain of modest amplitude and the corresponding downstream jet
streak does not appear well positioned for any enhanced deep layer
ascent to cause cyclogenesis. Despite this, some increased
isentropic ascent from 280K-290K will cause a swath of moderate to
heavy snowfall tonight through Tuesday, with the speed of the
system the primary limiting factor to snowfall. Favorable ascent
into the DGZ will likely yield briefly heavy snow rates, especially
in MN/WI/MI before weakening into New England, with moderate snow
accumulations expected. WPC probabilities for even 2" of snow are
quite limited for this event, but periods of moderate to briefly
heavy snow could create hazardous travel for the monday commutes
for parts of WI, MI, and into Upstate NY.

Immediately in the wake of this first system a second, and likely
more amplified, shortwave and accompanying vorticity lobe, will
swing southeast out of Alberta/Saskatchewan and dive into the
Northern Plains by this afternoon. This shortwave is progged to
deepen during its progression, and while it should maintain a
positive tilt through its lifetime, it will gain amplitude and dive
farther south than the initial impulse. The resulting impressive
height falls will overlap efficiently with the LFQ of an impressive
Pacific jet streak following in its wake, producing intense deep
layer lift which will help deepen a surface low /Alberta Clipper/.
There is continued spread among the various deterministic models
(generally a global vs. high-res dichotomy) and their accompanying
ensemble spreads, but in general the differences are small leading
to high confidence in a track from ND Tuesday aftn to lower
Michigan Wednesday morning to northern Maine/Quebec Thursday
morning.

In addition to the robust synoptic lift, ascent will become
enhanced through a progressive overlap of 285K-295K moist
isentropic ascent (mixing ratios as high as 4g/kg) with 850-600mb
fgen. This will be well aligned to the most impressive left-exit
diffluence, and directed into the primary snow growth region /DGZ/,
suggesting some banded snow with heavier snow rates occurring N/E
of the low- level circulation. This is reflected by the WPC
prototype snowband tool suggesting at least 1"/hr snowfall rates
diving across the region. While the temporal duration of these snow
rates, owing to the speed of the system, should cap the snowfall
amounts, some secondary banding is possible on the NW side of the
low in response to intensifying deformation, especially Tuesday
night into Wednesday. This may lead to some locally higher snowfall
totals, but at this time, WPC probabilities D1 for 4+ inches of
snowfall reach 50-70% from far eastern ND through much of northern
MN (south of the Arrowhead, however), with a more pronounced stripe
of 50+% probabilities for more than 4 inches on D2 across this same
area and extending into WI and lower MI. The transient nature of
the system will limit the high-end snowfall, but some areas
locally may
receive around 8 inches of snowfall.

Additionally, south of this axis of heaviest snow, a transition
zone of mixed precipitation, including freezing rain is expected,
leading to at least modest ice accretion. While light freezing rain
is expected from far eastern MT through lower MI, the greatest
chance (10-30%) for more than 0.1" of icing exists across eastern
ND and western MN.

As this storm exits the Great Lakes and moves into Ontario D3, the
downstream strong WAA will overrun a very cold antecedent airmass.
This will create a broadening plume of wintry precipitation,
falling primarily as snow from Upstate NY through northern New
England, with some mixing, including periods of freezing rain,
expected farther south into the northern Mid-Atlantic and southern
New England. Total accumulations of wintry precipitation should be
less than areas upstream, but WPC probabilities early D2 into D3
for 4+ inches of snow reach 50-90% for portions of western PA and
across the southern Adirondacks, Greens, Whites, and terrain of
northern ME.

Finally, behind this system a brief period of favorable NW flow
will create lake effect snow (LES) and upslope snow across the
eastern Great Lakes and into the Central Appalachians. While some
of this snow will fall on top of the synoptic snow noted above
(and is included in the snowfall probabilities), and additional 4+
inches of snow has a moderate risk of occurring across the
Chautauqua
Ridge from D2 into D3. Similar probabilities for 4+ inches of snow
exist
in the higher terrain of WV as well where upslope flow and gusty
winds
could create hazardous impacts. The Tug Hill Plateau, in
particular, has rather high chances (>70%) for snowfall totals
surpassing 8" with even low-to-moderate chances (30-50%) for
snowfall topping 12 inches.


Weiss/Mullinax