Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
228 AM EST Fri Nov 19 2021

Valid 12Z Fri Nov 19 2021 - 12Z Mon Nov 22 2021


...Pacific Northwest to the Northern Rockies...
Day 1...
Precipitation will continue to spread east from the Pacific
Northwest into the northern Rockies as a shortwave trough and its
associated surface wave move inland this morning.  As the wave
moves east, snow levels will drop through the day into the
overnight -- dipping below 4000 ft across the Idaho Panhandle and
northwestern Montana, and below 5000 ft across southern Idaho and
northwestern Wyoming by tonight.  WPC probabilities continue to
show the potential for locally heavy snow accumulations for
portions of the northern Rockies through Day 1 -- ending 12Z
Saturday.  These areas include the Bitterroot Mountains, and the
Swan and Teton ranges.  Probabilities indicate that accumulations
of 4 inches or more likely for portions of these regions.  For
parts of the northern Teton and southwestern Yellowstone region,
WPC probabilities indicate that localized amounts of 8 inches or
more can be expected.  Drier weather is forecast to spread from
west to east on Saturday and persist through the remainder of the
weekend as a strong ridge shifts east across the western U.S.

...Great Lakes...
Day 3...
An amplifying shortwave will dive southeast from Canada across the
Upper Midwest and Great Lakes Sunday night-Morning.  Strong
surface low pressure will develop quickly and move east of the
upper Great Lakes early Monday.  Strong cold air advection and
northwesterly flow on the back of the system will support
redeveloping lake effect snow showers, with the potential for
locally heavy accumulation developing across the Upper Peninsula
into northern Lower Michigan by early Monday.  

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

Pereira