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

Valid 00Z Sat Feb 20 2021 - 00Z Tue Feb 23 2021

...Great Lakes, Mid-Atlantic, and Northeast...
Days 1-3...

On Day 1, low pressure moving away from New England leaving broad
NW flow in its wake, while a secondary mid-level shortwave swings
across the Great Lakes during the day The combination of these two
features will lead to increasingly unidirectional W/NW flow and
strong CAA across the region, setting up a favorable LES
environment. Lake temperatures have cooled recently, and ice cover
has dramatically increased, both which will limit LES intensity.
However, favorable flow and the likelihood of some lake-enhanced
effective fetch across multiple lakes should still support periods
of moderate to heavy snow, especially downwind of Lakes Erie and
Ontario, as well as across the NW L.P. of MI. WPC probabilities
are high for 4 inches in these regions on D1, with locally more
than 6 possible.

After a respite on D2, a more significant low pressure system will
emerge out of the Midwest and extend a warm front eastward into
the Mid-Atlantic. Cold high pressure ahead of this system should
quickly erode and retreat eastward in response to increasing
mid-level divergence, so a strong overrunning scenario is not
expected. This will limit freezing rain the Mid-Atlantic, although
some light accretions are possible Sunday night into Monday.
Elsewhere, a swath of moderate snow is likely as WAA spreads
northward into the Great Lakes and Northeast. WPC probabilities
for 4 inches are as high as 30% D3 in northern PA extending into
parts of the higher terrain of NY and VT, with some enhancement
possible the latter half of D3 downwind of Lake Ontario behind
this system.

...Midwest...
Days 2-3...

A longwave trough moving through the West will shed a piece of
shortwave energy northeast through the Central Plains and into the
Midwest late Saturday into Sunday. This feature will be
accompanied by a robust poleward arcing jet streak placing
significant LFQ diffluence atop the region. The combination of
height falls, PVA, jet level diffluence, and at least modest WAA
ahead of the shortwave will drive surface low development and
expand precipitation northward. Guidance continues to feature a
rather large disparity in the timing and intensity of this
shortwave, but the trend today has been for a subtly deeper trough
and subsequent stronger low with heavier precipitation. This
heavier precipitation may also be accentuated by frontogenetical
banding which looks likely N/NW of the sfc low, but exact
placement at this time is still in question. The transient nature
of this system should limit snowfall amounts, but WPC
probabilities do indicate up to a 30% chance for 4 inches across
parts of IA on D2, shifting into Michigan on D3. Light freezing
rain may also occur ahead of this system at the onset of
precipitation, but accretion should be light.

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

A potent shortwave moving onshore the WA/OR coast early Saturday
will spill southeast through the Great Basin and into the Four
Corners by Sunday, spreading enhanced Pacific moisture eastward to
produce heavy snow across most of the Western Ranges from the
Olympics through the CO Rockies on D1. This shortwave will be
accompanied by a Pacific jet streak which will gradually sink
southward, providing moisture and enhancing ascent. Heavy snow is
likely in many of the ranges, with as much as 2 ft possible in the
NW CA ranges, the Cascades, and more than 1 foot likely in the
Tetons. Otherwise, WPC probabilities are high for 6 inches across
most ranges. As this upper trough continues to swing eastward on
D2, snowfall will generally wind down from west to east, with
lingering heavy snow likely in the CO Rockies and San Juans.

As the leading impulse moves southeastward, renewed moisture in
the form of a modest AR will approach the WA coast as confluent
mid-level flow angles orthogonally into the WA coast, while a 160+
kt jet streak dives southeast from British Columbia eventually
leaving the RRQ atop the Pacific Northwest. Snow will again
overspread the ranges from the Olympics to the Northern Rockies
late D2 and D3 with slowly lowering snow levels. WPC probabilities
are high both days for 8 inches, with up to 4 ft possible in the
higher terrain of the WA Cascades.

Weiss