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

Valid 00Z Fri Feb 19 2021 - 00Z Mon Feb 22 2021

Days 1 to 3...

...Great Lakes/Central Appalachians to the Mid-Atlantic and
Northeast...

One last round of energy associated with a modest mid-level trough
will be lifting northeast overnight and through early Friday
across the TN Valley, the Mid-Atlantic, and southern New England
as a multi-day winter storm impacting the South and the East
prepares to finally come to an end. As one wave of weak low
pressure exits away from the Mid-Atlantic seaboard, the upstream
energy will foster the development and some modest intensification
of a secondary low off the Southeast coast this evening which will
lift offshore of the Mid-Atlantic overnight and then offshore of
southeast New England on Friday. Some resurgence of warm-air
advection and moisture back into the lingering shallow cold air
over the interior of the Mid-Atlantic will foster some additional
concerns for freezing rain overnight. This will include areas from
central and northern NC north-northeast up along the I-95 corridor
to around Washington D.C., Baltimore and Philadelphia. While
additional ice accretions are not likely to be too significant,
there may be some additional 0.10" to 0.15" ice accretions for
areas of south-central VA and up toward the northern neck area of
VA where there is some support for a burst of locally heavier
precipitation. Areas of west and north of I-95 from northern VA to
NY and central/southern New England will also have some potential
for at least a little bit more sleet and snow, with the heavier
snowfall threat generally over areas of northern PA, NY and
central/southern New England where on Friday there will be a bit
stronger Atlantic moisture transport combined with cold air for a
more organized and steadier coverage of light to perhaps locally
moderate snow. The arrival of the aforementioned mid-level trough
will help further facilitate this. Locally a few inches of
additional snow are expected. On Saturday, the low center offshore
of New England should begin to exit out to sea with any additional
snow expected to be light and tapering off.

Meanwhile, cold and relatively moist northwest flow over the Great
Lakes will continue to foster areas of lake effect snow, with
locally several inches of additional accumulation possible going
through Friday and Saturday. This will be enhanced somewhat across
areas of far western NY in the wake of the departing East Coast
low center.


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

An influx of Pacific moisture and shortwave energy with low to
mid-level warm-air advection and ascent will support another round
of locally heavy snows over the Olympics and the WA/OR Cascades
through Friday and Saturday. Somewhat lighter snowfall
accumulations associated with the spillover of this moisture and
energy are expected across the Intermountain West and northern
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 going
through Saturday. By early Saturday, a somewhat stronger upper
trough will also move inland a little bit farther south and allow
for Pacific moisture and energy to overspread at least portions of
the northern Sierra-Nevada and interior areas of the Great Basin.
This will bring some locally heavy snow for the higher terrain,
and this snow will also develop farther east across portions of
the Wasatch and the northern and western CO high country as
stronger jet-aided ascent arrives with the upper trough along with
favorable upslope flow. Generally for the higher terrain of the
northern Sierra-Nevada east into the CO high country, there will
be some areas of 6 to 12+ inch amounts going through Saturday
night. Some of this snow will linger over the central Rockies on
Sunday, but should be rather modest as the upper trough exits the
region.


...Upper Midwest...

As upstream troughing traverses the Intermountain West through
Saturday, a wave of low pressure will begin the develop over the
central High Plains. On Sunday this low center and supporting
upper-level energy will be moving east-northeast toward the Upper
Midwest which will allow for a surge of warm-air advection and
related frontogenetical forcing to generate an area of generally
light precipitation along and north of the low track. No
significant snowfall is forecast at this time, but sufficient cold
air will be in place for an area of light snow to develop and
overspread portions of especially central/eastern IA, northern IL
and southern WI going through late Sunday with a couple inches of
snow expected at this time.

Orrison