Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Fri Dec 30 2022

Valid 12Z Fri Dec 30 2022 - 12Z Mon Jan 02 2023

...Western U.S. into the central High Plains...
Days 1-3...
An occluded system will continue to move into the northwestern
U.S. this morning, producing widespread precipitation from the
Pacific Northwest to the northern Idaho and western Montana
ranges.  Snow levels will be low at the onset, supporting
accumulating snow across the interior lowlands before snow levels
begin to rise later today.   Back along the trailing front, a
shortwave and accompanying plume of deep moisture is expected to
move inland near the Oregon-California border later today.  With
the deepest moisture centered across Northern California,
orographically-focused heavy precipitation is expected in the
mountains of Northwest California and the Sierra.  However, snow
levels will be quite high -- above 9000 ft across much of
California -- largely limiting heavy snow accumulations to the
high peaks of the southern Cascades and the Sierra.  This
shortwave is expected to move east of the Sierra this evening,
with ample moisture spreading east as well across the Great Basin
and into the Rockies.  This is expected to produce some locally
heavy snow accumulation across the higher elevations of the
northern Nevada mountains, including the Rudy Mountains, as well
as the southern Idaho, northern Utah, and western Wyoming ranges. 
WPC PWPF indicates that isolated snow amounts of a foot or more
are possible in these areas by Saturday morning. 

Meanwhile, wet weather is expected to continue across California
as an amplifying upper trough approaches the state early Saturday.
 This will focus a narrow stripe of deep moisture from Northern
into Central California as is pivots toward the region during the
day.  Snow levels will begin to crash as the system begins to move
inland, supporting heavy snow along the Sierra during the
afternoon and evening hours, with widespread accumulations of a
foot or more expected.  Further downstream, a lingering boundary
extending across the Great Basin into the Rockies will remain a
focus for organized precipitation, with additional heavy snow
accumulations possible across the northern Nevada mountains and
the southern Idaho, northern Utah, Wyoming, and western Colorado
ranges.

On Sunday and Sunday night, the shortwave in the West will
continue to amplify as it moves from California into the interior
West, with a closed low forecast to develop north of the Four
Corners Region Sunday night.  This will bring the threat for heavy
snow farther south and west, including the southern Utah mountains
and the Arizona High Country, as well as the Colorado and northern
New Mexico ranges.  Some of the heaviest amounts are most likely
over the San Juan Mountains, where deep southwesterly inflow and
strong ascent are expected to produce snow amounts of a foot or
more.  As the system continues to move east, southeasterly to
easterly flow on the north side of the 700 mb low will support
precipitation developing over the central High Plains back into
the central Wyoming and north-central Colorado ranges, where some
locally heavy accumulations are possible by early Monday. 

The probability for ice accumulations of 0.10 inch or more is less
than 10 percent.

Pereira