Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EST Sun Nov 22 2020

Valid 12Z Sun Nov 22 2020 - 12Z Wed Nov 25 2020


...Great Lakes/Northeast...
Day 1...
Low-to-mid level frontogenesis associated with an amplifying
upstream trough will continue to support precipitation spreading
north from the Ohio Valley into the southern Great Lakes region
this morning.  While precipitation is expected to begin as rain,
the frontogenetical forcing is expected to enhance precipitation
rates and dynamical cooling -- supporting a changeover to snow
across portions of northern Indiana, northwestern Ohio, and
southeastern Lower Michigan.  Several HREF members show the
potential for light to moderate snows shifting northeast across
the region through the morning hours, producing a stripe of at
least an inch or two centered across the region.  Meanwhile, low
pressure will continue to organize and track northeast through the
Ohio Valley on Sunday with precipitation spreading northeast along
the preceding warm front.  Retreating cold, dry air will support a
wintry mix at the onset across portions of northern Pennsylvania,
Upstate New York and northern New England before changing over to
rain.  Some light snow accumulations are likely, along with some
minor ice accumulations.

Day 3...
A low-to-mid level warm front associated with an approaching upper
trough will help generate widespread precipitation, with cold air
supporting snow, at least on the onset, across the upper
Mississippi Valley into the upper Great Lakes late Tuesday into
early Wednesday.  While model spread affords limited confidence in
the details, accumulating snows appear likely from eastern
Minnesota to the U.P. of Michigan, with a few inches most likely
across portions of northern Wisconsin into the U.P.

...Central Rockies...
Deep southwesterly flow ahead of an approaching upper trough will
support snow developing across the mountains of western and
central Colorado by late Sunday and continuing through Monday
before diminishing as the trough moves east of the region on
Tuesday.  Areas impacted will likely include the Elk and San Juan
Mountains and Sawatch Range.  Heaviest totals are expected to
center over the San Juan Mountains, where WPC PWPF indicates
widespread potential for storm totals of a foot or more.

...Pacific Northwest/Northern Rockies...
An upper trough and a pair of weak frontal systems are expected to
produce periods of mainly light snow from the Olympics and
Cascades to the northern Rockies Sunday and Monday.  A better
defined frontal system is expected to bring heavier precipitation
and a threat for heavier snow accumulations into the Olympics and
northern Cascades on Tuesday. 

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

Pereira