Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 PM EDT Tue May 7 2024

Valid 00Z Wed May 08 2024 - 00Z Sat May 11 2024

...Northern Rockies...
Days 1-2...

The upper low responsible for periods of heavy snow in the
Northern Rockies Monday night and Tuesday morning will begin to
track south from southwest ND this afternoon to southwest SD by
Wednesday morning. In addition, the upper low will begin weakening
this afternoon and steadily weaken through Wednesday. Despite the
weakening phase, there is still an anomalous NWlry IVT that is
above the 90th climatological percentile (according to NAEFS) and
will direct a steady diet of 850-700mb moisture into the Northern
Rockies this evening and through Wednesday. Given the time of year,
snow will be harder to accumulate on roads during the daytime
hours, but snow will still accumulate on trees and grassy surfaces
in areas where snowfall rates average close to 1-2"/hr in the
mountainous terrain above 6,000ft in the Lewis Range, Big Belt,
Little Belt, and Big Snowy Mountains of MT, above 7,000ft in the
Absaroka of MT and Big Horns in WY, and above 8,000ft in the
Absaroka, Wind River Range, and Laramie Range in WY. As the IVT
diminishes Wednesday night and the 500mb low continues to weaken
and tracks east, the strength of the upslope flow into these ranges
will lessen, thus decreasing snowfall rates Wednesday night and
into Thursday morning. Outside of the >9,000ft peaks in WY and CO,
most snowfall accumulations will be over with just minor
accumulations expected during the day Thursday.

WPC PWPF through Thursday morning shows high chances (50-70%) for
additional snowfall accumulations >18" in the Little Belt, Big
Snowy, and Absaroka Ranges in MT with similar probabilities
at/above 8,000ft in the Big Horns of WY. In fact, there are
moderate-to-high odds (50-70%) for >24" of additional snowfall in
these ranges. Similar high chance (>70%) probabilities for >8" of
additional snowfall are depicted in the Wind River Range, Laramie
Range, and Medicine Bow in WY above 8,000ft. The Probabilistic WSSI
(WSSI-P) shows moderate-to-high chances (50-70%) for Moderate
Impacts in the Big Belt, Big Snowy, Absaroka, and Big Horns through
00Z Thursday. These mountain ranges, particularly above 6,000ft in
the Big Belt and Big Snowy, along with the Big Horns >8,000ft, can
expect hazardous travel conditions with possible closures and
disruptions to infrastructure. The Little Belt Mountains and their
northern slopes are most likely to witness Major Impacts, indicated
by the WSSI-P showing moderate-to-high chances (50-70%) for
impacts that include considerable disruptions to daily life. This
includes dangerous to impossible travel and widespread closures.
It is also worth noting that blustery winds combined with periods
of heavy snow will reduce visibility and also make for hazardous
travel conditions. The WSSI-P Minor Impacts for Blowing Snow depict
moderate chances (40-60%) in these mountain ranges. Given the
heavy, wet snow and gusty winds forecast, spotty power outages and
localized tree damage may occur.

The probability of significant icing across the CONUS is less than
10 percent.

Mullinax