Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
140 PM EST Tue Nov 10 2020

Valid 00Z Wed Nov 11 2020 - 00Z Sat Nov 14 2020


...Upper Midwest...
As the associated upper level shortwave continues to lift
northeast -- assuming a negative-tilt as it moves from the central
Plains into the upper Mississippi valley -- ongoing precipitation
will move north across the Upper Midwest, with winter weather
impacts likely Tuesday evening into the overnight across much of
eastern Minnesota, northwestern Wisconsin and the western U.P. of
Michigan.  In addition to ample upper support, guidance shows good
low-to-mid level frontogenesis helping to support banded
precipitation lifting north across the region Tuesday evening into
the overnight.  WPC PWPF shows 50 percent or greater probabilities
for additional accumulations of 4-inches of snow or more during
the Day 1 period (00Z Wed - 00Z Thu), with 30 percent or greater
probabilities for 6-inches or more, across portions of
northeastern Minnesota and northwestern Wisconsin.  In addition to
snow, a wintry transition zone is likely to produce at least
measurable freezing rain accumulations over portions of northern
Wisconsin into the U.P.

...Western U.S....
A well-defined shortwave currently dropping southeast into the
Pacific Northwest is expected to help produce locally heavy
mountain snows as it moves quickly from the Cascades to the
northern Rockies Tuesday night into Wednesday.  Areas impacted
will likely include the Oregon Cascades, the mountains of
northeastern Oregon, as well as parts of the central and eastern
Idaho, western Montana and western Wyoming ranges.  WPC PWPF
suggests widespread coverage of snow amounts of 4-inches or more,
with locally heavy amounts of 8-inches or more across these areas.
 As this system moves east into the Plains, shortwave ridging will
support dry conditions across much of the Northwest into the
northern Rockies late Wednesday through Thursday.  By late
Thursday however, wet weather is expected to return to the
Northwest with the approach of another shortwave trough.  Models
show this system moving quickly east, however any break in
precipitation will likely be brief as the next system, positioned
on the leading edge of strong Pacific jet, approaches the
Northwest late Friday.  This combination of systems is likely to
bring heavy amounts back into the Olympics and Cascades, with
accumulations of a foot or more possible during the Day 3 period
(00Z Fri - 00Z Sat) across the higher terrain.

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

Pereira