Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Wed Oct 11 2023

Valid 00Z Thu Oct 12 2023 - 00Z Sun Oct 15 2023

...North Central Rockies & Black Hills...
Days 1-2..

An early season winter storm is set to unfold across the North
Central Rockies today and tonight, then across the Black Hills on
Thursday. The synoptic scale setup includes a "textbook" diffluent
left-exit region of a strong 110kt jet streak being positioned
over these aforementioned areas today and into Thursday. At 500mb
and 700mb, these geopotential heights are pegged to be as low as
the 2.5 climatological percentile over Wyoming, Colorado, and into
the central High Plains early Thursday morning. As the trough
amplifies, a closed low will form at 500mb overnight in southwest
Wyoming while the 700-850mb low forms in lee of the Rockies over
western Nebraska. As the 850mb low strengthens tonight, the LLJ in
the Great Plains will ramp up in intensity, delivering copious
amounts of 850-700mb moisture flux northward and wrap around the
700mb low, giving rise to a robust warm conveyor belt and
resulting deformation axis of precipitation stretching from the
North Central Rockies to the Black Hills. The IVT is quite
impressive with NAEFS showing the IVT topping the 97.5
climatological percentile over southwest SD and eastern WY. Thanks
to these towering mountain ranges being ideally positioned
orthogonally to the mean 500-850mb wind flow, strong vertical
ascent will be present not just due to the synoptic and mesoscale
forces aloft, but through strong orographic ascent.

Temperatures at higher elevations, particularly above 8,000ft,
appear sufficiently cold enough with the help of strong, dynamic
cooling to support heavy snowfall accumulations. At their peak,
snowfall rates could top 2"/hr tonight and into Thursday. The
mountain ranges with as much as a 50-80% chance for snowfall
accumulations >12" in a 24-hr span include the Absaroka, Wind
River, Bighorns, Laramie, and Medicine Bow. The Black Hills
meanwhile have up to 30-40% probabilities for >12" of snowfall.
WSSI is keying in on two particular impacts; Snow Load and Blowing
Snow. Snow Load looks to be a particularly big deal in the Black
Hills (especially above 6,000ft) as the WSSI shows Moderate to
even localized Major impacts possible. Blowing Snow is driven by
35-45kt winds at 850-700mb throughout the region mixing down
within the deformation axis. These wind speeds are as high as the
97.5 climatological percentile according to NAEFS, and the ECMWF
EFI is as high as 0.8-0.9 in the Black Hills and mountain ranges
of Wyoming. Latest WSSI shows Moderate Impacts throughout the
mountain ranges listed, including areas farther south along I-25
near Cheyenne where snowfall totals will be lighter, but the
combination of 50+ mph wind gusts and moderate snow could lead to
significantly reduced visibilities. The storm will track east into
the heart of the Midwest Thursday night, eventually taking its
precipitation shield with it and ending the winter storm in the
North Central Rockies and Black Hills by Friday morning.

Days 1-3, the probability of significant ice accumulation across
the contiguous U.S. is less than 10 percent.

Mullinax