Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
239 PM EST Tue Jan 12 2021

Valid 00Z Wed Jan 13 2021 - 00Z Sat Jan 16 2021

...Pacific Northwest and Northern Rockies...
Day 1...

The ongoing strong Atmospheric River will continue through
Wednesday before the driving upper jet pushes eastward and IVT
values drop rapidly. The upper jet streak responsible for driving
the moisture onshore will slowly shift east allowing the best
diffluent region to push inland, while the last in this series of
shortwaves crosses the WA coast Wednesday morning. The combination
of these two features will continue heavy precipitation across the
area, but by the latter half of D1 the most robust ascent, which
will be weakening quickly, will be focused across the Northern
Rockies, leaving very little snow during the aftn across the
Cascades and points west. WPC probabilities are high for 8 inches
in the WA Cascades, Sawtooth range of ID, and into the Northern
Rockies of MT, and Tetons of WY, with local amounts in excess of 2
ft likely in the WA Cascades above 6000 ft and more than 1 ft
elsewhere.


...Upper Midwest...
Days 2/3...

Complex pattern will evolve across the Northern Plains and Upper
Midwest as the final shortwave embedded within the AR mentioned
above pivots eastward and shears to the south. This is expected to
produce a split pattern with one impulse closes off over Manitoba,
and another sharpening into the Central Plains on Thursday. These
two impulses are then likely to interact/merge into an anomalously
deep closed low (500 heights falling to the bottom 1-percentile
according to NAEFS ensemble tables.) This upper low will then spin
slowly nearly in place Friday before slowly ejecting eastward
Friday night while spokes of vorticity and mid-level divergence
rotate around it. At the surface, low pressure over the western
Great Lakes will get captured and likely retrograde westward
Thursday night with subtle deepening, before filling as the entire
system becomes vertically stacked and shifts east D3.

Ahead of the wave, a brief surge of WAA/frontogenesis could
produce a burst of snow or a rain/snow mix, or some light freezing
rain Thursday morning/aftn. Most of this precipitation is expected
to be light, but a few hundredths of an inch of freezing rain are
possible, focused primarily across eastern MN into WI. Snow on D2
should be very light accumulating to just an inch or two in some
places. More significant snowfall is likely on D3, but significant
model discrepancies both with placement and timing of the
important features lead to lower than average confidence at this
time. The forecast tended towards the ECENS/GEFS means for
consistency and to reduce some of the spread, and it is likely
that periods of light to at times moderate snow will rotate around
the closed low as far west as ND Thursday night and Friday. A weak
but elongated TROWAL is progged to circle the upper low to the N
and W providing at least subtle enhancement in moisture and
forcing. While snowfall at any given time is expected to remain
light, persistent and long duration light snow should still
accumulate, becoming more efficient as the SLRs climb due to
subtly more intense ascent within a cooling column. WPC
probabilities for 4 inches are as high as 50% across parts of MN
and WI, highest along the Arrowhead close to the upper low center,
and south of Lake Superior near the Door Peninsula where some lake
enhancement is possible.


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

Weiss