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

Valid 00Z Sat Dec 05 2020 - 00Z Tue Dec 08 2020

...Significant Nor'easter to affect New England Saturday through
Sunday...

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

Further increase in confidence for the ingredients coming together
for the first widespread significant snow event of the season for
New England. A southern stream wave currently over the Mid-South
will phase with a northern stream trough drifting east over the
northern Great Lakes over the Mid-Atlantic to southern New England
tonight through Saturday. Rapid low level cyclogenesis will occur
tonight as the low shifts northeast over the Mid-Atlantic and
tracks northeast past or over Nantucket late Saturday before
stalling/slowing over New Brunswick on Sunday. A tight rain/snow
line is expected to develop near the central/northern New England
Coast on Saturday with an expectation for heavy, rapidly
accumulating snow developing from central Mass up the interior
portion of the New England coast (west of a strong coastal front)
and up against the White Mtns of NH and Maine through Saturday
night, slowly tapering off over northern Maine Sunday. There has
been notable jumping west and east in all guidance from run-to-run
for the low track, but as of 12Z there is good agreement among
global guidance so this forecast is based on a blend of the 12Z
ECMWF/GFS/NAM/UKMET/CMCregional which produces a middle ground
forecast.  Should the system track farther east or west the heavy
snow would track with it.

The mechanisms driving this evolution are split flow with energy
in both the northern stream and southern stream phasing over the
Mid-Atlantic producing a negatively tilting closed low over or
just off eastern New England by late Saturday. In response to this
amplifying trough, downstream jet streaks over the Canadian
Maritimes will begin to couple, producing an intense divergence
maxima which drives rapid surface pressure decrease. This energy
moving along a baroclinic zone supports a rapidly deepening low
pressure system, and intensifying precipitation spreading across
New England.

Downstream of the initial northern stream trough over the Great
Lakes, the antecedent column is marginal for snow, and
precipitation is likely to fall as rain initially, especially in
southern New England. As moisture spreads farther north it will
advect into a cooler column as well as dynamically cool, producing
snow from south-central through northern New England. The
exception is likely to be far eastern Maine (a little farther east
than from the 00Z consensus) where onshore flow and a surface low
track near the coast will produce warm air advection and rainfall
(with a narrow stripe of sleet possible in the rain to snow mix
zone in eastern Maine Saturday night). As the low continues to
intensify, mesoscale forcing will become increasingly intense as a
robust TROWAL develops through the warm conveyor belt wrapping NW
around the surface low, producing intense lift into the dendritic
growth zone, and dynamic cooling will occur. This should begin to
transition most of the precip from rain to snow down to the
coastal front near the MA/NH/ME coast, 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 eastern Mass coast and into CT/RI, including the I-95
corridor from Boston to Portland. There was generally a slight
west shift with the 12Z guidance, so the risk of heavy snow for
downtown Boston is a bit more uncertain while certainty has
increased farther west in the Worcester Hills. Day 1.5 WPC snow
probs encompass most of the snowfall and are moderate for 6 or
more inches in the Worcester Hills, and quickly go to high over
all but coastal and the CT Valley in NH through interior Maine.
Probabilities for 12 or more inches are moderate from the White
Mtns of NH and Maine up through northern Maine in Days 1.5 and 2.

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 has improved in the development of
this band, with the noted track jumps from run to run keeping
uncertainty present despite this beginning within 24hrs. Through
Sunday as the low winds down over New Brunswick, wrap around bands
over northern New England will slowly taper off.

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

Jackson