Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
346 AM EST Fri Feb 19 2021

Valid 12Z Fri Feb 19 2021 - 12Z Mon Feb 22 2021

Days 1 to 3...

...Lower Great Lakes/Mid-Atlantic and Northeast...

One last round of precipitation associated with an elongated upper
jet streak will continue early today from the Mid-Atlantic across
southern New England.  As the wave of low pressure develops off
the mid Atlantic coast, one more round of warm-air advection and
moisture back into the lingering shallow cold air over the
interior of the Mid-Atlantic will lead to additional freezing rain
into the morning hours over
eastern Virginia, with patchy freezing drizzle up across eastern
MD to NJ and metro New York/western Long Island.

The longer duration snow is expected over eastern New England as
the wave develops off the coast.  For a period today,  easterly
flow streaming onshore along the coast of MA and southern NH leads
to enhanced coastal convergence and results in a period of steady
accumulating snow, with several inches expected.  The winds veer
and convergence weakens tonight as the wave moves further
offshore.

An area of low pressure crossing lakes Erie and Ontario will
enhance low level convergence as it drifts east northeast today. 
This leads to a period of snow.  In its wake, boundary layer west
northwest flow across Lake Ontario sets up confluent flow that
leads to a few bands of lake effect snow that focuses on the
southeast corner of Lake Ontario Saturday into early Sunday.
A band of low level convergence along the eastern shore of Lake
Erie leads to a period of snow across areas from Buffalo NY and
then the south towns through Sat night.  On Saturday night/Sunday,
a low level ridge approaches from the west, advecting drier air
across first Lake Erie and then Lake Ontario.  This should lead to
reduced coverage/intensity, with lighter amounts on Day 3/Sun. 

...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 WA/OR Cascades, ranges of ID, and
the WY Tetons through Saturday.  The upper level jet gradually
sinks south in response to the forward progression of the upper
trough moving inland into the Great Basin, allowing the snow on
Sat to spread south from the Tetons into the UT Wasatch and then
the ranges of western CO.  As much as 2 feet of new snow is
expected for the Cascades, with as much as a foot for the northern
Rockies including the Bitterroots, Sawtooth and Tetons. The snow
tapers in the ranges of WY/UT/CO as the upper trough progresses
through late Sat night and Sun.

Snow redevelops in the the WA Cascades on Sunday, as the 00z ECMWF
indicates the upper jet becoming highly difluent with embedded
upper divergence maxima crossing northern WA into northern ID and
northwest MT.  Soundings show deep moisture and the 850-700 mb jet
increasing to 50 kt, providing sustained moisture advection
inland.  Because the favorable conditions persist for most of the
forecast period, locally heavy snow is expected to pile up 1 to 2
feet in the northern WA cascades, and up to a foot in the ranges
of northern ID and northwest MT.
The 3 day totals indicate 3-4 feet of snow is possible in the
northern WA Cascades. 

...Upper Midwest/Upper Great Lakes...

A 700 mb wave moves east of the WY/CO ranges onto the high Plains
Saturday evening, continuing east northeast to the upper MS Valley
early Sun and then the upper Great Lakes Sun night, with a
trailing front down into the lower OH Valley.
The 00z ECMWF suggests a possible closed circulation center
embedded within the wave, so precip coverage/intensity picks up on
Sunday.
Where it remains cold enough for snow, 2-4 inches is possible in
portions of Iowa to southern WI/adjacent northern IL and in lower
Michigan.  A lake enhancement in the vicinity of southern Lake
Michigan may lead to slightly higher totals in western lower
Michigan on Sun-Sun night.

Petersen