Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 PM EST Thu Nov 05 2020

Valid 00Z Fri Nov 06 2020 - 00Z Mon Nov 09 2020


...Western U.S....
A strong winter storm is expected to impact portions of the
northern Rockies and High Plains this weekend, with heavy snows
and strong winds likely to develop across north-central to
northeastern Montana Saturday night, continuing through Sunday.

A closed low now developing off of the Northwest coast will
continue to deepen as it digs farther south along the coast into
Northern California late Friday and Saturday.  While onshore flow
ahead of the system will support widespread precipitation from
western Washington into Northern California early in the period,
initially high snow levels will keep any heavy snow accumulations
confined largely to the high peaks of the central and southern
Cascades.  Meanwhile, southerly flow into a lingering low-to-mid
level front will support precipitation east of the Cascades
through northeastern Oregon into northern Idaho and western
Montana.  Here also, high snow levels will keep the heavy snow
threat confined largely to the higher peaks of the northwestern
Montana ranges.

Models show the low continuing to drop south through California on
Saturday, transitioning to an open wave as it pivots across
Southern California into the Southwest late in the day.  As the
flow becomes more alongshore, the threat for heavier precipitation
is expected to wane, with relatively light amounts expected across
California into Nevada, including generally light snow
accumulations across the Sierra and the Nevada mountains Friday
night into Saturday.  The Transverse and coastal ranges of
Southern California may be an exception, where onshore flow is
expected to bolster amounts, with heavy snow accumulations
possible over the higher peaks.  As the system begins to pivot
east, directing deeper moisture across the Southwest into the
Great Basin and Rockies, snows will likely begin to develop
Saturday across the northern Arizona high terrain and the Utah
ranges.  High snow levels at the onset are expected to keep
accumulations across these areas in check through late Saturday.  
However, heavy snows are likely to develop early Saturday across
San Juan mountains in southwestern Colorado.  With snows likely to
continue through the remainder of the weekend, accumulations of
two feet or more are likely across portions of San Juan mountains
before the end of the weekend.

As a second amplifying shortwave digs south into the Northwest
Saturday, the system over the Southwest is forecast to lift
northeast across the central Rockies into the northern High Plains
Saturday night.  By early Sunday, models show a powerful surface
cyclone forming over the northern High Plains, with wind-driven
heavy snows developing on the backside of the system.  Powerful,
easterly to northeasterly low level inflow, along with an upper
jet couplet are expected to support heavy snows spreading east
from mountains of northwestern Montana Saturday night and across
north-central into northeastern Montana on Sunday.  WPC Day 3 PWPF
(00Z Sunday - 00Z Monday) shows 50 percent or greater
probabilities for accumulations of 8-inches or more covering a
large portion of north-central Montana.    

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

Pereira