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

Valid 00Z Tue Nov 09 2021 - 00Z Fri Nov 12 2021


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

Over CA, confluent mid-level flow combined with the LFQ of an
approaching Pacific jet streak will drive a modest but transient
AR onshore, spreading precipitation eastward overnight into
Tuesday. 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 40-80%, especially over the highest
peaks. 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 70-80% for 8 inches, with 1-2 feet
possible in the highest peaks.

On Tuesday, a much more significant shortwave trough will shed off
on the southeastern side of a closed low just west of British
Columbia and move quickly across the Pacific Northwest/northern
Rockies before advecting into the Northern Plains Wednesday as
heights build quickly into the northeastern Pacific. 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 as it only weakens slightly to ~130-140kts as additional
shortwaves ride along the north side of the Pacific ridging. This
will place favorable LFQ diffluence for ascent atop the amplifying
trough and its associated height falls Thursday. The moisture
plume embedded within the AR will peak early 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/splitting 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 but still rising by the end of day 3.

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 with relatively lower snow levels compared
to Oregon and points south. By D3 the heaviest snow will trend
away from the Cascades and shift into the Northern Rockies and WY
ranges where WPC probabilities for 4 inches are 20-30%, while
portions of the CO Rockies have probabilities of 30-50%. By the
end of day 3, deepening Upper Midwest system will draw in enough
colder air to support snow over northern North Dakota/Minnesota as
the system occludes. WPC probabilities for 4 inches of snow are up
to about 30% along the Canadian border through 00Z 12 Nov (Thu
evening). Snow is expected to continue into Friday into the medium
range.


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

Fracasso/Weiss