Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 PM EDT Wed May 06 2020

Valid 00Z Thu May 07 2020 - 00Z Sun May 10 2020

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

A significant late-season winter storm may bring moderate to heavy
snow from Pennsylvania to Maine beginning late Friday.

A potent shortwave moving onshore the PacNW this afternoon will
dive quickly southeast into the mean longwave trough across the
east. By Friday afternoon it will likely be approaching the Ohio
Valley, while an anomalously deep 5H closed low pivots across
Ontario. As these features begin to interact, the parent trough
will begin to tilt negatively into Saturday, with the combination
of PVA, height falls, and a potent +3 standard deviation jet
streak combine to drive cyclogenesis from the Mid-Atlantic
northeast towards the coast of Maine. While initially this low
will deepen slowly Friday night, the likely capture of the lead
shortwave by a -3 standard deviation low (with respect to 5H
heights) combined with the upper ventilation and modest baroclinic
zone will drive rapid strengthening of this low as it moves slowly
near the coast of Maine Saturday. The 12Z ECMWF/NAM/GFS are all in
very good clustering of the surface low position at 72 hours,
leading to increasing confidence in a significant snow event,
especially for northern New England.

On Friday, as the shortwave pivots eastward, a band of enhanced
frontogenesis is likely from Ohio into Pennsylvania. While the
column is marginal for frozen precipitation across this area,
strong forcing should dynamically cool the column such that a
stripe of light accumulating snow is possible from Ohio into
western PA, with some higher accumulations (generally less than
2") possible in the higher terrain of central PA and WV. However,
beginning Friday night, WAA begins to increase into New York and
New England, while the surface low takes shape and deepens as it
lifts northeast. This will spread increasing coverage of
precipitation into the region. Again, the column initially will be
marginal for snowfall outside of terrain, but intense WAA could
produce a burst of snow capable of a few inches, and WPC
probabilities for 4 inches are 30% or less in the Catskills and
Adirondacks on D3. As the low intensifies and moves near eastern
MA into ME, moisture will begin to wrap back into New England on
modest theta-e ridging within a developing TROWAL, and
precipitation should become more widespread due to better
interaction of upper features. Additionally, an enhanced band of
deformation is likely to setup across Maine and into northern NH,
wrapping eastward as the low pulls away late in the forecast
period. While some light snow accumulations are possible on D3 in
the Berkshires, Worcester Hills, Southern Greens, and parts of
Southern NH, the heaviest snow is likely from the Northeast
Kingdom of VT into northern NH and much of central/northern ME.
Here, WPC probabilities are moderate to high for 4 inches, and in
the highest terrain some spots of 8" are possible.


The probability of significant icing is less than 10 percent.


Weiss