Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Fri Oct 25 2019

Valid 12Z Fri Oct 25 2019 - 12Z Mon Oct 28 2019


Days 1-3...

...Rockies...
Models continue to show upper level ridging in the West giving way
to an amplifying trough that is forecast to bring much colder air
and a chance for heavy snow that will move progressively south
from the northern to the central Rockies through the weekend. 

An upper level shortwave moving across the Alaska Panhandle into
western Canada Friday morning is expected to dive southeast into
the Pacific Northwest Friday - shifting the ridge in the West into
the eastern Pacific while drawing much colder air into the
northwestern U.S. by early Saturday.  Snow will begin to develop
Friday night into Saturday in the post-frontal upslope flow across
western Montana.  Through early Saturday, expect generally light
amounts across the ranges of northwestern Montana, followed by
heavier amounts later in the day farther south across southwestern
Montana and northwestern Wyoming.  Increasing upper divergence
along the left-exit region of the upper jet in addition to low to
mid level frontogenesis is expected to help support locally heavy
mountain snows across Yellowstone Country and Yellowstone NP, as
well as the Bighorn Mountains. 

By Saturday night and continuing into Sunday morning, the heavy
snow threat is expected to shift farther southeast into
southeastern Wyoming, with locally heavy accumulations expected
across the Shirley and Laramie mountains.  As temperatures fall,
low to mid level frontogenesis will help to extend the potential
for significant snowfall accumulations out into the High Plains
Saturday night, with several inches possible across portions of
southeastern Wyoming into the western Nebraska Panhandle and far
southwestern South Dakota.

As the upper level trough continues to amplify and cold air
plunges south, snows will move south as well, with heavy snows
moving into the central Colorado ranges Saturday into early Sunday.

For Days 1-3, the probability of significant icing is less than 10
percent.

Pereira