Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EDT Sat Oct 31 2020

Valid 12Z Sat Oct 31 2020 - 12Z Tue Nov 03 2020

...Great Lakes/Central Appalachians...
Days 1-3...

A shortwave and accompanying upper jet streak will drive a strong
cold front across the Lakes Saturday night through Sunday from
west to east. Behind this front, strong cold advection over still
warm lake waters will produce a period of favorable conditions for
Lake Effect Snow (LES). Favored belts south of Lake Superior into
the U.P. of MI will develop first on Saturday night, before
spreading into the L.P., and then downwind of Lakes Erie and
Ontario Sunday.

On Sunday, periods of moderate to heavy snow are likely across the
U.P. and northwest lower MI where N/NW winds will produce lake
effect snow bands. The heaviest snow is forecast to shift into the
eastern U.P. late as a modest deformation axis serves as a region
of secondary ascent. Later Sunday, snow will develop in northwest
lower MI as an effective fetch from Lake Superior moves across the
northern part of Lake Michigan to increase cross-lake fetch and
moisture fluxes, with ascent aided by upslope flow inland from
Lake MI in northwest lower MI.

On Sunday, LES will spread downwind of Lakes Erie and Ontario,
with a multiband setup likely. Very warm lake waters and 850mb
temps crashing below freezing to as low as -9C over Lake Erie will
produce significant instability as high as 1000 J/kg and lake
equilibrium levels above 700mb. Modest shear across these lakes on
NW winds will support multi-bands of snow showers.

As the mid level trough approaches the central Appalachians, the
low-mid level convergence and deformation combines with upslope
flow to produce showers, with rain quickly changing to snow. The
high amplitude trough of the NAM produces higher QPF, with better
clustering of the GFS FV3/03z SREF mean, and Canadian regional
GEM, which show potential for 4-8 inches of snow in the higher
elevations of central to northern WV. Snow should taper on Monday
as the upper trough departs and drier air aloft advects in from
the Ohio Valley.

On Sunday night into Monday, the upper trough continues east
across New York.  The GFS shows trajectories crossing Lake Ontario
into the Tug Hill and western slopes of the Adirondacks, so
additional snow showers are expected with potential for several
more inches. 

...Northern New York and Northern New England...
The trough moving across the Great Lakes Sunday will induce
cyclogenesis over eastern New England with the low moving north
across eastern Maine and into Canada Monday. Cold air being
advected south will cause precipitation to changeover from rain to
snow in the northern portions of VT, NH, and western Maine.
Accumulating snowfall is likely, enhanced by upslope flow on NW
winds into the terrain of the northern Adirondacks and northern
Green/White Mountains. Several inches are possible in windward
terrain.  Downslope flow should result in lower accumulations in
the Champlain Valley of NY/VT.

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

Petersen