Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
549 PM EST Wed Feb 17 2021

Valid 00Z Thu Feb 18 2021 - 00Z Sun Feb 21 2021

Days 1 to 3...

...Lower MS/TN Valleys to the Northeast...

As multiple rounds of energy traverse the Gulf Coast states
tonight and Thursday and lift toward the central Appalachians and
Mid-Atlantic region by Thursday evening, an axis of moderate to
perhaps locally heavy snow will develop and expand northeast out
of the Tennessee Valley and through the northern Mid-Atlantic
region. Areas of sleet and freezing rain will be breaking out just
south of this area across portions of the Piedmont of VA/NC and
SC. Initially this will all be associated with a strong surge of
warm-air advection and a moderate degree of Gulf of Mexico
moisture transport that will encounter a modified Arctic airmass
entrenched over the region, and aided by rather strong high
pressure over southeast Canada and northern New England. One wave
of low pressure crossing the Gulf Coast will be initially
associated with this precipitation focus, but by later Thursday
and into Thursday night, there will be a secondary area of low
pressure taking over and becoming dominant along the Southeast and
Mid-Atlantic coast. This low center will then lift northeast to
offshore of southeast New England on Friday and then go out to sea
on Saturday.

Generally, the greatest probabilities of heavy snow will be across
the Piedmont areas of the Mid-Atlantic from eastern WV and
northwest VA northeast into southern PA. Across this region there
is a 70 to 90% chance (per latest PWPF) of exceeding 4 inches of
snow from this evening through Thursday evening. However,
accumulating snow will extend well southwest into areas of the TN
Valley with locally a few inches expected. To the south of this
will be a notable concern for sleet and freezing rain, including
some areas that will initially be snow (i.e. Washington D.C.,
Baltimore and Philadelphia) and will gradually changeover to sleet
and then some freezing rain. The greatest threat of significant
freezing rain though will be for areas of central and southwest VA
and extending down into northwest NC where some sleet at the onset
should changeover to freezing rain with surface wet-bulb
temperatures forecast to drop below freezing in response to the
cold high pressure over New England that will be wedged in down
east of the Appalachians and driving low-level cold air advection
in a classic cold-air damming set-up underneath the mid-level warm
nose. There is locally a 70 to 90% chance (per latest PWPF) of
exceeding a 0.25" of ice accretion across these areas, and this
will include a threat to some areas that were hit particularly
hard by last weekend's ice storm (i.e. south-central VA). By late
Thursday night and Friday, the main focus of snow will be arriving
over especially southeast NY and southeast New England. Locally
several inches of snowfall are expected over these areas going
through Friday night with lesser amounts for the interior. As the
low center pulls away from the East Coast early Saturday, the
snowfall threat should come to an end.


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

In connection to some of the energy expected to impact the East
Coast over the next 1 to 2 days, there will be a period of some
snow, sleet and freezing rain later tonight and into Thursday for
areas of southwest TX including the Big Bend area, and then
extending east across portions of the Edwards Plateau. The
precipitation should generally be light, but locally a couple of
additional inches of snow is expected, along with some spotty
areas of .05" to .10" ice accretion. The greater ice threat will
tend to be for areas a little closer into the middle Rio Grande
Valley, but with the ice threat extending east more broadly out
across areas of south-central TX.


...Pacific Northwest to the Northern Rockies...

Unsettled weather is forecast to return during the day on Thursday
and continue through Saturday as a pair of mid-level Pacific
troughs arrive and break down the deep layer ridge that is in
place over the region. The influx of Pacific moisture and energy
with increasingly westerly flow will support another round of
locally heavy snows over the Olympics and Cascades with lighter
accumulations spreading east across the Intermountain West into
the Rockies. As much as 2 feet of new snow is expected for the
Olympics and the Cascades, with as much as a foot for the northern
Rockies including the Bitterroots, Sawtooth and Tetons. On
Saturday, a portion of the second Pacific trough will bring the
threat for some accumulating snow a bit farther south into the
Sierra-Nevada of northern California and the Great Basin.

Orrison