Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
224 AM EST Wed Dec 29 2021

Valid 12Z Wed Dec 29 2021 - 12Z Sat Jan 01 2022

...Western U.S...
Days 1-3...
A broad upper trough with several embedded shortwaves will
continue to support unsettled weather, including areas of heavy
mountain snow, across the western U.S. over the next couple of
days.  Drier weather is expected along the West Coast by the
weekend as the trough shifts east into the Rockies and High Plains
while an upper ridge moves onshore. 

Additional heavy snows are expected for the southern Sierra and
Southern California mountains as an amplified, positively-tilted,
upper trough develops and drops south across California later
today.  Additional accumulations of 1-2 feet are expected for the
southern Sierra beginning later today. 

Heavy snows are also likely for the Transverse and northern
Peninsular ranges.  Snow developing there later today is expected
to continue into Thursday as a upper low briefly closes off along
the coast of Southern California and northern Baja.  Accumulations
of 1-2 feet are likely, with several feet possible across the
higher peaks, before snow begins to diminish on Thursday.

Areas farther east are expected to see localized areas of heavy of
heavy snow late Wednesday into Thursday as well, especially across
the southern Nevada and southwestern Utah ranges, where guidance
continues to show low-to-mid level frontogenesis supporting
heavier totals.

Meanwhile, an amplifying northern stream shortwave is forecast to
dive into the Northwest on Thursday.  This will bring the threat
for locally heavy snow accumulations back into Washington and
Oregon Cascades, as well as the Blue Mountains.  As this system
continues to dig southeast and amplify the flow over the Great
Basin, the southern stream wave along the Southern California
coast will move inland across the Southwest on Friday, raising the
potential for heavy snows along the Mogollon Rim into the
mountains of southeastern Arizona and western New Mexico, into the
southern Rockies.

...Central Plains to the lower Missouri and mid Mississippi
valleys...
Day 3...
As the western U.S. trough shifts east, shortwave energy lifting
along the leading edge of the trough, in addition to low-to-mid
level frontogenesis, will support  precipitation over the central
Plains into the lower Missouri and mid Mississippi valleys.  While
the details are far from certain, WPC probabilities indicate that
at least light snow accumulations are likely by Saturday morning
from southern Nebraska and northern Kansas to southern Iowa and
northern Missouri, with mixed precipitation, including
accumulating ice, farther south.  Expect an increasing threat for
heavy snows to develop as the upper flow continues to amplify over
the Plains Saturday morning.

Pereira