Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Tue Oct 19 2021

Valid 12Z Tue Oct 19 2021 - 12Z Fri Oct 22 2021

...Wyoming and western South Dakota...
Day 1...
An upper low will continue to move east of the Great Basin into
the north-central Rockies this morning -- moving along the
Wyoming-Colorado border into the High Plains by late in the day. 
Left-exit region upper jet forcing will help support widespread
precipitation, including areas of heavy snow across portions of
central into eastern Wyoming.  In addition to strong upper
forcing, easterly to northeasterly flow on the backside of a low
level circulation moving across southeastern Wyoming into western
Nebraska is expected to help enhance precipitation rates across
the region.  Areas impacted are expected to include the southern
Absaroka and Wind River ranges, the southern Bighorn Mountains,
the Rattlesnake Hills, and the Shirley and Laramie mountains. 
Within these areas are probabilities that are 50 percent or
greater for accumulations of 8 inches or more. 

Models show the upper low beginning to lift east-northeast across
western Nebraska and South Dakota this evening and overnight. 
Snow is forecast to spread across northeastern Wyoming into the
Black Hills, where locally heavy totals are expected.  WPC
probabilities for accumulations of 4 inches or more are above 50
percent for portions of the Black Hills.  A transition from rain
to snow is expected overnight for areas east of the Black Hills,
with at least some light accumulations expected across portions of
west-central South Dakota.  Probabilities for at least 2 inches of
snow have increased east of the Black Hills, as good low-to-mid
level frontogenesis in combination with favorable upper jet
forcing may support some banded precipitation and the potential
for locally heavy precipitation rates overnight.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Pereira