Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Sat Feb 20 2021

Valid 12Z Sat Feb 20 2021 - 12Z Tue Feb 23 2021


...Pacific Northwest through the Northern and Central Rockies...
Days 1-3...

Multiple waves of deep layer warm/moisture advection are expected
to produce prolonged snow with a couple of days of heavy snow
leading to impressive snow amounts of several feet in the WA
Cascades.

As the leading impulse moves southeastward, renewed moisture
streams onshore in confluent mid-level flow, while difluent flow
aloft Sat night leads to divergence maxim a crossing western WA
State.  Snow increases in coverage and intensity in the Olympics
and northern WA Cascades,
with locally up to a foot of snow expected.

On Day 2, the moist confluent flow continues, with a 160+ kt jet
streak dives southeast from British Columbia eventually leaving
the right entrance region atop the Pacific Northwest. Snow will
again overspread the ranges from the Olympics to the Northern
Rockies with slowly lowering snow levels.
2 to 3 feet are forecast in favored mountains of the northern WA
cascades, with up to afoot inland over the ranges of northern ID
and northwest MT expected.

On Day 3, the upper jet continues inland and with an upper trough
developing in WA/OR, the jet max persists in northern ID and
western MT a little longer than WA.  With continued deep layer
ascent, heavy snows are highly likely across the mountains of ID
and northwest MT, with an additional 1 to 2 feet expected both in
these ranges and also the WA Cascades. Confidence is above normal
given the persistent deep moisture and strong, slow moving upper
jet. Also, the NAM soundings in the WA Cascades indicate steeper
lapse rates near 8 deg c/km Mon night, so response to lift will be
enhanced in WA State owing to favorable instability.

A potent shortwave moving across OR/CA early today will progress
southeast through the Great Basin and into the Four Corners by
Sunday morning, spreading enhanced Pacific moisture eastward to
produce snow across the ranges of NV/UT/CO Rockies on D1. This
shortwave will be accompanied by a Pacific jet streak which will
gradually sink southward, providing moisture and enhancing ascent.
Several inches are likely in favored terrain in these areas, with
locally as much as a foot of snow.

...Great Lakes Day 1...

On Day 1, snow showers will be widespread in western New York as 2
different streams of flow converge, leading to pooling and
moisture and lift.  One stream moves from west to east across Lake
Erie, and has already lead to widespread snow showers south of
Buffalo in near shore areas of southwest NY.  Streamers from this
area will continue to moves east into western NY today.  The
second stream of lake effect snow is from northwest flow across
Lake Ontario that moves onshore and inland into western NY and the
eastern Finger Lakes region.

Favorable flow and the likelihood of some lake-enhanced effective
fetch across multiple lakes should still support periods of
moderate to heavy snow inches in these regions today, with band of
4-6 inches expected.  As a low level ridge builds in western NY,
drier air advects across the region and the low level convergence
wanes as well, so snow shower coverage/intensity should decrease
overnight. 


...Midwest/Great Lakes, and Northeast Days 2-3...

A 700 mb trough moving through the northern Plains will reach near
the NE/IA border by 12z Sun, with the UKMET/ECMWF indicating a
closed low developing.  Near the circulation snow develops across
eastern SD and NE, continuing in tandem with the low across Iowa
and southern WI.
The continued steady progression across WI and MI and then into
the lower Lakes by 12z Mon leads to modest snowfalls across MI. A
brief enhancement off Lake MI in the Ludington area may lead to
4-5 inch totals with most of the rest of lower MI received 2-4
inches of snow.

On Monday, the wave continues to move east across PA, New York,
and then New England.  Low level southerly flow brings warm
advection  across
PA and NY, with the low level southerly flow enhancing ascent in
windward terrain from the Tug Hill to the northern Mohawk
Valley/southern Adirondacks of NY, where several inches of snow
are expected. The steady forward movement of this system precludes
a more widespread heavy snow event. Probabilities for 4 inches are
as high as 30% D3 in northern PA and terrain of VT/NH, peaking
near 70 percent in the southern Adirondacks of NY.

The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Petersen