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

Valid 00Z Sun Nov 01 2020 - 00Z Wed Nov 04 2020

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

A shortwave and accompanying upper jet streak will drive a strong
cold front across the Lakes beginning tonight and moving into New
England on Sunday. 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, 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. WPC probabilities are moderate to
high for 4 inches.

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, although an
effective fetch from Lake Huron may cause an enhancement SE of
Lake Erie as additional moisture gets transported downwind. This
is shown by many of the high res models. A local max of more than
6" is probable here, and WPC probabilities show a high risk for 4"
along the Lake Erie shore, 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 moderate to heavy snow in
the favored terrain of SW PA and along the Appalachians into WV.
Although the snow may be periodic 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. This feature has shown intensification today
in the model suite, and an enhanced area of snow is likely east of
Lake Ontario where fetch/LES enhancement is likely, but also
across much of central New England due to forced ascent. WPC
probabilities on D3 are high for 4 inches in the Adirondacks and
into the northern Greens of VT, with light accumulations possibly
spread across much of central New England.

...Northern New England...
Days 2-3...

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. The guidance has
trended SE with the surface low today. In addition to this low,
strong LFQ 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 as the
secondary shortwave moving in behind the closed low provides
additional forcing. The robust ascent and further SE low track
allows colder air to flood more rapidly into New England, and a
period of moderate to at times heavy snow is becoming more likely.
The precipitation may initially start as rain across much of NH/ME
on WAA, 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
isolate much higher totals. The recent ECMWF and GFS deterministic
models are much more aggressive across northern ME, so this will
need to be monitored, but at this time it appears that region may
experience a lot of its precipitation before or during changeover,
so the highest probabilities are confined to the terrain for now
where upslope flow will enhance the synoptic ascent.


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

Weiss