Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 PM EDT Wed Mar 15 2023

Valid 00Z Thu Mar 16 2023 - 00Z Sun Mar 19 2023

...Southern & Central Rockies...
Days 1-2...

A shortwave trough traversing the Four Corners region tonight will
provide sufficient lift within the atmospheric column, along with
ample Pacific moisture aloft, to generate heavy mountain snow
throughout most of the Colorado Rockies and into northern New
Mexico. Vertical ascent will be maximized along terrain oriented
orthogonally to the mean 850-300mb mean flow, making these areas
most likely to see heavier snowfall rates. Most of the terrain
mentioned are likely to experience 1-2"/hr snowfall rates tonight,
including the highest peaks of the Mogollon Rim and the White
Mountains of Arizona. WPC PWPF shows the San Juans with the best
odds (80% probabilities) of picking up >8" of snowfall between 00Z
Thus - 00Z Fri. By Thursday morning, a cold front racing south
through the central High Plains will introduce a surge in
850-700mb moisture flux into the Sangre De Cristo mountains.
Upslope northeasterly flow into the range, combined with the
aforementioned diffluent flow ahead of the upper trough, fosters a
favorable setup for heavy snowfall. WPC PWPF shows 80-90%
probabilities for >8" of snowfall on Thursday, with up to 70%
probabilities for >12" in the tallest peaks of the Sangre De
Cristo mountains. The Days 1-3 WSSI shows Major to even Extreme
impacts in the San Juans and neighboring Colorado Rockies, with
widespread Moderate impacts along the Sangre De Cristo. Minor
impacts doe extend north along the Palmer Divide and along the
Front Range west of Denver where WPC PWPF is suggesting there are
30-50% probabilities for >4" of snowfall through Thursday morning.

...Upper Midwest & Northern Great Lakes...
Days 1-3...

A pair of shortwave troughs; one emerging out of the Southwest and
the other out of the northern Rockies, will provide large scale
ascent throughout the northern and central Plains this evening. By
Thursday morning, the central Plains resides beneath two diffluent
flow regimes at 250mb; one beneath the right-entrance region of a
110 knot jet streak over northern Minnesota, and one beneath the
left-exit region of a 110 knot jet streak in Arizona and New
Mexico. At the surface, a wave of low pressure organizing in lee
of the central Rockies this evening will become the focus for a
developing winter storm tomorrow. Out ahead of the storm within
the warm sector, an impressive 60 knot 850mb jet Wednesday night
across the central Plains will introduce rich Gulf of Mexico
moisture into the Midwest. As the cold front to the north
approaches, it will become oriented from west-central Nebraska to
southern Minnesota late Wednesday night and into Thursday. A band
of heavy snow will then take shape along a sliver of 850-700mb
frontogenesis, which combined with the rich moisture present and
strong synoptic-scale ascent, will produce heavy snowfall rates.
The WPC Snowband Probability Tracker, utilizing the 12Z HREF,
shows the majority of members depicting 1-2"/hr snowfall rates
from central Nebraska, on north and east into the Upper Midwest.
Given the snow band is occurring during the day, it will be those
areas directly beneath the band that could see rapid
accumulations. The WSSI does depict Moderate impacts in areas
beneath the heavy snow band, suggesting there will be some
disruptions to daily life. This means for locations on the flanks
of the band snow will be harder to accumulate, making the actual
snowfall footprint for >4" totals more narrow. WPC PWPF shows
20-40% probabilities for >6" in northeast Nebraska and southwest
Minnesota, while the probabilities for >4" are larger,
encompassing more of northern Nebraska, southeast South Dakota,
and northwest Iowa. The strong cold frontal passage will also
produce 30-40 mph win gusts, which even in areas outside the
primary band of heavy snow, may lead to blizzard conditions in
some areas.

As the southern wave quickly tracks northeast out ahead of the
main trough over the Dakotas on Thursday, a secondary disturbance
diving south from central Canada will carve out a deeper trough
over the Upper Midwest and northern Great Lakes Thursday night
into Friday. The increasingly diffluent flow pattern aloft and
strong WAA over the Great Lakes will prompt the formation of a
deepening 850mb low over Wisconsin Thursday evening, then tracking
over the eastern Michigan U.P. Friday morning. On the north and
west side of the low, a shield of heavy snow will develop from
central Minnesota and the Minnesota Arrowhead to northern
Wisconsin and the Michigan U.P.. Areas likely to see the heaviest
snowfall for this event are northern Wisconsin and along the
northern half of the Michigan U.P.. WPC PWPF shows 40-60% chances
for >12" of snowfall Thursday evening and into Friday evening,
with notable 10-20% probabilities for >18" in these same
locations. The strong winds of Lake Superior will add to the
reduced visibilities which coincide with up to 2"/hr snowfall
rates in the heaviest bands. The WSSI shows far northern Wisconsin
and the northern coast of Michigan's U.P. with Major impacts on
Friday. WPC is initiating Key Messages for this Midwest winter
storm. They are appended to this discussion below.

...Key Messages for March 16-18 Winter Storm...

--A band of locally heavy snow develops late tonight over
west-central Nebraska with increasing winds as it tracks northeast
through the Upper Mississippi Valley Thursday.

--The system will then pivot east Thursday night while lake
enhanced snow occurs off Lake Superior before a transition to lake
effect snow that continues Friday into Saturday.

--A stripe of locally heavy snowfall is forecast from central
Nebraska through central Minnesota. Snowfall of 1 to 2 feet is
expected over northeast Minnesota, northern Wisconsin, and the
U.P. of Michigan.

--Snow-covered roads and reduced visibility will make travel
difficult across impacted areas. Whiteout conditions are possible
within the heaviest snow bands.

Mullinax