Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Mon Nov 09 2020

Valid 12Z Mon Nov 09 2020 - 12Z Thu Nov 12 2020


...Four Corners....
Day 1...

Anomalously deep upper low will pivot across the Great Basin and
the Four Corners on Monday. Embedded within this larger gyre, a
shortwave will open briefly as it ejects from AZ northeast, before
sharpening once again as it moves into the Southern Plains. This
feature will be accompanied by a poleward arcing jet streak
placing the favorable LFQ for ascent atop the Four Corners,
providing a brief but intense period of ascent coincident with
some enhanced pacific moisture. This combination will produce
periods of precipitation across the region, with snow occurring at
elevations above 3000-4000 ft. Periods of snow could be heavy at
times, especially within the San Juans due to orthogonal mid-level
flow to enhance ascent, but may persist longer across the CO
Rockies and even into the Uintas of UT. So despite weaker snow
rates, total accumulations Monday could be about the same as in
the San Juans, and WPC probabilities are moderate for 6", with
isolated amounts to 10" possible. As the low pulls away late
Monday, the forcing for ascent ejects eastward bringing an end to
the snow.


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

Yet another shortwave will dig into the Pacific Northwest tonight,
moving onshore WA state before elongating and rotating within the
larger cyclonic flow into the Northern Rockies by Wednesday night.
This shortwave will be accompanied by a weak Pacific jet streak to
transport moisture onshore, with confluent mid-level flow focusing
moisture from the WA/OR coast inland into MT and WY through
late-week. Periods of snow and snow showers are likely each day,
first in the Olympics and WA Cascades, and then spreading eastward
as far as the Tetons and Uintas by D3, with snow levels generally
1000-3000 ft, lowest near the Canadian border. The guidance
indicates periods periods of stronger omega which would enhance
snowfall at times, and 3-day totals could approach 2 ft in the
Cascades and Tetons. In general, however, WPC probabilities each
day are high for 6 inches, first in the Cascades, and then
spreading southeast through the forecast period.


...Central Plains to Upper Midwest...
Day 2...

A cold front will move quickly across the Central Plains and into
the Midwest today into Tuesday, leaving a sharp baroclinic zone
across the region. Atop this gradient, a potent shortwave will
lift northeast into the Midwest, while a secondary shortwave
closes off behind it to cause a slowing and interaction of the
two. At the same time, a jet streak downstream of the primary
trough axis to the west will intensify, placing a divergence max
in the LFQ atop the gradient as well, working in tandem to drive
surface cyclogenesis on Tuesday. While the guidance still features
considerable spread in timing and position, a blend that weighs
heavily towards the slower/stronger guidance was preferred today.
As this low develops, the column will initially be too warm for
frozen precipitation. However, as CAA continues behind the front
and dynamic cooling occurs through forcing and into the
strengthening low pressure, precipitation will expand and
changeover to snow, especially well NW of the surface low. The
fast movement of the system, antecedent warm temperatures, and
modest signal for banding suggests the heavy snow will be somewhat
limited. However, WPC probabilities do indicate a moderate chance
for 4 inches of snowfall, highest in the Arrowhead of MN.
Southeast of this snow, a transition area will occur and some low
level CAA could lead to light freezing rain accretion across parts
of IA and MN. WPC probabilities for 0.1" of accretion are
generally 10% or less.

Weiss