Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Tue Dec 15 2020

Valid 00Z Wed Dec 16 2020 - 00Z Sat Dec 19 2020

...Major Winter Storm to Impact the Mid Atlantic and Northeast
Wednesday and Thursday...

...Appalachians and Mid Atlantic into the Northeast...
(Days 1-2; 00Z Wednesday to 00Z Friday)

A major winter storm remains on track for a large portion of the
Northeast. In fact, for some areas, this storm seems likely to
deliver more snow than was observed all of last season (Winter
2019-2020), and is therefore one of the bigger snow events the
region has seen in awhile.

The genesis of the storm currently lies in the Southern Plains as
a relatively strong wave making steady eastward progress. This
wave will eject into the Mid South overnight, and a reinforcing
northern stream trough will bring on a negative tilt as the wave
begins to lift increasingly to the northeast on Wednesday and
Thursday. Overall, the synoptic situation is conducive for lee
cyclogenesis and a band of heavy snow along and north of the 700mb
low track. The interaction between the two waves will have an
important consequence, with a gradual deamplification of the
trough and diminishing mid-upper level height falls along the
track with time. Therefore, the expectation is that the heaviest
snow (and snow rates) are likely to be observed from Wednesday
afternoon into Wednesday evening over the Mid Atlantic and
southern New York, with heavy but somewhat lower amounts further
to the east into portions of New England.

Another consequence of the wave deamplification is that the
heaviest snow is also likely to be driven by the strong low-mid
level warm air advection ahead of the advancing trough. And there
are not likely to be strong factors inhibiting the northward
progression of the associated warm nose. This is likely to cause
issues with precipitation type mixing in and near the I-95
corridor from northern Virginia into New Jersey, including the
Washington D.C., Baltimore, and Philadelphia metro areas.
Additionally, models are showing indications of a dry slot
wrapping around the mid-level low; the 00Z ECMWF ensemble mean
(which was favored for the synoptic positioning of major features
such as this) projects the 700mb low to move from near Frederick
MD, to near Trenton NJ, to Long Island and then Cape Cod. Areas
south of that track may see increased chances of the ice crystal
layer drying out and further limiting snowfall. Therefore, the
forecast snow totals were trended lower today from northern
Virginia and Maryland into southeast Pennsylvania, New Jersey,
Long Island, Rhode Island, and southeast Massachusetts. Hi-res
models indicate that the near-surface cold air may be somewhat
resilient in the I-95 corridor from D.C. to Philadelphia given a
surface low track well to the south. Therefore, accumulating sleet
or light freezing rain will be a possibility in those areas, and
sleet could contribute more to on-ground accumulations than snow
in some places.

In contrast, the forecast snowfall was increased from northern
Pennsylvania and New York into much of New England. Most of the
available model guidance has shown steady northward shifts over
the past several model cycles, and that adds confidence that this
is a realistic trend. The biggest thing to emphasize in these
areas is the continued uncertainty and large model spread. This
has been a consistent issue over the past few days. For instance,
the overnight WPC PWPF ensemble at Albany, New York had about
1-in-4 members with over 18 inches of snow, and about 1-in-4
members with less than 6 inches of snow. The northward extent of
the very heavy snow remains in question, and that will likely
require continued refinement in the next forecast.

A swath of over 8 inches of snow appears very likely to extend
from the central Appalachians into southern New England, with
maximum amounts possibly reaching or exceeding 20 inches. The
heavier amounts approaching 20 inches would be most likely in
Pennsylvania or adjacent portions of nearby states, earlier on in
the wave deamplification process. Accumulating ice also is
expected further south, particularly in parts of western Virginia
and western North Carolina, with a swath of over 0.10 inches
likely from near Boone, to Lynchburg, to Warrenton.


...Western States...
(Days 1-3; 00Z Wednesday to 00Z Saturday)

Heavy snow is likely to affect higher elevations of the Western
U.S. over the next few days as a series of disturbances continues
to push through the region. The focus seems to be mostly on Day 2,
or Wednesday Night into Thursday, as a trough arrives and brings a
combination of Pacific moisture and strong forcing. The highest
probabilities of 1 foot of snow are in the Cascades, Bitterroots,
and Tetons, particularly in the Washington Cascades where they are
over 80 percent.


Lamers