Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 05 2020

Valid 12Z Sat Dec 05 2020 - 12Z Tue Dec 08 2020

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


...Significant Nor'easter begins this afternoon and persists
through Sunday...

Guidance has clustered nicely this morning on a surface low track
moving inside the benchmark (40N/70W) this aftn, just east of Cape
Cod tonight, and through the Gulf of Maine towards Nova Scotia
Sunday. As this occurs, the low will rapidly deepen, potentially
"bombing out" by deepening 24mb in 24 hrs. While a track this
close to the coast will bring rain to eastern sections, it is
extremely likely that many parts of southern, central, and
northern New England will receive significant to major snowfall
this weekend.

The synoptics driving this development include phasing of southern
and northern stream energy in time to capture the low and swing it
northward up the Mid-Atlantic and New England coastlines. At the
same time, height falls and robust coupling of downstream jet
streaks will provide ample mid and upper diffluence to promote
rapid surface pressure falls. The antecedent airmass is marginally
warm, so without an arctic high pressure in place ahead of the
system, precipitation spreading northward this morning will
initially be all rain across SNE, with snow developing where its
colder across NNE later today. However, intense forcing will cause
rapid dynamic cooling of the column, and in addition to better
track consensus, the models have also begun to show a more rapid
transition from rain to snow later this morning.

There are likely to be two maxima of snowfall with this event. The
more certain region is across northern NH and much of central
northern ME where the entirety of the event will be snow.
Precipitation will begin as snow this aftn as mid-level WAA and
robust synoptic ascent cause an expansion of precipitation. As the
low pulls northeastward and deepens, this region will also
experience a pronounced TROWAL through the WCB wrapping
cyclonically around the low, reaching as far west as the Northeast
Kingdom of Vermont. Snowfall rates may reach 1"/hr at times late
Saturday into Sunday as 700mb omega maximizes during the
WAA/TROWAL overlap, but otherwise an extended period of moderate
snow is likely across this region. Long duration snow will
accumulate efficiently however, and WPC probabilities from the NE
Kingdom of Vermont across the White Mountains of NH and into much
of northern ME are high for 8 inches, with many spots likely
reaching 12". In the mountainous terrain where upslope enhancement
will also occur, isolated totals to 2 feet are possible.

The other area of heavy snow is becoming more focused as a strong
frontogenetical band develops NW of the low across central/eastern
MA and into SE New Hampshire and SW Maine. There is very good
agreement among the high-res that this band will drop slowly
eastward as the low lifts northeast, potentially pivoting in place
across SW Maine into far NE Massachusetts as suggested by guidance
and the conceptual model for this event, and collocated with 700mb
omega maxima where WAA and the edge of the TROWAL overlap. Despite
marginal thermal profiles, intense dynamics are expected to
rapidly cool the column sufficiently for rain to change quickly to
snow even as surface temps remain near or even slightly above
freezing. The recent HREF mean and WPC snowband probabilities
indicate a high likelihood for 2"/hr snowfall rates within this
band. These rates are supported by intense omega of 30 ub/s into
the saturated DGZ coincident with a deepening near isothermal
layer in local soundings, and pockets of -EPV suggesting
thunder-snow and large aggregates which will accumulate rapidly.
Guidance has slowed the motion of this band as well, and a pocket
of very heavy snowfall is becoming more likely from Worcester
County, MA, to Essex County, MA northeast into Maine, potentially
impacting both the Boston and Portland metros. WPC probabilities
within this snowband are moderate to high for 8", with local
maxima potentially close to double this amount. There will be a
very sharp gradient in snowfall near the coast however, and any
deviation in the intensity of speed of this band could drastically
alter the snowfall in this area.

Elsewhere across New England, generally west of I-91 and south of
the CT/MA border, precipitation will be lighter and profiles
warmer. Light to moderate accumulations are likely in the southern
Greens and Berkshires where WPC probabilities are moderate for 4
inches. Otherwise, much of the precipitation will fall as rain,
with some light snow as the event winds down.

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

Weiss