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

Valid 12Z Sun Nov 01 2020 - 12Z Wed Nov 04 2020

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

A shortwave and accompanying upper jet streak will drive a strong
cold front across the lower Lakes and moving into New England
today. 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 in the
U.P. of MI will develop first, before spreading into northwest
lower MI, and then downwind of Lakes Erie and Ontario later this
afternoon and tonight. 

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 today, 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. WPC probabilities are moderate to high for 4 inches.

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. Modest shear across these lakes
on NW winds will support multi-bands of snow showers, although an
effective fetch from Lake Huron may cause an enhancement SE of
Lake Erie as additional moisture gets transported downwind. A
local max of more than 6" is probable here, and WPC probabilities
show a high risk for 4" inland from the Lake Erie shore in
northwest PA/adjacent southwest NY, and a moderate risk into the
Tug Hill east of Lake Ontario.

As the mid level trough approaches the central Appalachians, 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. Although the snow may
be short-lived across this area, at times briefly heavy rates are
possible and WPC probabilities have increased to show a
low-moderate risk of 4".

On Monday night into Tuesday, a secondary but potent trailing
shortwave is progged to cross from the Great Lakes into the
northeast once again. An enhanced area of snow is likely east of
Lake Ontario where fetch/LES enhancement is likely, so several
inches is expected int he Tug Hill, with isolated maxima to a foot
o snow possible.
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. WPC probabilities on day 2 are high for 4 inches in the
Adirondacks and into the northern Greens of VT.

The event winds down on day 3/Tue as low level ridging crosses the
lakes and drier air aloft advects into New York/New England.

...Northern New England...
Days 1-2...

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. 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.

The precipitation may initially start as rain across much of NH/ME
on warm air advection, but will rapidly change over as the column
cools. WPC probabilities have increased and now show a moderate
chance for 6" in the terrain of VT/NH/ME, and upslope enhancement
could lead to higher totals.

Behind the departing synoptic system, moist cyclonic flow crossing
the mountains of northern VT/NH should bring periods of snow
showers Monday. The stronger 700 mb vertical velocity maxima in
northern VT leads to greater snow fall potential there.

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

Petersen