Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Sun Nov 01 2020

Valid 00Z Mon Nov 02 2020 - 00Z Thu Nov 05 2020

...Great Lakes/Central Appalachians/New York...
Days 1/2...

A potent upper trough takes on a negative tilt over the eastern
Great Lakes this evening with rapid surface cyclogenesis in the
Gulf of Maine tonight that lifts north into the Gulf of St.
Lawrence Monday. Strong cold advection over rather warm Lake Erie
and Ontario waters (generally mid 50s to 60F lake temps) will
produce a period of rather favorable conditions for multi-band
Lake Effect Snow (LES) off these two lakes through tonight. Day 1
snow probabilities for 8 or more inches are in northwest PA and in
the Tug Hill area of NY.

As the upper trough approaches the central Appalachians this
evening, the low-mid level convergence and deformation combines
with upslope flow to produce additional periods of snow in the
favored terrain of SW PA and along the Appalachians into WV/MD.
Some low Day 1 probabilities for 4 or more inches are present in
the highest Allegheny Highlands.

A secondary but potent trailing shortwave is progged track
southeast from north of the Great Lakes Monday night, crossing New
England early Tuesday. An enhanced area of snow is likely east of
Lake Ontario where fetch/LES enhancement is likely, so several
inches is expected in the Tug Hill area, with moderate Day 1.5
snow probabilities for a foot in the highest part of the Tug Hill,
but given mesoscale processes a foot is likely where the largest
band sets up Monday night.

Moist northwest flow with 300 mb divergence maxima and 700 mb
vertical velocity maxima favors accumulating snow showers across
the western to northern Adirondacks into the northern Green
Mountains where there are low to moderate Day 2 snow probabilities
for 6 or more inches in the highest Adirondacks and Green Mtns.

The event winds down on Tuesday as low level ridging crosses the
lakes and drier air aloft advects into New York/New England.

...Northern Maine...
Day 1...

The upper trough moving east from the the Great Lakes Sunday will
induce cyclogenesis over eastern New England with the low moving
north across eastern Maine and into Canada Monday. In addition to
this low, strong diffluence within a robust jet streak and rapid
height falls will likely lead to a broad precipitation shield,
with some enhancement and prolonging of precipitation likely in a
mid level deformation zone. However, guidance has shifted farther
north with the precipitation shield, likely ending precip Monday
afternoon over northern Maine where before Monday afternoon was
looking like a prime time to have additional wrap around snow.

Therefore Day 1 snow probabilities are now low for 4 or more
inches over the northern White Mtns and across northern Maine.


The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.


Jackson