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

Valid 00Z Mon Dec 13 2021 - 00Z Thu Dec 16 2021

...Western U.S....
Days 1-3...
As an upper trough continues to amplify and approach the western
U.S., heavy precipitation is expected to redevelop and spread
farther south across California.  From late today through Monday,
heavy precipitation including mountain snow, is expected from
southern Oregon to central California.  With snowfall rates at or
above 2 inches/hour in some locations, very heavy snowfall amounts
are expected across the northern to central California ranges,
including the Klamath, southern Cascades, and the Sierra Nevada. 
As the trough approaches the coast, 500 mb heights are expected to
drop more than 2 standard deviations below normal across western
Oregon and northwestern Californian -- supporting accumulating
snow near the coast, with the potential for locally heavy amounts
along the higher peaks of the coastal ranges.  Meanwhile,
unsettled weather is expected to continue farther north and west
across the remainder of the Pacific Northwest into the northern
Rockies.  Moisture and mid-level energy embedded within the
leading edge of the upper trough will support periods of heavier
snow across portions of the northern Great Basin into the northern
Rockies. 

On Tuesday, models show the upper trough continuing to amplify as
it moves onshore, supporting additional heavy precipitation across
central California while extending farther south into Southern
California and east into the Great Basin.  As locally heavy snows
develop over the higher elevations of the Transverse Ranges, heavy
snows will continue to gradually shift south along the Sierra
Nevada. Farther north and east, a strong baroclinic zone pushing
east will support areas of moderate to heavy moving across the
Great Basin and northern Rockies.

By Tuesday night, heavy precipitation across California is
expected to quickly diminish as the upper trough moves east into
the Four Corners region.  A period of heavy snow is expected
Tuesday night into Wednesday for the higher elevations of northern
Arizona, as well as the Utah and western Colorado ranges.  The
system is expected to move rather quickly however, with snow
diminishing from west to east as the upper trough ejects into the
central Plains by late Wednesday.

Organized heavy precipitation, including mountain snow is forecast
to quickly return to the portions of the Northwest and Northern
California as another well-defined shortwave drops south from the
Gulf of Alaska on Wednesday.  The latest guidance suggests that
strong onshore flow and upper forcing ahead of the system will
help focus additional heavy snows across southern Oregon and
Northern California, including the Klamath Mountains and the
southern Cascades. 

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


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

...Ongoing Atmospheric River will bring heavy precipitation to
much of the Western U.S....
Widespread precipitation across the Pacific Northwest will
continue to expand southward through central 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 may eclipse 2"/hr.

...Widespread impacts to travel and infrastructure...
Travel will become dangerous, especially for the mountain passes,
and is discouraged. Some road closures are expected. Whiteout
conditions, downed trees, and power outages are all likely.
Avalanches will be possible across the Sierra.

...Heavy rain expected for portions of California...
Several inches of rainfall will create the potential for flash
flooding and debris flows, particularly near recent burn scar
areas and along the central California coast.


Pereira