Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Tue Mar 30 2021

Valid 12Z Tue Mar 30 2021 - 12Z Fri Apr 02 2021

...Northeast...
Days 2-3...

A shortwave moving across the Great Lakes Wednesday will sharpen
as reinforcing vorticity energy rotates through its base. This
will drive a negative tilt to the trough as it advects eastward,
with the 500mb trough likely to close over Ontario causing
features to move only slowly eastward through the end of the
forecast period. Strong height falls accompanied by robust
divergence, PVA, and intensifying diffluence within the RRQ of a
poleward extending jet streak will drive surface cyclogenesis
Wednesday night. This low is likely to develop over the
Mid-Atlantic and then move northeast across Southern New England
and into Maine by Thursday night. The trend this morning has been
for a surface low that is slightly further SE, meaning there is
likely to be more precip falling into the cold air, and snow
accumulations and increased.

As the low lifts northeast, it will be accompanied by moist and
warm advection from the Gulf of Mexico with widespread and ample
precipitation spreading northward. Initially, the column will be
too warm for any wintry precipitation. However, a cold front
sinking southeastward will lead to CAA and full-column cooling,
aided by what is likely to be an intense deformation axis on the
NW side of the low, into which robust WAA will drive strong omega
through the DGZ. This suggests that a period of heavy snow is
likely as precip changes over from rain to snow, and model-cross
sections indicate a chance for CSI banding across upstate New York
Thursday morning. While the heaviest snow is likely in the terrain
where temperatures will be colder and cool more quickly, WPC
probabilities for moderate to heavy snow have expanded slightly to
encompass some lower terrain as well where the dynamic cooling
will overcome the marginal low-level thermal structure. WPC
probabilities have increased, and are now high for 6 inches on
days 2.5-3 in the Finger Lakes region, the Tug Hill Plateau, and
Adirondacks of New York. In the Adirondacks, locally more than 10"
is likely where some upslope enhancement will occur as NW flow
develops behind the low. WPC probabilities for 4 inches are
moderate for much of upstate New York outside of the Hudson River
Valley, and parts of VT.

Lighter snows adding up to a few inches are likely as the low
pulls away in the favored upslope regions of WV, and downwind of
Lake Erie and Ontario.

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

Weiss