Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 PM EST Mon Jan 11 2021

Valid 00Z Tue Jan 12 2021 - 00Z Fri Jan 15 2021

...Upper Midwest...
Day 3...

A closed low will deepen over central Canada Wednesday before
plunging into the Northern Plains and Upper Midwest Thursday.
While guidance still varies considerably in the strength and
evolution of this feature, a blend of the ECENS/GEFS means which
were near the center of the guidance cluster were preferred for
this iteration. This suggests an anomalously deep closed mid-level
low will drive a cold front and arctic intrusion into the region,
with a wave of low pressure developing along the baroclinic
boundary. This low is likely to become vertically stacked and
intense across MN/WI by the end of D3, with precipitation
spreading across much of the Upper Midwest. Ahead of the front,
WAA will drive expanding precipitation, some of which is likely to
be freezing rain into MN/IA/WI, although the duration and
intensity of freezing rain should be light. As the low wraps up,
the combination of WAA and spokes of vorticity rotating around the
primary mid-level gyre will produce periods of moderate snowfall.
WPC probabilities for 4 inches are as high as 20% in northern MN
for D3, with additional snow possible beyond this forecast period.


...Pacific Northwest and Northern Rockies...
Days 1-3...

A strong Atmospheric River (AR) is beginning tonight and will
persist into Wednesday before shunting to the east. Strong
850-500mb confluent flow will drive IVT as high as 800 kg/m/s
onshore, focused into OR, for a duration of at least 24 hours.
Impulses embedded within this flow will enhance ascent already
robust through the persistent LFQ of an upper jet streak
positioned along the WA/OR coast, with associated frontal
boundaries aiding in low-level convergence. QPF is likely to be
excessive in this persistent moist flow with intense lift, and at
the high elevations this will lead to exceptional snowfall.
However, WAA on subtly backed SW 700-500mb flow ahead of the
fronts will raise snow levels to as high as 7000 ft Tuesday,
crashing back to as low as 3000 ft behind the cold front late D2
into D3, but by this time most of the precipitation will have
ended. This implies that the exceptional snowfall, which could
exceed 5 ft in places, will be confined above 7000 ft, and in the
ranges most orthogonal to the most intense moisture/forcing
overlap which would be the WA Cascades.

The heaviest snow on D1 is likely in the WA Cascades where WPC
probabilities are high for 12 inches, and locally 3 ft is
possible. In the other ranges including the Olympics, Northern
Rockies, and Blue Mountains of OR, WPC probabilities are high for
6 inches. On D2, the pattern remains mostly unchanged with a
continuation of the strong AR and another impulse moving inland.
This continues the heavy snow risk across these same ranges with
high 12 inch probabilities persisting in the WA Cascades, and 6"
probabilities of greater than 40% extending into the Tetons of WY.
By D3 the jet streak shifts eastward and longwave ridging bulges
up from off the coast of CA. This will bring an end to the heavy
snowfall. During this event, snowfall at pass levels in the WA
Cascades could reach 6-12".


...Eastern Great Lakes...
Days 1-2...

A series of weak shortwaves will move across the Great Lakes
through Wednesday leaving broad cyclonic flow in their wake. This
will produce modest but favorable conditions for LES downwind of
Lake Erie and Ontario Monday night through Wednesday morning
before forcing wanes the latter half of D2. Briefly heavy snow
rates are possible, especially into the Tug Hill Plateau east of
Lake Ontario. However, low EL's at least modest directional wind
shear, and a DGZ which is lacking deep moisture should preclude
significant snow amounts. There are some low probabilities for
1"/hr snowfall rates during this time, but WPC probabilities
indicate an extremely low chance for 4" of snow on either D1 or
D2, with total accumulation remaining below 6" for the 2 days.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Weiss