Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Thu Apr 09 2020

Valid 00Z Fri Apr 10 2020 - 00Z Mon Apr 13 2020


...Northeast...
Day 1...

Strong nor'easter deepening along the coast of Maine tonight will
lift away quickly after 12Z, followed by wraparound light and
upslope snow. The heaviest snow for this period will be across
parts of northern and central Maine where a strong deformation
band will pivot, accompanied by possible thunder-snow and hourly
rates of more than 2"/hr. WPC probabilities for 8 inches are high
across much of northern and central Maine, and isolated amounts to
15" are possible. A few inches of snow is also likely in the
favored upslope regions of the Northern Adirondacks, Greens, and
Whites where WPC probabilities for 4 inches remain high behind the
departing system.


...Southern California...
Day 1...

Upper low which has been plaguing Southern California the past
several days will pivot cyclonically one last time tonight before
ejecting slowly to the east late Friday. Continued moist advection
south of this low combined with a final shortwave impulse and
associated ascent will drive heavy snow in the transverse ranges
of Southern California, mostly during the first half of D1. WPC
probabilities are high for 6 inches, with the highest amounts in
the San Bernadinos. Late D1 and beyond, the forcing and moisture
get shunted to the east, finally bringing an end to the snowfall.


...Northern Rockies through the Central Plains...
Days 2-3...

A strong cold front will dig into Montana and then dive through
the Northern and Central Plains through Sunday beneath a shortwave
pivoting across Southern Canada. As this drops southward, a potent
vorticity impulse and jet streak will approach from the NW, with
the favorable synoptic ascent interacting with the baroclinic
gradient below. A wave of low pressure is likely to develop and
move SE along this boundary, further providing lift and enhancing
the upslope component into the terrain on NE winds. Guidance still
differs on the axis position of the heaviest snowfall, but a swath
of heavy snow is likely from the Northern Rockies near Glacier NP
through the terrain of WY and into northern CO. In these ranges,
WPC probabilities are high for 8 inches. In the foothills of the
terrain and into the Plains, the column will initially be marginal
for snow, but as the cold front continues southward, cooling will
occur and precip will changeover to snow. A stripe of heavy snow
is likely in this area, and WPC probabilities for 4 inches are
high for SE Montana through the High Plains of WY and into SD/NE
on D3.

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

Weiss