Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 AM EST Mon Jan 04 2021

Valid 12Z Mon Jan 04 2021 - 12Z Thu Jan 07 2021

...Pacific Northwest/California/Northern Rockies...
Days 1-3...

A modest AR through persistent mid-level confluent and primarily
zonal flow, with 2 sharp shortwaves embedded within this flow
producing periods of heavy snow across the West.

The first is is a sharp and amplified shortwave which will drag
onshore this aftn, accompanying a cold front which will gradually
decay with eastward progression onshore. Ahead of this front,
mid-level flow will back to the SW driving enhanced WAA/moist
advection but at the same time raising snow levels to around
4500-5000 ft. Height falls, PVA, and this moist advection will
spread precipitation from the Sierra northward through the WA
Cascades and eastward into the ranges of NW WY. Precipitation is
likely to be heavy, and WPC probabilities are high for 12 inches
above 5000 ft in all of these ranges, with up to 2 ft possible in
the WA Cascades, Sawtooth, and Blue Mountain ranges.

This first impulse moves quickly eastward during D2 followed
quickly by shortwave ridging as wavelengths remain short. This
will cutoff the ascent/moisture briefly, but the next system will
approach the coast late D2 and then move onshore with another cold
front D3. Once again a backing of the mid-level flow will drive
WAA and moist advection into the West, with heavy snow
redeveloping. This second impulse does not appear to have as much
amplitude as the first, so the heavy snow will generally be
confined north of CA, and WPC probabilities are high for 8 inches
in the WA and OR Cascades, spreading inland to the Blue Mountains
of OR. Event total snowfall in the highest terrain may exceed 4 ft
in the Cascades. with 12+ inches possible at pass level including
Snoqualmie, Stampede, and Stevens.


The probability of significant icing (0.25" or more) is less than
10 percent all 3 days.

Weiss