Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Fri Dec 04 2020

Valid 12Z Fri Dec 04 2020 - 12Z Mon Dec 07 2020

...Northeast...

...Significant nor'easter to affect parts of New England Saturday
and Sunday...

All the ingredients are coming together for the first widespread
significant snow event of the season for New England. While
guidance still features above normal spread, consensus among the
preferred non NAM (and non hi-res outside the 00Z/4 HRRR) is for a
rapidly deepening cyclone to move off the Mid-Atlantic coast and
then lift just inside the benchmark (40N/70W) before slowing
through the Gulf of Maine on Sunday and eventually departing into
the Canadian Maritimes. NW of this system a swath of heavy snow is
becoming more likely, and many parts of New England are expected
to receive plowable snowfall this weekend.

The mechanisms driving this evolution are split flow with energy
in both the northern stream and southern stream phasing over the
Mid-Alantic producing a negatively tilting closed low over New
England by Sunday morning. In response to this amplifying trough,
downstream jet streaks will begin to couple, producing an intense
divergence maxima which drives rapid surface pressure drops. This
energy moving along a baroclinic zone supports a rapidly deepening
low pressure system, and precipitation spreading into the
northeast.

The antecedent column is marginal for snow, and initially
precipitation is likely to fall as rain, especially in southern
New England. As moisture spreads further northward it will advect
into a cooler column producing snow across central and northern
New England. The exception is likely to be eastern Maine where
onshore flow and a surface low track near the coast will produce
WAA and rainfall. However, as the low continues to intensify,
mesoscale forcing will become increasingly intense as a robust
TROWAL develops through the WCB wrapping NW around the surface
low, producing intense omega into the DGZ, and dynamic cooling
will occur. This should begin to transition most of the precip
from rain to snow, and there is likely to be an intense fgen band
collocated with the TROWAL just NW of the center. As the low
deepens and lifts northeast, this band should collapse
southeastward slowly, with the isallobaric acceleration into the
low potentially causing heavy snow all the way to the coast,
including the I-95 corridor from Boston to Portland. Recent WSE
plumes have indicated an increase in the means and 90th percentile
for eastern MA/NH/ME, with a decrease in NY and western New
England, and some enhancement in the terrain where orographics
will produce higher snowfall. WPC probabilities for 6" are highest
in northern NH and ME, where locally more than 15" is possible in
the higher terrain. Elsewhere across the terrain of upstate NY and
much of central New England, WPC probabilities are low to moderate
for 4 inches.

Although the heaviest snowfall may be across northern New England
due to a longer period of snow and less mixing, the heaviest snow
rates may occur within the strong fgen band, with rates
approaching 2"/hr possible as it pivots E/SE Saturday evening. A
secondary max of snowfall is possible from Worcester county, MA
eastward into Essex County, MA and towards Portland, ME where this
band may linger. Confidence at this time is low in the exact
placement of this band, but is high that it will develop.
Additional accumulations in excess of 4" are likely across this
area, potentially much higher, and this could include the Boston
to Portland metro areas. By Sunday afternoon the low will be
racing away into Canada, and only light snow will linger across
parts of eastern and northern Maine.

For Days 1-3, the probability of significant icing (0.25-inches or
greater) is less than 10 percent.

Weiss