Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EDT Mon Apr 18 2022

Valid 00Z Tue Apr 19 2022 - 00Z Fri Apr 22 2022

...West Coast to the Northern Rockies...
Days 1-3...

Two shortwaves accompanied by Pacific jet streaks will impact the
West through late week, separated by a brief period of shortwave
ridging. The first of these will lift onshore WA state tonight
into Tuesday while the best Pacific jet energy surges onshore near
the OR/CA border. Snow levels will be falling through D1 behind a
surface cold front which moves onshore tonight, and heavy snowfall
is likely to spread through the Olympics, along the length of the
Cascades into the Sierra, the Shasta/Trinity area, and into the
Northern Rockies, generally above 2000-3000 ft. WPC probabilities
for more than 6 inches feature a moderate to high risk in these
areas, with locally more than 12 inches possible. The Cascade
passes may also experience moderate snowfall accumulations,
including Snoqualmie, Santiam, and Willamette Passes.

A brief respite on D2 occurs as shortwave ridging moves across the
Northwest, but the next system will move in quickly late D2 and
especially D3 with an even stronger closed low digging into the
Pacific Northwest accompanied by another frontal system and upper
level jet streak. Moist advection with this next system will be
pushed a little farther south than with the first impulse, so the
heaviest snow is expected in the CA ranges. WPC probabilities D3
are high for 6 inches or more in the Sierra and Shasta/Trinity
area, with more than 12 inches appearing likely in the higher
terrain of the Sierra.


...Northern MN...
Day 2...

A potent closed mid-level low will track just north of the
Canadian border Wednesday, producing downstream divergence and
height falls into the Upper Midwest. A wave of low pressure
beneath the closed low will occlude to a secondary trough over MN,
ahead of which robust WAA surging northward from the Gulf of
Mexico will drive a shield of precipitation into the state. The
thermal profiles for this event appear very marginal for snow, and
much of the strongest ascent becomes focused beneath the DGZ
suggesting dynamical cooling will not help overcome the low-level
warmth. This suggests that the greatest chance for any significant
snow accumulations will be confined to area N/NW of Duluth where
WPC probabilities for 4+ inches are as high as 40%. As is typical
with southerly flow event, local enhancement is likely in the Iron
Ranges of the Arrowhead along the north shore of Lake Superior
where additional moisture and orographic enhancement will produce
some local maxima of snowfall where WPC probabilities for more
than 6 inches reach 50%.


...Northeast...
Day 1...

A deepening low pressure system will continue to move up the coast
and into Southern New England tonight and Tuesday before ejecting
into Canada by Wednesday. This low will deepen in response to
phasing shortwave energy merging into a closed upper low, with the
resultant downstream divergence helping to deepen the cyclone. As
this low strengthens and moves northeast, pronounced WAA will
expand the precipitation shield northward through PA, Upstate NY,
and much of New England. The surface low tracking across Long
Island and northward through eastern VT keeps most of southern New
England and eastern New England all rain except in the highest
terrain of Maine. However, VT, Upstate NY, and northern PA will
likely receive significant snowfall. The heaviest snow will occur
within a strong deformation band which is progged to lift
northward with snowfall rates that may exceed 2"/hr according to
the HREF probabilities. Despite a marginal thermal structure in
the low-levels of the column, this intense ascent and heavy snow
could produce accumulations even into the lower valleys of VT and
Upstate NY. However, the heaviest snow is likely above 2000 ft in
the Poconos, Catskills, Adirondacks, and west into the Tug Hill
Plateau where WPC probabilities for more than 6 inches are 70-90%,
and locally more than 12 inches is likely in the highest terrain.
A continued subtle westward expansion of the precip noted in
guidance this aftn has led to an increase in snowfall
probabilities as far west as the Finger Lakes, where WPC
probabilities for more than 4 inches are over 50%. As the low
pulls away late D1, additional moderate snow through upslope NW
flow and on lake enhanced moist ascent into the Chautauqua Ridge
could also add up to more than 4 inches of accumulations before
snowfall winds down into D2.

Although less significant, the guidance has remained aggressive
with the northward push of a robust dry slot early Tuesday which
will rapidly dry the DGZ from the top down. Regional forecast
soundings indicate a loss of cloud ice across much of eastern
Upstate NY and VT by early Tuesday, and this could manifest as a
period of light freezing rain/drizzle. WPC probabilities for 0.1"
of freezing rain are less than 5%, but a few hundredths of an inch
of accretion are possible in the Catskills and Adirondacks.

The probability of significant icing across the CONUS is less than
10 percent.


Weiss