Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Sun Dec 19 2021

Valid 00Z Mon Dec 20 2021 - 00Z Thu Dec 23 2021

...Northwest U.S....
Days 1-3...

An area of low pressure offshore northern California and an
associated wavy stalling frontal boundary through Oregon into
western Montana will be the focus for widespread precipitation
during the day 1 period. A plume of higher moisture, upwards of
0.75" PWs, will support modest snowfall accumulations for the
higher elevations of the Oregon Cascades through central/northern
Idaho. At the surface, trapped colder air marginally near freezing
could lead to pockets of freezing rain. The upper low will wobble
offshore through Tuesday but its associated moisture plume will
gradually refocus onshore later Tuesday into Wednesday,
particularly on Oregon/northern CA initially then southward toward
central California. This will support locally heavier snowfall
accumulations from the Klamath Mountains, the Shasta Cascade
region, and then the northern/central Sierra Nevada.

WPC probabilities for at least 6 inches are above 80 percent for
the northern OR/southern WA Cascades and the Clearwater Region
during the day 1 period. After a brief lull on day 2,
probabilities for at least 6 inches ramp up above 80 percent for
the WA Cascades and the Sierra Nevada and northern California with
moderate probabilities over the northern Idaho and far northwest
Montana mountains.


...Northern Tier/Great Lakes...
Days 2-3...

Shortwave trough energy dropping into the northern Plains early
Tuesday will lead to a fast moving surface low tracking from
central North Dakota to the U.P. of Michigan. Along and north of
the low, a band of snowfall is expected. Forecast soundings in the
region show a favorable setup for light, fluffy snow with deep
saturation within the preferred DGZ and coincident with the
maximum lift through the column. This should lead to a higher SLR
event with the WPC SLR adjusted toward 18-20:1. With between
0.15-0.20" (locally 0.25") of QPF, this is expected to produce a
stripe of 3-4" (locally 4-5") of snow during the Day 2 period from
eastern North Dakota to the North Shore of Minnesota. As the wave
progresses eastward across the Great Lakes region, westerly to
northwesterly flow will support enhanced snow totals downwind the
Great Lakes, particularly Lake Superior where WPC probabilities
for at least 4 inches approach 70-80 percent across the eastern
U.P. of Michigan.


For days 1-3, the probability of ice accumulation greater than
0.25" is less than 10 percent.

Taylor