Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EST Thu Dec 03 2020

Valid 00Z Fri Dec 04 2020 - 00Z Mon Dec 07 2020

...Northeast...
While significant differences remain, the 12Z models did move into
better agreement -- presenting a stronger signal for a heavy
snowfall event impacting a large portion of interior New England
Saturday and Sunday. 

Models show a southern stream low pivoting east into the lower
Ohio and Tennessee valleys by late Friday, where it is expected to
begin to interact with a northern stream trough amplifying over
the Great Lakes.  While previous runs differed on the timing and
degree of phasing, the 12Z runs presented better agreement.  The
general consensus of the models (including the 12Z GFS) now show
the southern stream low beginning to assume a negative-tilt as it
reaches the Mid-Atlantic coast early Saturday, with a surface low
beginning to quickly deepen along the coast.  However, there are
still some differences with the degree of phasing and the track of
the surface low as it moves off of the Mid-Atlantic coast on
Saturday.  The GFS, which had previously shown less phasing and a
more suppressed surface low track, made trends toward a more
phased solution, raising confidence on the potential for a
significant winter storm for at least some portion of New England.
 In contrast, the ECMWF, which had been presenting one of the more
wrapped-up solutions and northwesterly low track from the
Mid-Atlantic to the Northeast coast, shifted considerably
southeast with its 12Z run.  These changes in continuity, along
with continued model spread, limit forecast confidence in the
northern extent of precipitation, the rain/snow line and their
associated impacts.  While these differences in the details
remain, there is now increased confidence in the threat for heavy
snow across portions of New England, especially from Berkshires
northeastward into northern New England, with the heaviest totals
most likely over northern Maine. 

By Day 3, despite the differences along the Mid-Atlantic to
southern New England coast, the models actually move into better
agreement by late Saturday -- showing a strong surface low moving
from just east of Cape Cod north through the Gulf of Maine toward
Atlantic Canada.  By this point, low-to-mid level frontogenesis
along with strong upper forcing will likely support moderate to
heavy precipitation on the northwest side of the low.  While it
remains to be seen how close to the coast the rain/snow line will
reach, heavy snow does appear likely across much of interior New
Hampshire and Maine.

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

Pereira