Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Sat Nov 21 2020

Valid 12Z Sat Nov 21 2020 - 12Z Tue Nov 24 2020


...Pacific Northwest/Northern Rockies...
Shortwave ridging is expected to support dry conditions across the
region through Saturday before giving way to an upper trough and a
pair of frontal systems that will bring wet weather back into
western Washington and Oregon on Sunday.  A weakening system
moving inland is expected to bring mainly light snow to the
Olympics and Cascades on Sunday.  This will be quickly followed by
a second system moving inland Sunday night into Monday morning. 
While both system are expected to produce relatively light
amounts, combined totals may reach several inches across the
higher elevations of the Olympics and northern Cascades.

...Central Rockies...
Southwesterly flow ahead of an amplifying trough moving across the
Great Basin/Southwest will support the increasing potential for
light to moderate precipitation, including high elevation snow
across western to central Colorado and far northern New Mexico on
Monday.  Across the higher elevations of the San Juan Mountains,
amounts of a foot or more are possible by early Tuesday.

...Great Lakes/Northeast...
A warm front ahead of a deepening surface low lifting from the
Ohio Valley toward the lower Great Lakes, along with favorable
mid-to-upper level forcing will support precipitation moving
northeast across the region on Sunday.  Cold air in place ahead of
the system will support a wintry mix, with minor ice accumulations
and/or a few inches of snow expected from southern Lower Michigan
and northern Ohio to northern New England, before strong warm
advection produces a changeover to rain across most areas.

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

Pereira