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

Valid 12Z Tue Dec 15 2020 - 12Z 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...

A Pacific trough moving into the northern BC coast with the
associated cold front crossing the Pacific Northwest coast this
morning will reach the the northern Rockies later today. A surge
of Pacific moisture will cause locally heavy mountain snow across
the WA Cascades and northern ID/northwest MT Rockies are where
there are high probabilities for 8 or more inches of new snow.


...Central/Southern Plains...

A southern stream shortwave trough shifting east across NM will
close into a low over the TX Panhandle this morning and shift
across OK this afternoon/evening. An inverted trough at the
surface will shift east across TX today and an associated increase
in warm advection and moisture transport back into the colder air
focused over portions of southern NE and western KS into the
morning with the focus then ahead of the mid-level low over
western OK where more moderate to locally heavy snow is expected.
Day 1 snow probabilities are now likely for 4 or more inches over
western OK and the northeastern TX Panhandle with 20 percent
probabilities for 6 or more inches in western OK. A reinforcing
wave shears apart the low as it tracks over MO/AR tonight with
less precip and just light snow over eastern OK and the northern
AR Ozarks.


Days 2/3...

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

Major winter storm on tap for portions of the central
Appalachians, the Piedmont area of the Mid-Atlantic and much of
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 today will
eject through the Mid-South tonight as a reinforcing trough brings
on a negative tilt as it turns east-northeast across the southern
Appalachians on Wednesday. This will promote cyclogenesis along
the Carolina Coast with low pressure then lifting north and
strengthening along the Mid-Atlantic coast Wednesday evening
before shifting east-northeast offshore and south of Long Island.
Meanwhile, a cold, 1038mb 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 east of the
surface ridge and along 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 generally west of
Washington D.C. and Baltimore and just north or over 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. There
has been a bit of a northerly shift in 00Z model guidance earlier
on Wednesday before shifting east offshore as before. Still expect
a rather strong comma-head/deformation zone impacting especially
areas of central/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 and central WV, the cold air will
be much more shallow by comparison with freezing rain and locally
some sleet as the event ensues on Wednesday. The thermal profile
supports sleet spreading up near or just west of I-95 as well from
Washington D.C. to Philadelphia and toward NYC. But the threat of
ice north of Philadelphia is notably lower.

WPC QPF for this event is a multi-model blend weighting heavier
with the 00Z ECMWF, and lesser weighting to the 00Z NAM/GFS.
Overall this essentially shifts the heavy QPF focus farther west
over the northern Mid-Atlantic compared to the previous forecast.
Thermally, a blend of the 00Z ECMWF (which is rather warm now in
the central Mid-Atlantic), 00Z GFS, and 00Z 3kmNAM was preferred.
Based on this combination of moisture and thermals, the
possibility for maximum storm total snowfall in the 18 to 24 inch
range is from the eastern WV Panhandle, western MD and
south-central to east-central PA and closer to a foot for far
southeast NY and southern New England. Where freezing rain occurs,
the greatest ice accretions are expected over northwest NC up
through southwest to central VA and from southern into central WV
where Day 1.5 ice probabilities are 30 to 40 percent for a quarter
inch of ice.


...Western U.S...

The next in a long line of disturbances in this La Nina-type
pattern is a long trough and a fairly strong cold front arriving
to the West Coast north from San Francisco Wednesday night that
then pushes east across the Intermountain West Thursday and
reaches the north-central Rockies Thursday night. This will bring
a strong surge of Pacific moisture and strong forcing for heavy
snowfall to occur over the higher terrain of the entire Cascades,
the central and northern Sierra Nevada, and also the northern
Rockies including the Sawtooth, Bitterroots, Tetons, as well as
the northern Wasatch of Utah. Locally 1 to 2 feet of new snow is
expected for these areas through Thursday night as the upper-level
trough and axis of lower snow levels advances inland.

Jackson