Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 PM EST Mon Dec 14 2020

Valid 00Z Tue Dec 15 2020 - 00Z Fri Dec 18 2020

...Major Nor'easter/Winter Storm to Impact the Mid-Atlantic and
southern New England states Wednesday and Thursday...

Day 1...

...Pacific Northwest/Northern Rockies...

An offshore Pacific trough and associated cold front will be
arriving across the Pacific Northwest and the northern Rockies on
Tuesday which will bring a surge of Pacific moisture a swath of
locally heavy snow across especially the WA Cascades where locally
over 1 foot of new snow will be possible. Somewhat lesser amounts
are expected for the Oregon Cascades and the Bitterroots.

...Central Rockies into the Central/Southern Plains...

Southern stream shortwave energy ejecting out of the Four Corners
region will amplify as it reaches the southern Plains on Tuesday
and will drive surface low pressure across western and central TX.
The energy over the higher terrain of the CO Rockies should yield
several inches of snow overnight, with low probabilities of
exceeding 4 inches. An increase in warm advection and moisture
transport back into the colder air focused over portions of
southern NE, western KS and northwest OK should allow for a swath
of light to moderate snow to break out across these areas with
locally a few inches of snow possible. Low probabilities of
exceeding 4 inches are denoted.


Days 2/3...

...Western U.S...

A new full-latitude trough and a fairly strong cold front will
arrive across the West Coast by late Wednesday and then traverse
the Intermountain West through Thursday. This will bring a strong
surge of Pacific moisture and strong forcing for heavy snowfall to
occur over the higher terrain of the Cascades, the central and
northern Sierra-Nevada, and also the northern Rockies including
the Sawtooth and Bitterroots. Locally 1 to 2 feet of new snow is
expected for these areas by late Thursday as the upper-level
trough and axis of lower snow levels advances inland.


...Appalachians/Mid-Atlantic to New York and Southern New
England...

The latest forecast continues to advertise a significant winter
storm for portions of the central Appalachians, the Piedmont area
of the Mid-Atlantic, and much of south-central/southeast NY and
southern New England, as a nor'easter develops and lifts up along
the East Coast Wednesday into Thursday. The wave that crosses the
southern Plains Tuesday will eject through the Mid-South Tuesday
night and is forecast to take on a negative tilt as it arrives
across the Appalachians on Wednesday. This will promote
cyclogenesis off the Southeast coast with low pressure then
lifting north and strengthening along the Mid-Atlantic coast and
then arriving offshore of southeast New England by early Thursday.
Meanwhile, a cold, 1036+mb high will be focused over Quebec and
will be nosing south down across the interior of the Northeast and
the Mid-Atlantic with a rather classic cold air-damming event
expected to be in place as the low center moves up the coast.

As strong warm-air advection/isentropic ascent occurs ahead of the
upstream trough along with increasingly moist Atlantic inflow over
the cold air, a large swath of moderate to heavy snow is expected
to develop for portions of the central Appalachians and the
adjacent Piedmont areas of WV/VA/MD and PA on Wednesday with the
focus west of the I-95 urban corridor, and the bigger cities of
Washington D.C., Baltimore and Philadelphia. By Wednesday night
and early Thursday the focus of heavy snow will overspread areas
of southeast NY and southern New England including the New York
City and Boston metropolitan areas. The latest model guidance
favors a rather strong comma-head/deformation zone impacting
especially areas of eastern PA, southeast NY, northern NJ and
southeast New England on Thursday as the low center further
deepens and then begins to edge east-northeast and farther
offshore. Meanwhile, down across areas of northwest SC, western
NC, southwest VA, and southern WV, the cold air will be much more
shallow by comparison, and it appears likely that there will be a
period of freezing rain and locally some sleet as the event ensues
on Wednesday. Some of this may sneak up near or just west of I-95
as well from Washington D.C. to Philadelphia. But the threat of
ice farther north closer to New York City and certainly Boston
will be notably lower.

WPC favored a a multi-model blend approach for the precipitation
forecast with a tad more weighting to the ECMWF, and lesser
weighting to the NAM and GFS. The NAM is farther west and warmer
with the evolution of this system, with the GFS a bit colder and
farther east. While not totally dismissing the NAM and GFS, a
model consensus somewhat closer to the ECMWF and ECENS mean was
favored for QPF and was applied to the thermal profiles.

Based on the latest forecast, the expectation is for storm total
snowfall to be as much as 12 to 18 inches for the interior of
especially the northern Mid-Atlantic, far southeast NY and
southern New England. Where freezing rain occurs, the greatest ice
accretions are expected over northwest NC, southwest to central VA
and over parts of southern WV where locally as much as a tenth to
quarter inch of ice will be possible.

Orrison