Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Sun Dec 12 2021

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

...Pacific Coast and Intermountain West...
Days 1-3...
An active period of weather will impact a large portion of the
western U.S. through the middle of the upcoming week. 

A mid-level trough originating from the Gulf of Alaska will
amplify into a closed low and pivot southward along the Pacific
Northwest coast through Monday. As this feature opens into
Tuesday, a secondary shortwave rotating through the base will
amplify and dig onshore the CA coast late Tuesday, before finally
ejecting the entire trough eastward into the Great Basin late D3.
This evolution will be accompanied by increasingly confluent
mid-level flow beneath an intensifying subtropical jet streak
placing persistent RRQ diffluence atop the region. Moisture within
this impressive plume will surge onshore, and IVT is progged to
exceed the 99th percentile late D2 into D3 according to the NAEFS
ensemble tables. With snow levels generally low behind a series of
surface fronts moving eastward, heavy snow is likely in most of
the terrain from the Olympics eastward to the Central Rockies and
southward as far as the transverse ranges of Southern CA.

For D1, the heaviest snowfall is expected from the Olympics and WA
Cascades southward through the northern and central Sierra where
WPC probabilities are high for 8 inches, generally above 2000 ft.
The heaviest snowfall D1 is likely in the Shasta/Trinity ranges
and Sierra where SW flow will favorable upslope to enhance
snowfall to locally in excess of 2 feet, which is also possible in
the Sawtooth NE of the Snake River valley.

As the trough sharpens and digs southward D2 the most robust
IVT/AR surges onshore, angled ideally into the Sierra for
exceptional snowfall. WPC probabilities on D2 are above 90% for 18
inches across much of the Sierra with snow levels climbing as high
as 5000 ft, and some places could received 3 to 5 feet of snow as
snowfall rates reach 2-4"/hr according to the WPC snowband
prototype tool within possible thunder snow. Further north, heavy
snow will continue near Mt. Shasta, the OR Cascades, and the
Sawtooth of ID/Blue Mountains of OR where WPC probabilities for 12
inches are above 70%. Across the rest of OR/WA lowering snow
levels will allow for light accumulations as low as 1000 ft. These
snow levels will continue to fall into D3, and it is possible the
city of Portland, OR could get some light snow Tuesday, although
WPC probabilities for 1 inch are less than 10% at this time.
However, as the main trough finally begins to eject eastward,
heavy snow will persist in the Sierra while expanding eastward
across much of the Great Basin and as far east as the
Wasatch/Uintas of UT and the San Juans of CO, and as far south as
the San Gabriels, San Bernadinos, and the Mogollon Rim. WPC
probabilities for 6 inches are high in the higher terrain of all
of these ranges, with locally 2 feet possible in the Sierra,
bringing storm total snowfall there to 6-7 feet above 7000 ft.

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 into CA and eastward into the Northern Rockies today. A
more impressive surge of moisture will shift onshore CA Monday and
Tuesday spreading heavy precipitation into the Great Basin.

...Heavy mountain snow likely...
Significant snow accumulating to 1-3 feet is likely in the
Olympics, Cascades, and Northern Rockies. Heavy snow of 3-5 feet
or more is likely in the ranges of CA including the Sierra Nevada.
Snowfall rates at times may eclipse 2"/hr.

...Widespread impacts to travel and infrastructure...
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. Dangerous avalanches will become possible across the
Sierra.

...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.


Weiss