Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
415 AM EST Mon Jan 02 2023

Valid 12Z Mon Jan 02 2023 - 12Z Thu Jan 05 2023

...Western U.S....

Day 1 and Day 3...

The strong shortwave trough is currently centered over the Four
Corners region this morning, and the favorable forcing for ascent
downwind of this feature will continue through today, The
combination of this enhanced lift, orographic effects from the
terrain, and the low-mid level moisture enhancements will help
produce moderate to locally heavy snow across the mountain ranges
through tonight. The greatest accumulations are expected for the
San Juans in Colorado, the Uinta Mountains in Utah, and across
much of central Wyoming. Further to the west, another quick moving
system approaching California will bring mountain snows to mainly
the Sierra where peak accumulations of 8-12"+ are expected.

Heading into the day 3 period, another strong atmospheric river
event is set to impact the region. A closed mid/upper level low
will approach the coast before slowing down and the favorable
plume of higher moisture is expected to be positioned favorably
for enhanced precipitation for California, including heavy
mountain snow for the Sierra. WPC probabilities are already above
70-80 percent for at least 12 inches of snow for the day 3 period
with signals in place showing potential for 1-2 feet possibly for
the highest peaks.

...Central Plains and Upper Midwest...

Days 1-3...

Shortwave energy crossing through the Four Corners and Rockies
this morning will lead to a developing area of low pressure in the
lee of the Rockies this morning. That low will deepen as it tracks
northeast toward the Upper Midwest Tuesday into Wednesday before
slowing down. The upper level shortwave is then forecast to close
off and stall over the Upper Midwest while the primary surface low
lifts east/northeast. That upper level feature will be slow to
exit the region through the end of day 3.

North and west of the low track a swath of moderate to heavy snow
is expected from portions of eastern WY and northeast CO through
the Plains and Upper Midwest. The latest model guidance has been
slowed down and perhaps shifted slightly southward with the axis
of heaviest snow during the day 1 period. Favorable overlap with
the strongest lift within the DGZ and conditional symmetric
instability supports intense snow rates in the deformation band,
perhaps between 1-2"/hr at times. At this time, the highest totals
are expected across central/northern Nebraska, southern South
Dakota and from southwest to central Minnesota where a large area
of 8-12"+ is likely. Based on the latest ensembles and WPC snow
probabilities, isolated/localized totals greater than 12" are most
likely from northern NE and southern SD toward central Minnesota
where probabilities are 20-30 percent. As the surface low lifts
away, the influence of the upper level low closing off will be the
main driver for additional snow totals on day 3 across portions of
the Upper Midwest. Confidence in the placement of the individual
vorts around the upper low still remain a bit uncertain but the
signal for an additional several inches of fluffy snow appears
possible as soundings show better saturation within the DGZ and
environment becomes favorable for higher SLRs.

South/east of the heavy snow axis will be a zone of wintry mixed
precipitation with the potential for significant ice accumulations
from portions of eastern Nebraska through northwest Iowa and
southern Minnesota, possible extending into western Wisconsin. The
greatest probabilities for at least 0.25" of ice accumulation is
from eastern NE to northwest IA and remain in the moderate level
(40-60 percent). A secondary area of freezing rain and ice
accumulation exists for central/north-central WI where one to two
tenths of accretion is currently anticipated.

Taylor


...Key Messages for January 1-3 Winter Storm...

-A large swath of moderate to heavy snow, sleet, and freezing rain
is expected across the Plains and Upper Midwest as an area of low
pressure intensifies as it tracks northeast through Tuesday.

-Intense snow rates of 1-2"/hr may be accompanied by thunder,
especially in southern South Dakota and far southwest Minnesota.
This will lead to rapidly accumulating snowfall of more than 12
inches from the Panhandle of Nebraska through southwest Minnesota.

-These intense rates combined with gusty winds will produce areas
of blowing and drifting snow, resulting in snow-covered roads,
reduced visibility, and difficult travel.

-Localized significant ice accumulations exceeding 0.25" are
likely from portions of northeastern Nebraska through southern
Minnesota. This may lead to dangerous travel and isolated power
outages.

-The multiple precipitation types and storm track uncertainty will
affect the locations and possible impacts due to snow and ice.
Continue to stay updated to the latest forecast information.