Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 AM EST Mon Nov 08 2021

Valid 12Z Mon Nov 08 2021 - 12Z Thu Nov 11 2021


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

Brief shortwave ridging will develop across the Pacific Northwest
today, raising snow levels and limiting heavy snow to the highest
terrain of the Cascades. However, to the south, focused across CA,
confluent mid-level flow combined with the LFQ of an approaching
Pacific jet streak will drive a modest AR onshore, spreading
precipitation eastward. Snow levels will generally rise through
D1, from 3000 ft early to above 8000 ft late within the warm flow.
This will limit heavy snowfall to the higher terrain of the Sierra
D1 where WPC probabilities for 6 inches are 40-50%. The exception
is likely to be in the Trinity/Shasta/Siskiyou ranges where
upslope flow and more impressive snowfall rates will likely trap
some lower snow levels, and the heaviest snow on D1 is expected in
this area where WPC probabilities are as high as 80% for 8 inches,
with 1-2 feet possible in the highest peaks.

A much more significant shortwave trough sheds from a closed low
in the Gulf of Alaska and moves quickly across the Pacific
Northwest before advecting into the Northern Plains Wednesday,
while a rapidly deepening shortwave digs out of Alberta Canada D3.
This will be accompanied by intense Pacific jet energy, as a
zonally oriented jet streak reaching 150 kts surges onshore and
into the High Plains, placing favorable LFQ diffluence for ascent
atop the amplifying trough and its associated height falls. The
moisture plume embedded within the AR will peak D2 across CA and
the Great Basin before shifting northeast towards the Northern
Plains D3, with snow level rises occurring within the region of
greatest IVT ahead of a weakening surface cold front. Snow levels
will rise to 9000 ft in the Sierra, and 5000 ft or more into the
Intermountain West, but remain 2000-4000 ft in the Cascades and
Olympics.

On D2 the heaviest snow is likely in the Olympics, WA Cascades,
Sawtooth, and Tetons where WPC probabilities for 4 inches are
high, and more than 12 inches is likely in the higher terrain. At
pass level, several inches of snow is likely at Snoqualmie,
Stevens, and White Pass. By D3 the heaviest snow continues in the
Cascades, but also shifts into the Northern Rockies, WY ranges,
and CO Rockies where WPC probabilities for 4 inches are 20-30%.

As these shortwaves consolidate late D3 into D4, robust WAA will
spread precipitation into MN/ND, potentially bringing heavy snow
to the northern parts of these states just beyond this forecast
period. 

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

Weiss