Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EST Sat Dec 11 2021

Valid 00Z Sun Dec 12 2021 - 00Z Wed Dec 15 2021

...Pacific Northwest to the Northern Rockies and California...
Days 1-3...
An active period of weather will impact a large portion of the
western U.S. through the remainder of the weekend and into early
next week. 

A series of shortwaves embedded within the leading edge of a broad
upper trough will continue to support precipitation spreading east
from the Pacific Northwest into the northern Rockies.  As this
energy continues to move through the base of the trough, this will
shift the focus of better moisture advection and heavier
precipitation farther south from western Washington and Oregon
into Northern California tonight.   

Following a relative lull on Sunday, a second surge of deeper
moisture associated with an amplified shortwave will impact
southwestern Oregon and Northern California on Monday.  This will
bring additional heavy snows to the Klamath Mountains and the
southern Cascades.  Meanwhile, heavy snows are expected to extend
farther south from the northern into the central Sierra Nevada on
Monday.

The shortwave will continue to amplify as it moves onshore,
bringing heavy precipitation farther south along the Sierra Nevada
and into Southern California, as well as farther east into the
Great Basin on Tuesday.  This will include locally heavy mountain
snows for the Transverse Ranges of Southern California and the
Nevada mountains.  In addition to deeper moisture, this will also
bring a reinforcing shot of colder air, supporting accumulating
snow close to the coast with heavy accumulations possible in the
coastal ranges of northwestern California and southwestern Oregon.

Snowfall accumulations of 1-3 feet are likely for Olympics,
Cascades, and portions of the northern Rockies.  Beginning Sunday,
accumulations of 2-4 feet are likely for the Klamath Mountains and
the Sierra Nevada.  

For Days 1-3, significant icing is not expected.

~~~Key Messages for Dec 11-14 Winter Storm across the Western
U.S.~~~

...A prolonged Atmospheric River will bring heavy precipitation to
much of the Western U.S....
Heavy precipitation across the Pacific Northwest will spread
southward and eastward tonight. A second, even more impressive,
surge of precipitation will lift onshore CA Monday and spread into
the Great Basin by Wednesday.

...Heavy mountain snow likely...
Significant snow accumulating to 1-3 feet is likely in the
Olympics, Cascades, and Northern Rockies. Heavy snow of 2-4 feet
or more is likely beginning Sunday in the ranges of CA including
the Sierra Nevada.

...Widespread hazardous travel...
Travel will become dangerous, especially for the mountain passes,
and is discouraged. Roads are likely to experience closures.
Whiteout conditions, downed trees, and power outages are all
possible.

...Heavy rain expected for the valley locations in California...
Several inches of rainfall will create the potential for flash
flooding and debris flows, particularly near recent burn scar
areas.


Pereira