Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Sat Nov 06 2021

Valid 00Z Sun Nov 07 2021 - 00Z Wed Nov 10 2021

...Pacific Northwest and Northern California to the Northern
Rockies...
Days 1-3...
An upper trough will continue to broaden across the western U.S.
into early next week as a series of shortwaves move inland ahead
of a deep upper low settling southeast through the Gulf of Alaska.
 A series of leading waves are expected to lift across the
Northwest into central Canada overnight and Sunday, as another
well-defined wave approaches the Washington coast late Sunday.  In
addition to heavy snows for portions of the Olympics and Cascades,
a low-to-mid level baroclinic zone pushing east is expected to
support some some moderate to locally heavy snows across portions
of the northern Rockies, with the heaviest accumulations expected
across the northwestern Wyoming into southwestern Montana ranges. 
For the Olympics and Cascades, WPC Day 1 guidance -- ending 00Z
Monday -- shows high probabilities for accumulations of 8 inches
or more, mainly for areas above 4000 ft.  WPC guidance also shows
some locally higher probabilities for 6 inches or more for parts
of Yellowstone National Park.

The previously noted shortwave approaching the Northwest late
Sunday, is forecast to move quickly across the region --
supporting some additional locally heavier amounts for the
Olympics and northern Cascades.  A shortwave ridge briefly builds
across the Northwest on Monday -- providing a relative lull ahead
of the next system.  For the Day 2 period -- ending 00Z Tuesday --
WPC probabilities suggest that any threat for additional
accumulations of 8 inches or more will be largely confined to the
higher peaks of the Olympics and northern Cascades.

On Tuesday, a strong wave is forecast to move into northern
California -- bringing locally heavy snows to the Klamath
Mountains into the southern Cascades and the Sierra.  For the Day
3 period -- ending 00Z Wednesday -- WPC guidance shows higher
probabilities for snow accumulations of 6 inches or more for
portions of the Klamath Mountains into the southern Cascades,
mainly for areas above 3000 ft.  Similar probabilities are noted
for the Sierra, but primarily above 7000 ft.  Onshore flow and
increasing divergence ahead of the approaching low over the Gulf
of Alaska will support the return of increasing probabilities for
heavy amounts across the Olympics, with WPC probabilities
indicating additional amounts of 6 inches or more are likely above
4000 ft.

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

Pereira