Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
513 PM EDT Mon Oct 19 2020

Valid 00Z Tue Oct 20 2020 - 00Z Fri Oct 23 2020


...Northern Rockies/High Plains to the Upper Midwest...
Day 1...

A shortwave trough moving southeast across British Columbia and
Alberta today will continue dropping southeastward into the
northern U.S. Rockies and adjacent High Plains.  Southerly flow
over cold air in place to the north of a stalled frontal boundary
is forecast to support snow across the higher elevations of the
northern Rockies which eventually spreads out into the adjacent
High Plains with some freezing rain for valleys and much of
central MT through this evening. As low pressure at the surface
and an inverted surface trough extending from the low spreads
eastward, generally light snow will spread from the eastern
Dakotas into northern and central Minnesota and northwest
Wisconsin.  Given some frontogenesis associated with the warm
advection, there could be some pockets of briefly heavier snowfall
rates.  But the overall warm conditions and the time of day may
limit the amount of snowfall along the southern and eastern
periphery of the area.

Day 2...
The system should continue lifting northeast on Tuesday
evening...with any lingering snowfall tapering off from west to
east as the low moves away.


...Northern Rockies to northern Great Plains...
Days 2/3...

The next wave rounding the deep low over Hudson Bay amplifies as
it digs over the Pacific Northwest on Wednesday, reaching the
northern Great Basin Wednesday night. This promotes less side
surface cyclogenesis over Wyoming on Thursday which shifts south
to CO Wednesday night. This provides yet another opportunity (and
perhaps the strongest yet) for snow in the northern Rockies as
Gulf of Mexico sourced moisture lifts up the Plains and wraps
around the developing lee-side cyclone. Furthermore, a ridge from
a 1040mb high shifts south into northwestern MT Wednesday night,
further promoting low level convergence and upslope flow. Day 3
snow probabilities for 6 or more inches are moderate to high for
much of central MT. Probabilities for a foot or more are moderate
to high for the northern Absarokas and Crazy Mtns of southern MT. 

An inverted surface trough under a developing southwesterly jet
spreads this focus east over the Dakotas Wednesday night into
Thursday...reaching as far east as northern Minnesota during the
day on Thursday.  There is growing confidence that there will be a
stripe of 8+ inches embedded within a broader area with 4 to 6
inch snowfall amounts in the Dakotas...with the probability of 4
or more inches spreading into northwest Minnesota by the end of
the end of Day 3.

For Days 2/3, the probability of significant icing is less than 10
percent.

Bann