Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
504 AM EDT Tue Oct 20 2020

Valid 12Z Tue Oct 20 2020 - 12Z Fri Oct 23 2020


...Northern Plains to Lake Superior...
Day 1...

A shortwave trough swings east across the northern Plains today
and Lake Superior tonight. There is little surface reflection from
this beyond an inverted trough. However, mid/upper level forcing
is strong enough to produce the current swath of precip spanning
SD that will spread east across central MN and northwest WI today
before reaching the UP of MI this evening. Given some
frontogenesis associated with the warm advection, there should be
pockets of heavier snowfall rates. But the warm air advection,
time of day (crossing the MSP metro in the midday/afternoon)
should limit the amount of snowfall along the southern and eastern
periphery of the area. Day 1 snow probs for six or more inches are
moderate in a swatch across south-central MN to north-central WI
as well as some low probs for the North Shore of Lake Superior in
MN.


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

The next wave rounding the deep low just north Hudson Bay
amplifies as it digs over the Pacific Northwest on Wednesday,
reaching the northern Great Basin Wednesday night before shifting
east over the northern Great Plains through Thursday night. This
promotes lee side surface cyclogenesis over Wyoming late tonight
which shifts south to CO Wednesday night before shifting east over
the north-central Plains Thursday, reaching the UP of MI Thursday
night. This provides yet another opportunity (and perhaps the
strongest yet this week) 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 until it
is cut off under the high Thursday/Thursday night. Day 2 snow
probabilities for 6 or more inches are moderate to high for higher
elevations of much of central MT. Probabilities for a foot or more
are moderate to high for the northern Absarokas and Crazy Mtns of
southern MT. 

A much stronger inverted surface trough and surface low than with
the wave crossing MN/WI today shifts east across SD/NE Wednesday
night and MN/IA Thursday...before crossing the UP of MI Thursday
night. There is now a 60% chance for 8 or more inches across
southern ND on the north side of this inverted trough in the Day
2.5 snow probabilities. This swath with a moderate risk for 8 or
more inches the spreads across northern MN in the Day 3
probabilities.


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

Jackson