FOUS11 KWBC 200753
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 AM EST Sat Dec 20 2025
Valid 12Z Sat Dec 20 2025 - 12Z Tue Dec 23 2025
*** Strong Atmospheric River to impact California and the Sierra
Nevada through next week ***
...Great Lakes... Days 1-2...
Maximum impact level from WSSI: Minor
Weather pattern across the Great Lakes quickly shifts away from
strong cold air advection as a shortwave crosses over the region
with an associated surface low tracking north of Lake Superior
towards southern Quebec by Sunday morning. This allows for some
moderate warm air advection snow across the Upper Great Lakes
followed by lake effect snow overnight behind a potent cold front.
This west-northwest flow returns to Lake Ontario and Lake Erie by
Sunday as well. All in all snowfall amounts from this system don't
appear that impressive given it's progressive nature. WPC
probabilities for at least 6 inches of snowfall are highest across
the eastern U.P. of MI and between 40-70%. Chances for at least 4
inches of snow downwind of Lake Ontario are low (20-40%) along its
southern shore and the Tug Hill through Day 2.
...Cascades, Northern Rockies, & Northern California... Days 1-3...
Maximum impact level from WSSI: Major to Extreme
Deep upper troughing in eastern Pacific below the 10th
climatological percentile through at least early next week
continues to allow for an influx of westerly flow and Pacific
moisture into the western U.S. along with increasing IVT within a
wavering Atmospheric River. As moisture flux wanes across the
northern Rockies on Day 1 the next surge enters the region by the
start of Day 2 with snow levels starting around 5000ft across the
Sawtooths before steadily increasing throughout the region,
including the Absarokas, Tetons, and Wind River Range, up to
around 8000ft within the core of the moisture plume extending all
the way back to the central Pacific. Moisture will be plentiful as
PWs increasing to above the 99th climatological percentile over
northern UT and into WY. The heaviest snowfall is expected
throughout the ranges of central ID and western WY. WPC
probabilities for at least 18 inches of snowfall through Tuesday
morning are high (70-90%) across the Sawtooths of ID and Wind River/Teton/Absaroka ranges of WY above 8000ft. Snowfall totals in
this region could top 2 to 3 feet.
Farther north and away from the immediate AR plume, persistent
westerly flow and lower snow levels are forecast to impact the
Pacific Northwest. Snow levels across the Cascades are expected to
start the forecast period around 2000ft and only briefly rise to
around 3000ft tonight before falling yet again to 2000ft on Monday.
This places all major passes at risk of heavy accumulating snowfall
and treacherous driving conditions. Snoqualmie (3022ft) and
Stevens Pass (4061ft) of particular note could see total snow
accumulations between 1 to 2 feet. WPC probabilities for at least 2
feet of snowfall are high (70-90%) across the WA Cascades above
4000ft, with high chances for at least 12 inches of snow above
3000ft. Moisture also continues to stream into the northern ID and
northwest MT ranges, where elevations above 5000ft have high
chances for at least 8 inches of snowfall over a 72-hr period.
*** Strong Atmospheric River to impact California and the Sierra
Nevada through next week ***
...Sierra Nevada.. Days 1-3...
Maximum impact level from WSSI: Major to Extreme
Multiple AR impulses through Tuesday are expected with snow levels
potentially dropping down to around 7500ft as these moisture
plumes are aimed at the northern/central Sierra Nevada, with the
strongest surge forecast on Sunday ahead of a strong shortwave.
This may impact some of the passes across the crest (e.g., Donner
Pass at 7239ft) as snow levels dip/oscillate. The AR event will
continue beyond this forecast period into the medium range. WPC
probabilities for at least 12 inches of snowfall on days 1-3
(ending 12Z Tuesday) are between 70-90% and primarily above 8500ft
elevation. Snow levels will trend a bit lower into Monday and
Tuesday as WPC probabilities for at least 4 inches of snow are >30%
to elevations around 7500ft. This is possibly the start of a very
active and impactful weather pattern across California, with
additional high elevation heavy snow likely through the middle to
end of next week. See our latest Key Messages that cover the event
through the end of next week, and CPC's Key Messages into week 2.
The probability of significant icing is less than 10 percent.
Snell
...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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