Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Thu Feb 18 2021

Valid 12Z Thu Feb 18 2021 - 12Z Sun Feb 21 2021

Days 1 to 2...

...Central Appalachians to the Mid Atlantic and Northeast...

A strongly confluent flow today leads to a southwest to northeast
upper jet crossing from the Ohio Valley to NY/New England.
Low level deformation/frontogenesis extends across the central to
south Appalachians to the mid Atlantic and then up to long Island
NY.

An axis of moderate to perhaps locally heavy snow will develop
through the northern Mid-Atlantic region. Generally, the greatest
probabilities of heavy snow will be across the Piedmont areas of
the Mid-Atlantic from northeastern WV and northwest VA northeast
into southern PA. Across this region there is a 70 to 90% chance
of exceeding 4 inches of snow. Snow changes to sleet across the DC
to Baltimore/Philadelphia/NJ corridor, cutting down on
accumulations.

Upper jet-induced divergence maxima cross southeast NY to southern
New England, where a broad area of 3-6 inches of snow is expected.
As the wave lifts off the coast on Fri, return easterly flow
causes convergence along the eastern shore of MA, including the
Boston area.  Enhanced ocean effect snow should bring a few more
inches of snow tomorrow before the winds veer and the low level
coastal convergence weakens late Fri.

Areas of sleet and freezing rain will occur across portions of the
Piedmont of VA/NC. This will all be associated with a strong surge
of warm-air advection and Gulf of Mexico moisture transport that
will encounter a modified Arctic airmass entrenched over the
region.
Higher than normal deep moisture exists with precipitable water
values in the GFS indicating 1-2 std deviations above normal
across southern VA and adjacent northern NC, where the longer
duration freezing rain will occur. The icing threat decreases
heading south across central North Carolina as temps remain above
freezing.

There is a 70 to 90% chance (per latest PWPF) of exceeding a 0.25"
of ice accretion across southwest to south central VA to the NC
border. This will include a threat to south central VA, which was
hit particularly hard by last weekend's ice storm. The icing
threat wanes on Fri as the wave of low pressure moves offshore,
with ascent decreasing in its wake.


...Pacific Northwest to the Northern Rockies and Great Basin...

An influx of Pacific moisture and energy with low-mid level warm
advection and ascent will support another round of locally heavy
snows over the Olympics and WA/OR Cascades with lighter
accumulations spreading east across the Intermountain West into
the Rockies. As much as 2 feet of new snow is expected through Fri
for the Olympics and the Cascades, with as much as a foot for the
northern Rockies including the Bitterroots, Sawtooth and Tetons.
On Saturday, an upper trough moves east across the Great Basin and
upper jet crosses southern UT and CO.
Upper divergence in the left jet exit region will bring several
inches of in part of the UT Wasatch and also the ranges of western
CO.
The Olympics and WA Cascades are expected to receive another round
of higher elevation snow as upper divergence maxima cross
northwest to southeast across northern WA. Resurgent low-mid level
moisture/warm advection and ascent aid in producing lift, which is
augmented by the topographically induced  lift in windward
terrain. A low probability of a foot is forecast for Saturday in
the northern WA Cascades.

...Southwest to South-Central Texas...

A period of some snow, sleet and freezing rain is expected today
where a low-mid level deformation and frontogenesis maxima is
located in portions of southwest to south central TX; including
the Big Bend area.  An area of 2-5 inches of snow is expected
centered near Del Rio TX.
A broad are of light freezing drizzle extends along the mid level
front across south central TX to northern LA.
The front is progressive so the lift will be short-lived, and so
will the icing threat.


Petersen