Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EST Wed Feb 03 2021

Valid 12Z Wed Feb 03 2021 - 12Z Sat Feb 06 2021

...Midwest/Great Lakes...
Days 1-2...

Split stream shortwaves will advect eastward across western Canada
and the Pacific Northwest today before moving into the Northern
Plains Thursday morning. As they advect east, deepening is likely,
as the northern stream shortwave is progged to dive sharply
southeastward towards the Great Lakes while closing off in
response to interaction from the southern stream energy lifting
northeast through the base of the amplifying longwave trough.
While these features intensify, a strengthening subtropical jet
streak will lift across the Central Plains, placing the
increasingly diffluent LFQ atop the region, which will act
together with height falls and mid-level divergence to drive deep
layer ascent. This robust ascent will impinge upon a lingering
surface baroclinic boundary draped NE to SW into the Plains, along
which a surface low is likely to develop. As this low lifts
northeast, rapid intensification is likely, and it is possible the
surface low will "bomb out" by Friday morning while lifting into
Ontario, Canada.

Strong WAA/isentropic ascent will spread moisture northward while
expanding in coverage. Initially, this may lead to a period of
freezing rain from Iowa and Illinois northeastward. However, as
the precip lifts further north, it should quickly change to snow
due to a cooler column north, or rain further south. As the low
deepens and lifts northeast, it is likely that intense
frontogenesis will develop on the back side of the system
coincident with strengthening deformation. This should dynamically
cool the column from NW to SE, and lead to a rapid changeover for
most of the area that is rain for the first half of the event. In
addition, very strong winds are likely, and near blizzard
conditions are possible despite limited snow accumulations for
parts of the region. The heaviest snow Thursday is likely across
WI and the U.P. of MI where strong WAA into the cold column will
produce snowfall rates that may eclipse 1"/hr, and WPC
probabilities are greater than 50% for 6", with higher
probabilities shifting into the L.P. of MI Friday. As the low
pulls away, intense cold advection across the Lakes will produce
efficient LES downwind of Lake Superior and Michigan, with the
favored snow-belts on N/NW flow showing a moderate risk for 6"
again on D3.

While accumulations will likely be much less for eastern IA and
northern IL, and WPC probabilities only indicate a 20-40% chance
for 4 inches, the rapid changeover due to intense fgen/deformation
could create intense snowfall rates combined with strong winds.
This could include the Chicago and Milwaukee metro areas, creating
a treacherous evening commute on Thursday.


...New England...
Day 3...

The same low pressure system moving through the Great Lakes on
Thursday will occlude to a triple point across the Mid-Atlantic
Friday, with subsequent low pressure development likely into New
England D3. Strong WAA ahead of this secondary low will spread
precipitation northward late Thursday night through Friday. While
initially this could lead to a narrow temporal window of freezing
rain as warm air overruns cold surface high pressure, it is likely
that much of the region south of VT/NH/ME will changeover to rain
on Friday. Further north into these states, however, a period of
heavy snow is possible, and WPC probabilities are 20-40% for 4
inches, highest in the terrain.


...Western United States...
Days 1-3...

A potent trough digging into the Pacific Northwest today will
amplify into a deep longwave trough over the central CONUS by
Friday morning, leaving long duration NW flow across much of the
West. Within this NW flow, Pacific moisture will continually
spread across the region through 500-700mb confluent flow, and a
persistent upper jet arcing southward from the Gulf of Alaska
through the Central Plains. Embedded within this flow, periodic
omega maxima will drop southeast, first with the leading
shortwave, and later with minor impulses and associated divergence
maxima.

As the lead shortwave drops southeast from the Pacific Northwest
into the Central Plains D1, it will drive a surface low onshore
before weakening over Oregon this morning. Heavy snow is likely
south of this feature into the Shasta/Trinity/Siskiyou ranges of
Northern California, as well as with the best height falls into
the Central Rockies, Wasatch, Tetons, and Big Horns. WPC
probabilities for 8 inches are moderate to high in these ranges.
After a brief respite behind this system, late D2 and D3, forcing
will intensify once again from NW to SE, aided by a cold front
banking against the Northern Rockies from the northeast and as
surface wave moving along it. This will produce heavy snow late D2
in the WA Cascades and Northern Rockies, with 8 inch probabilities
as high as 80%, and more than 12 inches likely above 3000 ft. As
the cold front sags slowly southward and moisture/forcing continue
to overlap and drop southeastward, heavy snow will spread on D3
from the Northern Rockies through the WY ranges and back into the
Colorado Rockies. Snow levels are expected to drop to below 2000
ft in these ranges, and WPC probabilities are again high for 8
inches.


Weiss