Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 PM EST Thu Nov 21 2019

Valid 00Z Fri Nov 22 2019 - 00Z Mon Nov 25 2019

...Southern Rockies into the High Plains...
Day 1...

Deep upper low spinning over the Four-Corners will fill and begin
to eject northeast tonight. Ahead of this feature, broad mid-level
divergence and increasing LFQ diffluence combined with 700mb moist
flow will induce ascent, with the best forcing slowly spreading
northeastward into KS/MO Friday. This will produce heavy snowfall
continuing in the mountains, including the Colorado Rockies, the
San Juans, and the Sangre De Cristos, where WPC probabilities are
high for more than 6 inches of additional snowfall. Further east
in the High Plains of CO and into Kansas, isentropic overrunning
combined with LFQ jet diffluence will induce a swath of snowfall.
Much of this snow should be light, and WPC probabilities are less
than 20% for 4 inches, but up to 50% for 2 inches.

..Ohio Valley through the Northeast...
Day 3...

Uncertainty abounds in the Saturday/Sunday timeframe regarding a
wave of low pressure moving out of the TN Valley an then
redeveloping near the coast of New England. For the ongoing
forecast trended towards the GEFS and ECENS mean for p-type and
low position. As the low lifts out of the Southeast, overrunning
could produce some light freezing rain and snow in the Ohio Valley
Saturday night, but WPC probabilities are low for more than 1" of
snow and a few hundredths of ice accretion.

As the low redevelops off the coast in response to northern stream
and southern stream shortwave interaction aloft, more significant
moisture may spread northward into New England and upstate New
York. Ascent will become enhanced through height falls and jet
level diffluence, and intensifying deformation NW of the surface
low. Temperatures at the surface will initially be cold so that
WAA aloft will lead to freezing rain everywhere but the terrain
and northern New England where snow is more likely. As the low
pulls away at the end of D3 /Sunday/ colder air should crash back
to the south changing precipitation back to snow. While
uncertainty is greater than normal, WPC probabilities currently
indicate a moderate risk for 0.1" of freezing rain, and low
probabilities for 4" of snow, confined to the higher terrain.

Weiss