Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EST Tue Jan 05 2021

Valid 00Z Wed Jan 06 2021 - 00Z Sat Jan 09 2021


Day 1...
...Northern Plains...
An upper trough centered east of the Rockies this afternoon will
continue to move east across the Plains through the evening and
overnight.  Models show light to moderate precipitation developing
near a low-to-mid level center closing off and moving east across
South Dakota.  While many areas may begin as a wintry mix, an area
of enhanced vertical motion centered along the central to eastern
North Dakota/South Dakota border, may support a small area of
mostly snow at the onset this evening-- enhancing snowfall
accumulations.  Elsewhere, expect most areas to change over to
snow overnight, producing a stripe of mainly light snow
accumulations from southeastern South Dakota to eastern Nebraska
and western Iowa.  While most areas will likely only see amounts
up to an inch or two, cannot rule out the potential for locally
heavier accumulations.

...Pacific Northwest to the Northern Rockies...
Precipitation will continue to spread south and east this evening
into the overnight ahead of an approaching upper trough and its
associated frontal band.  Strong southwesterly flow and favorable
upper jet forcing is expected to help support organized heavy
precipitation across western Washington, which includes mountain
snow.  Heavy snow accumulations are likely across the Olympics and
Washington Cascades, with significant accumulations extending down
to the pass levels.  Relatively lighter, but significant
accumulations are expected farther east into the mountains of
northeastern Washington and the Idaho Panhandle.

Days 2-3...
...Ozark Region to the Southern Appalachians...
Models show a southern stream low closing off and moving east from
the ArkLaTex region Wednesday night.  As the low moves across
southern Arkansas/northern Louisiana, rain changing to snow, with
a few inches of accumulating snow can be expected across the
higher elevations of southern Missouri and northwestern Arkansas
during the overnight hours. 

Then as the low continues east into southeastern U.S.,
accumulating snow is likely to develop over the southern
Appalachians by late Thursday.  While model spread is limiting
confidence in the details, there is the potential for locally
heavy accumulations, especially along portions of the southern
Blue Ridge.

...Pacific Northwest/Northern California...
A strong low is expected to close off over the eastern Pacific and
move toward Vancouver Island late Thursday into Friday.  A
well-defined mid level shortwave/upper jet to its south is
expected to help focus the next round of organized heavier
precipitation, including some locally heavy snow from the southern
Cascades to the Klamath Mountains and northern Sierra.

Pereira