FOUS11 KWBC 222058
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 PM EST Mon Dec 22 2025

Valid 00Z Tue Dec 23 2025 - 00Z Fri Dec 26 2025

*** Strong Atmospheric River followed by low pressure systems to
impact California and the Sierra Nevada through Friday ***

...Northeast... Days 1-2... Maximum impact level from WSSI: Moderate

A shortwave trough crosses northern Ontario Tuesday and New England
Tuesday night on a potent 150+kt NWly jet below a deep and
sprawling vortex centered north of Hudson Bay. Precip blossoms
late tonight in warm air advection over the upper Ohio Valley and
the Northeast Tuesday. Day 1 snow probs for >4" are 30-60% for the
Catskills west toward the Finger Lakes and the southern slopes of
the Adirondacks. Light freezing rain (a few hundredths accretion)
on the southern edge of the precipitation shield (from central PA
to central WV) is possible late tonight into Tuesday morning.

The surface low crosses New England Tuesday evening with continued
snow for Tuesday night in terrain including the Whites, Greens and
Adirondacks as the WAA pivots to deformation banding with
topographic enhancement (which should boost snow rates into the
1"/hr range) on the back side of the low. Day 1.5 snow probs for

6" are 30-80% over this terrain with 50% probs for over 8" in the

highest terrain.

Most of Maine is north of the surface low track which will have
ocean enhanced snow Tuesday night before the back side banding
shifts down the coast to produce ocean enhanced snow for eastern
Mass Wednesday morning. Day 2 probs for >6" are 40-70% for much of
the central Maine coast west well into the remote interior
portions of the state.

...California/Sierra Nevada... Days 2-3...
Maximum impact level from WSSI: Extreme

Atmospheric River (AR) over northern CA this afternoon pivots north
to OR and the northern Rockies tonight. High snow levels over
8000ft lift north with the core of highest precip from the
northern Sierra Nevada through the CA Cascades/Trinity Alps keeping
Day 1 quiet in CA from a snow perspective.

However, the now north-south oriented AR pushes across the CA coast
late Tuesday, reaching the full extent of the Sierra Nevada with
heavy precip and snow levels around 7000ft Tuesday evening with
particularly heavy snow starting roughly after midnight (with rates
exceeding 3"/hr per the 12Z HREF) above the snow level. Snow
levels drop under height falls from the approach trough through the
day Wednesday, though precip rates decrease on the Sierra Nevada
as the AR focuses over southern CA with 9000ft snow levels for the
SoCal ranges. Day 2 snow probs for >18" are 40% for the higher
central Sierra and 70-100% for the southern Sierra and the highest
White Mtns along the border with NV.

Expect a lull in Sierra Nevada snow Wednesday night as the dry
slot works its way across. However, by Thursday rates increase
again ahead of the upper trough axis. This time snow levels are
around 6000ft on the Sierra Nevada under the lower heights of an
upper low centered off northern CA. Snow rates of 2"/hr or more can
be expected for much of Thursday. Broader coverage of the >18"
snow probs are depicted for Day 3 with 50-90% values along the full
length of the Sierra Nevada as well as the CA Cascades including
Donner Pass on I-80.

Key Messages for the CA atmospheric river and subsequent low that
maintains impacts through Friday are linked below. CPC has further
KMs for next week.

...Pacific Northwest, Great Basin and Rockies... Days 1-3...
Maximum impact level from WSSI: Major

Upper low that is directing AR into CA is centered over the Queen
Charlotte Sound (BC) and a reinforcing shortwave trough crosses
Vancouver Island by this evening bringing a round of moderate
precip to the WA Cascades this evening. Snow levels already down
around 2500ft drop to around 1800ft before rates decrease overnight
providing impactful snow well below pass level. Day 1 snow probs
for >6" are 40-60% roughly at pass level in western WA, but it's
really all for this evening.

AR across northern CA to western WY this afternoon pivots
north to southern OR to the Bitterroots through Tuesday. Snow
levels rise in this moisture axis to around 7000ft keeping Day 1
snow probs for >6" limited to the highest Sawtooths, Tetons, and
Absarokas. The AR continues to pivot north-south along the West
Coast through Tuesday night with snow levels rising above 4000ft on
the WA Cascades by Wednesday. Day 2 snow probs for >4" are 40-60%
in the WA Cascades.

The focus for precip shifts interior on Wednesday as the AR
extending the length of CA shifts east over the Great Basin and the
Rockies for Wednesday night/Thursday. Snow levels are generally
high, 8000-9000ft with this moisture shield shifting inland. Day 3
snow probs for >6" are limited to the highest Great Basin ranges,
the Sawtooths, Uinta, Wind River, and San Juans where values are
40-70% (80% or greater for the higher Wind Rivers).

Jackson

...Atmospheric River/Winter Storm Key Messages are in effect for
California as linked below...

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

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