Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Wed Mar 31 2021

Valid 12Z Wed Mar 31 2021 - 12Z Sat Apr 03 2021

...Northeast...
Days 1-2...

...Late season significant winter storm likely for parts of
Upstate New York and Northern New England...

A shortwave moving across the Great Lakes will amplify in response
to a vorticity lobe swinging through its base atop the Central
Appalachians Thursday morning. This will cause the trough to take
on a negative tilt and close off, while at the same time a
poleward extending jet streak intensifies leaving a strengthening
divergence maxima over the region. The subsequent combination of
height falls, mid-level divergence, and upper ventilation will
drive surface cyclogenesis across the Mid-Atlantic, and this
surface low will strengthen as it lifts into Maine and then Canada
by Friday.

An impressive moisture plume noted by PW anomalies of +2 standard
deviations surging northward from the Gulf of Mexico will be wrung
out by the robust deep layer ascent across the Mid-Atlantic and
Northeast. Initially, this will be all rain for the region.
However, a cold front dropping southeast out of Canada will begin
to cool the column, while persisting SW flow aloft will maintain
moisture lifting atop the front. This column cooling will then
become enhanced by what is likely to be an intense deformation
axis overlapping strengthening fgen through the ageostrophic
response of the upper jet streak and the sharpening low-level
baroclinic gradient. This is favorable for a strong band of
snowfall, and both the HREF snow rate probabilities and the WPC
snowband prototype page indicate the potential for 1"/hr snowfall
as the column cools both through both dynamic and advective
processes. With the exception of the GFS which has become a
progressive outlier with its 500-700mb trough axis, the guidance
has come into better agreement tonight in depicting this
impressive band of snowfall developing late tonight across PA/NY
and shifting northeast into Thursday. Despite the hostile
antecedent conditions due to warmth and rain, these snow rates
should quickly begin to accumulate, first in the terrain and later
into the lower elevations, and as such the heaviest snowfall is
likely in the Adirondacks, Tug Hill Plateau, and northern
Catskills where WPC probabilities are above 50% for 6 inches.
Lighter accumulations are expected from the Laurel Highlands
northeast through much of Upstate New York except the Hudson River
Valley, and into much of northern and central Vermont.

As the low pulls away Thursday night and Friday morning, NW flow
should produce some upslope snow showers as well as periods of LES
downwind of Lake Erie and Ontario, with light additional snowfall
accumulations likely.

Additionally, there is likely to be a period of freezing rain
across parts of northern NH and ME where rain transitions before
turning over to snow. Heavy rates, warm antecedent conditions, and
freezing rain occurring during the April afternoon hours should
limit accretions. However, WPC probabilities are as high as 40%
for 0.1" of accretion across northern ME.


Weiss