Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EST Wed Dec 16 2020

Valid 12Z Wed Dec 16 2020 - 12Z Sat Dec 19 2020

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

...Appalachians and Mid-Atlantic and across the Northeast...
(Days 1-2; 12Z Wednesday to 12Z Friday)

A major winter storm develops today and continues into Thursday
for a large swath from the south-central Appalachians, interior
central Mid-Atlantic and nearly all of the northern Mid-Atlantic
and through south-central New England. This storm is expected to
deliver more snow than was observed all of last season (Winter
2019-2020) from northwest Virginia, the eastern West Virginia
Panhandle, central and eastern Pennsylvania, far northern New
Jersey, and at least portions of the New York City metro area.

A southern stream trough shifting east into the Mid-South early
this morning will intensify as it takes on a negative tilt and
swings increasingly to the northeast across the Tennessee Valley
today and the Mid-Atlantic tonight before crossing southern New
England Thursday morning. A weak leading impulse is allowing
surface cyclogenesis off the Georgia coast now which will become
the dominant surface low later today as the mid-level wave
connects with it this afternoon. Energy translates to this
developing coastal low that will track north along the
Mid-Atlantic coast tonight. Another important component to this is
the 1037mb surface high over south-central Quebec that is
supplying ample dry/cold air to the entire Northeast with an
associated surface ridge extending the lee side of the
Appalachians into Georgia. Leading precipitation overrunning this
cold air damming wedge of a surface ridge will start as a mix of
snow and freezing rain with the freezing rain occurs near the Blue
Ridge of NC up through at least central VA this morning (as of 08Z
KGSO in NC is 32F with light freezing rain). As the low
intensifies as it tracks north from Cape Hatteras this afternoon
the baroclinic zone inland over the central Mid-Atlantic
intensifies and low level frontogenesis allows heavier
precipitation and a tighter rain/mix/snow line. Heavy snow is
expected to begin on the cold side in the Potomac Highlands and
northern Shenandoah Valley this afternoon. Of notable importance
is the development of a 700mb low over northern Virginia early
this evening which is an indication of the strength of the system
and forms a good signal for where the heaviest snow will begin and
will continue along and just west of the 700mb low track. 

Thermal preferences with this system a blend of the 00Z ECMWF/GFS
which have a similar eastern boundary to snowfall (west/north of
DC then along the I-95 corridor north from Baltimore and most/all
of Long Island and southern New England.

As noted by the main snow line, strong low-mid level warm air
advection ahead of the advancing trough/low will cause any wintry
precip to change to rain over much of the Washington D.C. metro
and east-northeast across southern New Jersey. Additionally, a dry
slot wrapping around the lower mid-level low will track up this
rainy sector tonight. Given a low track off/along the Delmarva
coast tonight, cold air could be retained essentially above the
fall line in the central Mid-Atlantic with accumulating sleet
tracking from northern Virginia this afternoon to north-central
Maryland and southeast Pennsylvania and the NYC metro area
overnight.

There continues to be a slight northward trend in the axis of the
heaviest precip with the 00Z ECMWF a little farther north in
Pennsylvania than the 00Z GFS. The greatest swath of snow is from
south-central to northeast Pennsylvania where Day 1 WPC PWPF is 40
to 60 percent for 18 or more inches of snow. This is 40 percent
for the central Hudson Valley in the Day 1.5 probabilities.
Overall the coverage and intensity of the snowfall forecast is
similar to the previous shift with an increase in interior
southern New York.

A swath of over 8 inches of snow is expected from the eastern West
Virginia Panhandle to the Berkshires of Massachusetts with maximum
amounts possibly reaching two feet in central Pennsylvania. The
risk for 8 or more inches has increased to likely for the rest of
Massachusetts. Accumulating ice after 12Z has a 40 to 50 percent
chance of reaching a quarter inch in a swath along/east of the
Blue Ridge from southwest Virginia to north-central Virginia as
well as southern West Virginia west of the Allegheny Highlands.


...Western States...
(Days 1-3; 12Z Wednesday to 12Z Saturday)

An active pattern remains with the northwest CONUS with the next
wave the most meridional for the foreseeable future with trough
axis crossing the central California coast and north late tonight
into Thursday morning. Ample Pacific moisture will produce heavy
snow for higher elevations of much of the central and northern
portions of the Western U.S. over the next few days. The Day 1.5
snow probabilities are moderate to high for 8 or more inches for
the Olympics, Cascades, central and northern Sierra Nevada, and
western slopes of the ID Rockies and Tetons. This shifts to the
northern Wasatch for Day 2. The next wave reaches the Pacific
northwest Friday with Days 2 and 3 snow probabilities moderate to
high for 8 or more inches for the northern Washington Cascades. 72
hour probabilities for 18 or more inches are high in the
Washington Cascades.


Jackson