FOUS11 KWBC 210754
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 AM EST Sun Dec 21 2025

Valid 12Z Sun Dec 21 2025 - 12Z Wed Dec 24 2025

*** Strong Atmospheric River to impact California and the Sierra
Nevada over the next several days ***

...Sierra Nevada... Days 1-3...
Maximum impact level from WSSI: Major to Extreme

A long-lived Atmospheric River event will contain waves of
precipitation as embedded systems move inland from the Pacific over
the next several days (and beyond). An area of surface low
pressure will move inland across NorCal today and bring heavier
snow to the northern/central Sierra into early Monday with snow
levels around 8000-10,000ft from north to south. Snow may impact
some of the passes across the Sierra Crest though the heaviest snow
rates may be above 9000ft (2-3"/hr). Thereafter, the moisture
plume will lift northward and snow levels may lower a bit into
Tuesday with less accumulation overall except for the highest peaks
in the Shasta/Siskiyous and farthest north Sierra. The next surge
of moisture is expected by the end of day 3 as the lingering AR
plume orients more north-south in response to height falls just off
the West Coast. Snow levels also fall with this next round to
around 6000-8000ft from north to south. WPC probabilities for at
least 12 inches of snowfall are 50-90% above around 8000ft through
12z Wednesday. This very active and impactful weather pattern
across California is expected to continue through at least the next
several days, with additional high elevation heavy snow likely into
the end of this week. See our latest Key Messages that cover the
event through the end of next week, and CPC's Key Messages into week 2.

...Upstate New York... Day 1... Maximum impact level from WSSI: Minor

Clipper system racing across southeast Canada and a strong cold
front pushing across the Great Lakes and Northeast will provide the
chances for isolated snow squalls and a heavy lake effect snowband
downwind of Lake Ontario through tonight. Westerly flow around 40
kts at 850 mb and temperatures at this level around -15C will aid a
single snowband off Lake Ontario into Oswego and northern Oneida
counties. Flow bends towards a more northerly direction around 00Z
tonight before the band lifts back north on Monday. This wavering
will prevent significant snowfall accumulations, but moderate
amounts are still possible specifically prior to 00Z tonight. WPC
probabilities for at least 6 inches of snow are around 30-60% for
this region extending east from the southeast shores of Lake Ontario.

...Northeast... Days 2-3... Probability of at least Minor Impacts: >60%

Upper level pattern over CONUS by late day 2 includes an anomalous
ridge centered over the central U.S. and strong jet soaring to the
north along the U.S-Canadian border before aiming southeastward
across the Northeast, where slight troughing remains. An embedded
shortwave crossing north of the Great Lakes on Tuesday within a
divergent left-exit region of a strengthening 150kt jet streak will
aid in blossoming precipitation across the Northeast by Tuesday.
Broad WAA ahead of the system over the still cold Northeast will
bring generally light snow to much of the Northeast and northern
Mid- Atlantic away from the immediate coast as early as Tuesday
morning. Some light freezing rain is also possible on the southern
edge of the precipitation (central PA and perhaps into parts of
WV) though the highest QPF will lie over Northern/Upstate/Central
NYS to southern New England. Snow is expected to track across New
England on day 3 as the shortwave crosses over the region and could
help spin up a surface low in the Gulf of Maine. This may lead to
an inverted trough or quickly developing TROWAL maximizing QPF
potential across southern Maine, but high uncertainty remains
within the day 3 timeframe. WPC probabilities for at least 4 inches
of snow are currently 20-40% from NYS through much of New England
(excluding regions south of I-90).

...Cascades, Northern Rockies, & Northern California... Days 1-3...
Maximum impact level from WSSI: Major to Extreme

Digging heights near 135W in the eastern Pacific will help lift a
shortwave over the Northwest tonight as well as direct a resurgent
Atmospheric River into northern California with spillover
extending into the Northern Rockies. Snow levels will remain on the
lower side north of the moisture plume, but continued upslope into
the Cascades and Rockies will still yield modest snow totals for
these regions, especially days 1-2. The incoming shortwave tonight
will briefly raise snow levels in the WA Cascades from around 2000
to 3000ft, but then fall again with some CAA behind the system.
This will continue to impact the passes in the region (e.g.,
Snoqualmie and Stevens) where significant snow is likely. Heavy
snow in these regions of the Northwest and Northern Rockies may be
focused around two waves -- the initial one today/tonight with
another Monday night into early Tuesday. WPC probabilities for at
least 12 inches of snow for the three-day period are >50% above
3000ft or so.

Farther east, snow levels around 6000ft over the Northern Rockies
will rise today as heights rise and moisture increases. SW flow and
sufficient moisture will support lighter but still appreciable
snow compared to the Cascades over northern Idaho into
northwestern Montana, generally above 5000ft. The strongest
moisture flux will be in the core of the AR where PW and IVT
anomalies lie near/above the 99th percentile, generally in an arc
from the southern OR Cascades into the central/southern Idaho
ranges and into Wyoming. There, WSW to W flow will maximize
accumulations over parts of the Bitterroots and especially into the
Tetons, Absarokas, and Wind River Range. WPC probabilities for at
least 24 inches of snow over the three-day period are high (>70%)
over western Wyoming. High mountain peaks/ridges above
9000-10,000ft may see several feet of snow before precipitation
begins to wane on Tuesday.

The probability of significant icing is less than 10 percent.

Snell/Fracasso

...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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