Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 PM EDT Fri Oct 22 2021

Valid 00Z Sat Oct 23 2021 - 00Z Tue Oct 26 2021

...Pacific Northwest and Northern California to the Central
Rockies...
Days 1-3

An active pattern continues across the West as waves of heavy
precipitation move onshore from the Pacific Ocean and spread
eastward.

The first will occur D1 /Friday night and Saturday/ as a weak
shortwave and associated surface cold front push onshore. The
mid-level flow during this period is nearly zonal, but broadly
confluent, which will drive PWs of +0.5 to +1 standard deviation
above the climo mean. Within this moistening column, weak PVA and
modest LFQ diffluence will produce ascent to spread precipitation
along and ahead of the front across northern CA, OR, and WA. Low
and mid-level flow nearly orthogonal to the front will keep the
system progressive, so total snowfall should be modest and
generally above 6000 ft. WPC probabilities on D1 for 4 inches are
30-40% in the highest peaks of the Cascades, and 10% in the
Sawtooth Mountains.

A much more significant event begins the latter half of D2
/Sunday/ and D3 as an extremely strong Atmospheric River moves
onshore CA and spills northeast into the Rockies. An anomalously
deep mid-level low, progged to exceed the climatological minimum
geopotential height at 500mb according to the NAEFS ensemble
tables, will drift eastward south of British Columbia Sunday into
Monday before opening and lifting onshore WA State Monday night.
This feature will help amplify a full-latitude trough pushing
across the Pacific, causing downstream upper jet intensification
to 150kts. Intense deep layer ascent through upper diffluence and
impressive height falls will combine with strong WAA and PWs
reaching +4 standard deviations to produce widespread heavy
precipitation beginning Sunday morning. A prolonged overlap of
ascent and moisture will lead to intense snowfall and snowfall
rates, especially in the Sierra where orographic enhancement will
also occur, although heavy snow is also likely in the Sawtooth and
Blue Mountains, as well as the WA Cascades.

For D2 /Saturday night and Sunday/, WPC probabilities for 6 inches
are moderate in the highest peaks of the WA Cascades above 6000
ft, and in the Sierra Nevada above 10,000 ft. By D3, the AR surges
onshore and excessive snowfall and snow rates are likely. The
heaviest snow will occur in the Sierra where WPC probabilities for
18 inches are above 80%, and the 10th percentile of the WSE is
over 18 inches in a few locations. With persistent onshore flow
and strong ascent, lowering snow levels, and increasing SLR, it is
likely that some peaks above 9000 ft will receive in excess of 2-3
feet of snow D3, with event totals possibly reaching 4 feet. As
snow levels crash behind the cold front and longwave trough axis,
light to moderate accumulations will occur as low as 5000 ft.
Across the other ranges of the West, snowfall accumulations of
more than 8 inches are likely as shown by WPC probabilities in
excess of 50% in the Sawtooth, WA Cascades, Olympics, and parts of
the Blue Mountains and Northern CA ranges. WPC probabilities of
50% for 4 inches extend into the Uintas, Tetons, and CO Rockies as
well on D3.

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss