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Winter Weather Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1855Z May 06, 2024)
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 PM EDT Mon May 6 2024

Valid 00Z Tue May 07 2024 - 00Z Fri May 10 2024

...Cascades through Rockies...
Days 1-3...

A significant late season winter storm, one that dates back to
producing over 2 feet of snow in portions of the Sierra Nevada
this past weekend, is now tracking through the Central Rockies and
will deepen into an impressive cyclone over the northern High
Plains this evening. NAEFS is lit up with some remarkable
anomalies for 00Z this evening that include MSLP values (<984mb)
that fall below the observed CFSR database in southwest ND and
north-central MT, 850mb and 700mb heights that are below the
observed CFSR database for all 00Z observations, and 700mb winds
topping the 99.5 climatological percentile across most of WY and
southern MT. At the same time, a secondary 500mb disturbance
tracking into the Pacific Northwest will keep snow levels lowers
than normal while also providing a steady stream of 850-700mb
moisture flux into the Cascade Range and northern Rockies. Day 1
WPC PWPF shows moderate-to-high chances (50-70%) for snowfall >8"
above 5,000ft in the Cascade Range and Blue Mountains.

This setup will initially be responsible for periods of moderate-
to-heavy snow tonight and into Tuesday morning throughout the
Northern Rockies. One area that has trended snowier in recent runs
of guidance are the Black Hills, where many members of guidance
show >12" of snow, and in some cases >24". This seems far-fetched
given how late in the season it is, but this is a highly anomalous
storm system that is directing copious amounts of moisture overhead
with excellent synoptic scale ascent and topographically- enhanced
snowfall rates expected. Latest WPC PWPF shows moderate- to-high
chances (50-70%) for snowfall totals >12" in the Black Hills
through Tuesday afternoon. Residents in the Black Hills,
particularly living at or above 6,000ft, should monitor the
forecast closely as roads could be quite treacherous by early
Tuesday morning. As Tuesday progresses, this Pacific Northwest
disturbance will help to "tug" on the storm system in western ND
and retrograde west into eastern MT. The end result is a TROWAL
axis over western and central MT that combined with northerly
upslope flow leads heavy snow in the Lewis Range, Little Belt, Big
Snowy, and Absaroka starting near midday Tuesday and lasting
through Tuesday night.

Snowfall rates will pick up in intensity late Tuesday afternoon
and into Tuesday night after sunset over the northern Rockies,
allowing for better opportunities for accumulation with the loss of
daytime heating. By early Wednesday morning, the TROWAL pivots
more to the south to put the Big Horns more at risk for heavy snow
during the day, then the threat for heavy snow shifts as far south
as the Tetons and Wind River Range Tuesday night into Wednesday.
The slow progression of this upper low will keep snow in the
forecast across most of these ranges through Wednesday night and
into Thursday morning, but rates will gradually diminish throughout
the day Thursday as the upper low weakens and finally advances
farther east. In terms of where the heaviest snowfall looks to
occur, generally at/above 6,000ft in the Lewis Range, Sawtooth,
Bitterroots, and Blue Mountains. Heavy snow is likely at elevations
as low as 5,000ft in the Little Belt and Big Snowy Mountains in
central Montana where the strongest upslope enhancement is
expected. Farther south, the Absaroka and Big Horns above 7,000ft
are favored to see heavy snow, with the Tetons and Wind River Range
above 8,000ft favored for heavy snowfall totals.

In terms of totals and expected impacts, as much as 1-2 feet of
snow are expected in the higher terrain >6,000ft in the Northern
Rockies of western MT and northern ID, as well as the >7,000ft
elevations in both northern and western WY. The WSSI shows Major
Impacts (considerable disruptions to daily life) are anticipated in
the Big Belt, Little Belt, and Big Snowy Mountains of central
Montana with snow amount the primary driver in the WSSI algorithm.
Localized Major Impacts are also forecast along the peaks of the
Lewis Range and the Bitterroots due to the expected heavy snowfall
totals. It is worth noting that many of the mount ranges listed in
this discussion sport Moderate Impacts from Blowing Snow, which
will make for treacherous travel conditions from both drifting snow
and significantly reduced visibilities through Wednesday night.

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