Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 AM EST Fri Nov 23 2018

Valid 12Z Fri Nov 23 2018 - 12Z Mon Nov 26 2018

...Western U.S....
Day 1...
A potent mid-level shortwave with a subtropical moisture
connection is poised to move into Northern California later this
morning, delivering another round of heavy precipitation,
including mountain snows to portions of Northern California, the
Pacific Northwest and northern Rockies.  Snow levels are expected
to momentarily rise before a mid-upper level low dropping south
from Vancouver Island begins to draw colder air across the region
later today.  This low will continue to drop southeast across the
northern Intermountain West and Rockies Friday night.  Strong
upper forcing along the leading edge of the upper jet will support
heavy snow accumulations along the western Wyoming into the
northern Utah ranges Friday night into Saturday, with storm-total
accumulations likely to exceed a foot across a wide area.  Snows
will spread east into the Colorado and eastern Wyoming ranges,
with the probabilities for heavy amounts increasing across the
region on Saturday.  Snows are expected to diminish across the
Rockies as the trough swings further east into the Plains Saturday
night.

...Central Plains to the Great Lakes...
Days 2-3...
As the trough in the West digs southeast across the central
Rockies, light to moderate snows are expected to spill east out
across eastern Wyoming into South Dakota and Nebraska late
Saturday into early Sunday.  This trough is expected to continue
amplifying as it moves across the central Plains on Sunday, with
several models showing a mid level low closing off as it reaches
the mid Mississippi valley late in the day.  Strong low to mid
level frontogenesis north of the associated surface low, along
with left-exit region upper jet forcing is expected to support a
stripe of moderate to heavy snows shifting northeast from portions
of eastern Nebraska and Kansas Sunday morning to lower Michigan by
Monday morning.  Forecast certainty was limited by the spread in
the models solutions, which became significant during the Sunday
night to Monday morning time period.  With limited confidence, WPC
preferred a solution slower than the GFS, (which was faster in
comparison to much of the remaining guidance), and closer to the
ECMWF.

...Southern and Central Appalachians into the Piedmont...
Day 1...
Isentropic lift along with mid-upper level dynamics associated
with an upper trough moving across the Tennessee valley will
support widespread precipitation across the region Saturday
morning.  With a cold air wedge remaining in place, precipitation
is expected to begin as a wintry mix with some light icing and
snow accumulations expected before strong southerly winds erode
the level cold air.  While the probabilities for ice accumulations
of 0.25 inch or more remain near zero, WPC probabilities support
increasing confidence that it will be an icy morning for several
areas, with WPC probabilities 50 percent or greater for measurable
ice from western North Carolina to southwestern Pennsylvania.

Pereira