Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri Oct 23 2020

Valid 12Z Fri Oct 23 2020 - 12Z Mon Oct 26 2020


...Pacific Northwest to the Northern and Central Plains...

As a well-defined system drops south from western Canada into the
western U.S., widespread snows, with moderate to heavy
accumulations, are expected from the Pacific Northwest to the
north-central U.S. over the coming days.

Heavy snows will initially develop from the northern Cascades to
the northern Rockies as an amplifying upper level shortwave drops
south through western Canada into the northwestern U.S. on Friday.
 In addition to the higher elevations of the northern Cascades,
areas impacted by the heaviest snow accumulations on Friday into
early Saturday will likely include the northern Idaho and western
Montana ranges.  As much colder air begins to spill south into the
region, a low level circulation spinning south, along with
favorable upper jet forcing, are expected to support heavier
accumulations across this area.  As the system continues drop
south, increasing low level easterly flow will help snows develop
farther east as well across central Montana into the High Plains
Friday night into Saturday. 

By early Saturday, a low-to-mid level low developing over
southeastern Montana/northeastern Wyoming will help spread snows
farther east into the Dakotas.  Meanwhile, continued favorable
upper level forcing and low level baroclinicity upstream will
support moderate to heavy snows spreading south across the western
and central Wyoming mountains.  WPC PWPF indicates storm totals
accumulations of a foot or more are likely across a large section
of the northern Idaho, western and central Montana, and western
Wyoming mountains.

By early Sunday, models show a split in the upper level pattern
beginning to develop over the western U.S., with a deep
positively-tilted upper level trough centered over the central
Rockies and Great Basin Sunday into early Monday.  Lift supported
by the upper trough, along with low level circulation and
frontogenesis dropping south across Colorado will encourage heavy
snows developing and moving south across the Colorado ranges, with
significant snows also possible farther east across the High
Plains into western Nebraska.  WPC PWPF indicates widespread
accumulations of 8-inches or more are likely across the central
Colorado mountains, including the Front Range, by early Monday.  

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

Pereira