Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Fri Apr 24 2020

Valid 12Z Fri Apr 24 2020 - 12Z Mon Apr 27 2020


...North-Central Rockies...
Day 1...

A shortwave trough digging southeast along the leading edge of a
110kt Pacific jet streak will dive into the Central Plains this
evening bringing an end to the regional forcing for ascent. The
strongest ascent through this afternoon will be associated with
the LFQ of the upper jet and collocated height falls, which are
expected to be focused in the CO Rockies. Moisture across the
region is modest, but the forcing should wring out precipitation
efficiently, and this will be snow above generally 7000 ft. High
probabilities for 4 inches are confined to a small portion of the
CO Rockies, with lower probabilities extending from the
southernmost Absarokas of MT, to as far SE as the Front Range in
CO.


...Northeast...
Day 3...

A shortwave trough swinging across the Ohio Valley Saturday will
close off early on Sunday before lifting towards New England late
on D3. A surface low beneath this feature will occlude out to
secondary low development near the Virginia Tidewater, and this
low is expected to then lift rapidly to the E/NE, with this
double-barreled structure forecast to move south of New England
towards Monday morning. Outside of the GFS/GEFS which are
displaced NW and warm, the remaining guidance is pretty well
clustered around this solution. As some modest coupled jetting
develops, the primary surface low should intensify, which when
combined with Canadian High pressure over Ontario will allow cold
air to funnel southward into New England. While the column may
initially be marginal for anything but rain, as the latter half of
D3 develops, increased forcing and column cooling is likely to
cause a p-type changeover from rain to snow. First this will occur
in the terrain, and by the end of the forecast period, snow is
possible even down into the valleys if forcing is strong enough to
cause dynamic cooling of the lowest levels. The heaviest snow is
likely in the higher elevations, generally above 2000 ft, in the
southern Adirondacks, Greens, and Whites, and WPC probabilities in
these areas suggest a low chance for 6 inches. Moderate WPC
probabilities for 2 inches extend into most of the other terrain
including the Catskills and Berkshires. The evolution of this
system will need to be monitored as late season snow is possible
even into the lower elevations of New England, upstate New York,
and into parts of northern PA.


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

Weiss