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

Valid 12Z Thu Dec 03 2020 - 12Z Sun Dec 06 2020


...Northeast...
Day 3...

A significant winter storm is possible Saturday into Sunday, but
confidence is lower than normal. A southern stream impulse
ejecting out of the Plains this morning will open and lift
northeast while a northern stream shortwave digs out of central
Canada. The interaction of these two will likely lead to phasing
and a deep longwave trough, but the details of exactly where and
when this will occur are uncertain. The ECMWF continues to be
faster and further NW than the remaining guidance, while the
operational GFS and NAM are slow/east. For this iteration, the
UKMET and GEFS were used heavily as they provided a middle ground
with a reasonable solution.

As the energy lobes phase, a coupled upper jet structure will also
develop, and the collocation of these features will drive a
rapidly strengthening surface low lifting northeast from the
Southern Appalachians to near Cape Cod by the end of D3. This
track suggests the coast will remain liquid due to WAA and a
column above freezing, but heavy snow is possible well inland into
Northern New England and the terrain of Upstate New York and
Central New England. A track further SE would bring the snow
further south, while a track further NW would push rain
potentially into Canada. Using a blend of the UKMET/GEFS, there
has been an increase in snow amounts for northern VT/NH/ME, and
WPC probabilities now indicate a high risk for 4 inches in the
Northeast Kingdom of VT as well as the White Mountains of NH and
ME. A lower chance for 4" exists in the Adirondacks, Catskills,
and Berkshires, as well as much of the rest of Central New England
and northern Maine. There is potential for these amounts to be
significantly higher, but confidence in placement is too low to
introduce higher probabilities at this time.

Additionally, some light snow is likely in the terrain of WV,
where WPC probabilities suggest a less than 10% chance for 4
inches of snow accumulation as northerly flow behind the system
cools the column and provides some upslope enhancement.


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

Weiss