FOUS11 KWBC 230833
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Tue Dec 23 2025

Valid 12Z Tue Dec 23 2025 - 12Z 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 today and New England
early Wednesday on a potent 150+kt NWly jet below a deep and
sprawling vortex centered north of Hudson Bay. Precip will be
ongoing this morning in warm air advection over the upper Ohio
Valley and the Northeast. Surface temperatures become marginal
quickly after sunrise and limits the potential for heavy
accumulations from PA to southern New England. Light freezing rain
(a few hundredths accretion) is also possible this morning on the
southern edge of the precipitation shield (from central PA to central WV).

The surface low crosses New England this evening with continued
snow into Wednesday morning for the 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 snow probs for

6" are 40-80% over this terrain with 50% probs for over 8" in the

highest terrain.

Heaviest snowfall associated with this system is expected across
the coastal plain and areas just inland across Maine as low
pressure redevelops along a surface trough over the Gulf of Maine.
This surface trough, or also referred to as a "Norlun Trough", will
lead to very strong and narrow low-level convergence capable of
producing snowfall rates of 1-2"/hr. However, given the narrow axis
these setups can be extremely tricky to forecast. Current
expectations are for heavy snow to occur between Portland and Bar
Harbor, along with areas just inland. Surface temperatures could
also be a concern for regions right along the coast as southerly
flow draws both moisture and a slightly warmer marine airmass. Days
1-2 probs for >8" are 50-80% for much of the central Maine coast
west well into the remote interior portions of the state. Maximum
amounts within the heaviest band are likely to exceed a foot, which
is depicted in the NBM 75th percentile for areas just inland of the
coastal plain.

...Great Lakes...
Day 3... Chances of at least minor impacts from WSSI-P: 40-50%

By late in the forecast period (Thursday night), the next shortwave
to round the top of the anomalous central U.S. upper ridge begins
to cross over the Northern Plains and Upper Midwest. Even though
the system is fairly unorganized through early Friday morning, the
origins of the shortwave and associated mid-to-upper level moisture
trace back to the tropical Pacific and Atmospheric River set to
impact California. In fact, NAEFS ESAT depict IVT above the 99.5th climatological percentile Thursday evening over the western Great
Lakes. Still, the greatest forcing at this time appears along the
MN-Canadian border and just north into Canada, limiting snowfall
impacts. However, a potent warm nose around 800mb per model cross
sections will allow for precipitation throughout parts of the
Upper Midwest and Great Lakes to fall as sleet or freezing rain. As
850mb FGEN rapidly strengthens across southern MI Friday morning
in response to a developing low pressure center over the Midwest,
precipitation is expected to blossom and fall mostly as sleet or
freezing rain. WPC probabilities for at least 0.1" of ice accretion
through 12Z Friday (more possible afterwards as well) are currently
low (10-20%) across the central L.P. of MI. Expect higher chances
once all of Friday is in the forecast period.

...California/Sierra Nevada...
Days 1-3... Maximum impact level from WSSI: Extreme

Strong Atmospheric River is set to impact CA during the Day 1
period with heavy snowfall across the Sierra Nevada. Additional
systems containing heavy high elevation snow and falling snow
levels are expected through the end of the week. In total, multiple
feet of snowfall are expected along the entire Sierra Nevada
(heaviest in central and southern sections), with snowfall also
impacting most major passes into northern CA.

The meteorological setup for this significant event includes a
currently north-south oriented AR combining with a potent shortwave
lifting along the WEst Coast today while on the eastern periphery
of an anomalous eastern Pacific upper trough. This shortwave and
associated potent surface low pressure will draw moisture northward
out of the tropics and produce IVT above the climatological record
for late December according to the NAEFS ESAT by early Wednesday.
Additionally, snow levels start out Tuesday night between
7500-8500ft across CA (highest in the southern Sierra) and fall to
below 6000ft in northern CA by Wednesday night as levels dip to
around 7500ft in the southern Sierra. Where snow does occur across
the Sierra Nevada it will come down extremely heavy as 00z HREF
depicts rates greater than 2-3"/hr after 09Z Wednesday and lasting
into Wednesday evening. Snow levels continue to drop under height
falls from the approaching trough through Wednesday night into
Thursday though as precip rates decrease on the Sierra Nevada as
the AR focuses over southern CA with 9000ft snow levels for the
SoCal ranges. This will lead to a dry slow until the next surge
ahead of a separate Pacific low approaches on Thursday.

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. Snow levels also
drop to around 4000ft by the end of Day 3 across northern CA and
allow for accumulating snow potential for the I-5 pass near Mount
Shasta. 72-hr snow probabilities for >30" snow are between 50-90%
for much of the Sierra Nevada. Totals are likely to exceed 4-8 feet
above 7000-8000ft.

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 2-3... Maximum impact level from WSSI: Moderate

Outside of light snow across the northern Great Basin and Cascades
on Day 1, the next surge of moisture is forecast to enter the
interior West and Northwest ranges on days 2-3. The strong AR
extending the length of CA shifts east over the Great Basin and the
Rockies for Wednesday night/Thursday along with its anomalous
moisture. Snow levels are generally high, 8000-9000ft with this
moisture shield shifting inland outside of the Cascades where
levels increase to 5000ft Wednesday and drop back to around 3000ft
Thursday. Days 2-3 snow probs for >6" are limited to the highest
OR/WA Cascades, Great Basin ranges, the Sawtooths, Uinta, Wind
River, and San Juans where values are 40-70% (80% or greater for
the higher Wind Rivers and Sawtooths).

Snell/Jackson

...Atmospheric River 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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