Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Thu Oct 22 2020

Valid 12Z Thu Oct 22 2020 - 12Z Sun Oct 25 2020


...Northern Plains and Upper Midwest...
Day 1...
Ongoing snows will continue to shift east across the Dakotas and
Minnesota as an upper level shortwave digs south across the
northern Rockies.  Mid level frontogenesis in combination with
right-entrance region upper jet forcing will help to support a
stripe of heavier snows from southeastern North Dakota and
northeastern South Dakota to central Minnesota on Thursday. 
Heaviest accumulations are expected to center from the eastern
North and South Dakota border to the Arrowhead of Minnesota --
where WPC PWPF shows 50 percent or greater probabilities for
additional accumulations of 4-inches or more during the Day 1
period.

...Pacific Northwest/Northern Rockies and Plains...
Days 2-3...
Another well-defined upper level shortwave is expected to drop
south through western Canada into the region, with a surface to
mid level cyclone dropping southeast across Washington on Friday. 
Consensus of the overnight guidance has shown a slight northerly
shift in the track of the low -- resulting in higher snow levels
and overall lighter accumulations across the northern Cascades. 
Snow levels have come up some across eastern Washington as well,
but there remains a good signal for significant accumulations
across the higher elevations of northeastern Washington, with
heavy accumulations likely across portions of the northern
Rockies.  With ample cold air in place, moist westerly inflow
along with favorable upper jet forcing will help support the
development of heavy snows across the mountains of northern Idaho
into western Montana by late Friday.  Then by Friday night,
increasing low to mid level easterly winds and frontogenesis will
support developing snows east of the Rockies over the High Plains.
  As the center moves east, snows spread along with the trailing
front dropping south through the western Wyoming ranges on
Saturday.   Snowfall accumulations of a foot or more are likely
across large portions of the northern Idaho, western and central
Montana, and western Wyoming ranges.  Snows will continue to
spread east across Montana into the western Dakotas as a mid level
circulation develops and moves east across southeastern Montana on
Saturday.  By Saturday night, snows will once again begin to
spread east from the Dakotas into Minnesota.

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

Pereira