Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Fri Oct 30 2020

Valid 00Z Sat Oct 31 2020 - 00Z Tue Nov 03 2020

...Great Lakes...
Days 2/3...

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 CAA atop 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 Monday.

On Sunday, periods of moderate to heavy snow are likely across the
U.P. and NW L.P. where N/NW winds will produce lake effect snow
bands. The heaviest snow is forecast in the western U.P. early,
then shifting into the eastern U.P. late as a modest deformation
axis serves as a region of secondary ascent to produce WPC
probabilities that are moderate for 4 inches. Later Sunday, snow
will develop in NW L.P. as an effective fetch from Lake Superior
moves across the northern part of Lake Michigan to drive heavy
snow showers in that region as well.

On Monday, LES will spread downwind of Erie and Ontario, with a
multiband setup likely. Very warm lake waters and 850mb temps
crashing below -5C will produce significant instability as high as
1000 J/kg and lake EL's above 700mb. Modest shear and the limited
fetch across these lakes on NW winds will support multi-bands with
snow rates that could be significant. Warm ground temps initially
could limit some accumulation, and the temporal duration of deep
moisture is somewhat reduced. Still, WPC probabilities for 4
inches have increased, and are moderate in NW PA, western NY, and
into the Tug Hill Plateau.

A secondary impulse moving through the broad cyclonic flow may
redevelop a band of LES east of Lake Ontario late D3 which could
also be significant and add moderate snow accumulations.


...Northern New England...
The trough moving across the Great Lakes Sunday will spawn weak
cyclogenesis over eastern New England which will race northeast
across eastern Maine and into Canada Monday. Cold air being drawn
behind this system will cause precipitation to changeover from
rain to snow in the northern portions of VT and NH, and more
impressively across northern Maine. Some snowfall is likely,
enhanced by upslope on NW flow into the terrain late D3. WPC
probabilities are modest for 4 inches, greatest in the high
terrain of the White Mountains.


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

Weiss