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

Valid 12Z Sun Dec 19 2021 - 12Z Wed Dec 22 2021

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

A weakening shortwave shearing eastward across British Columbia
will help drag a cold front inland toward the Divide today before
stalling parallel to the mid-level flow as heights begin to rise.
A modest plume of moisture -- PW values around 0.75 inch or about
+1 sigma -- will support some moderate snowfall at higher
elevations of the Oregon Cascades eastward through the Clearwater
Region of Idaho. Marginally colder temperatures in lower
elevations east of the Cascades could support a period of light
snow and/or freezing rain. Just off the coast, a trailing upper
low well west of the Oregon/California coast will maintain a
steady feed of moisture into the Pacific Northwest but decreasing
in magnitude. This low pressure system is forecast to meander off
the coast and slowly push its cold front into OR/CA Tuesday as the
in-situ boundary weakens. This will bring deeper moisture into
Northern California on Tuesday which will support locally heavier
snow back into the Klamath Mountains, the Shasta Cascade region,
and the northern/central Sierra Nevada.

For the three-day period, WPC probabilities of at least 6 inches
of snow were moderate to high over the Cascades and Blue Mountains
into the northern Rockies, including a large portion of northern
Idaho into western Montana at and above the passes higher than
4000 ft. Probabilities are moderate for at least 6 inches over
much of the Sierra Nevada and northern California on day 3.


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

Upper trough swinging out of southwestern Canada late Monday will
support a quick-moving area of snowfall northeast of a surface low
along a wavy boundary from southeastern Montana across the central
Plains early Tuesday. Surface low should lift toward the U.P. of
Michigan by late Tuesday and into Canada by the end of the period.
Model guidance was in fairly good agreement on a swath of about
0.10-0.25" QPF along and north of a GFK-DLH-PLN line in the brief
warm advection. Column will be cold (sfc T in the 10s) and modest
lift in the DGZ could support some higher snow ratios to make for
a fluffy few inches of snow. WPC probabilities of at least 4
inches of snow were low to moderate over the U.P. of Michigan
(20-50%) on Tuesday ending early Wednesday, perhaps enhanced by
Lake Superior as the system passes by and winds turn
northwesterly.


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

Fracasso