Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Tue Apr 30 2024

Valid 00Z Wed May 01 2024 - 00Z Sat May 04 2024

...Pacific Northwest and Northern Rockies...
Days 1-2...

A large upper low over the northwestern U.S. and southwest Canada
will inject a steady stream of Pacific moisture and positive
vorticity advection over the Pacific Northwest and Northern Rockies
through tonight. Lift will be maximized aloft thanks to the
divergent left-exit region overhead through tonight while mean
850-300mb wind flow out of the west will support upslope
enhancement
in the Oregon Cascades, the Sawtooth, Tetons, and Absaroka. By
Wednesday morning, low pressure over southern Alberta will pivot
southward and shift low level winds out of the N-NE. This, in
addition to the pivoting deformation axis of precipitation
advancing southward throughout the day, will lead to upslope
enhancement on the eastern slopes of the Lewis Range (including
Glacier NPS) through Thursday. Cyclonic flow on the western
periphery of the low will also result in upslope snowfall
enhancement in mountain ranges such as the Absaroka, Tetons, Little
Belt, Big Snowy, and other neighboring mountain tops of at least
6,000ft in elevation. Snow will finally come to an end by Thursday
night as high pressure builds in from southwest Canada.

For the duration of this snow event, WPC PWPF shows high chances
(>70%) for snowfall amounts >12" in the Little Belt Mountains and
along the Lewis Range with moderate-to-high chances for >18"
(including Glacier NPS). Mountain ranges with moderate-to-high
chances for >8" of snowfall include portions of the Oregon
Cascades, the Absaroka, Tetons, and tallest peaks of the Big
Horns. The WSSI depicts mostly Minor Impacts in these mountain
ranges with the Lewis Range, given their higher probabilities for
>12" snowfall totals, most likely to witness Moderate Impacts
through Thursday.

...Central Rockies...
Days 2-3...

By Wednesday afternoon, low pressure forming in lee of the Central
Rockies and positive vorticity advection out ahead of a longwave
trough centered over the Great Basin will utilize a sliver of
Pacific moisture aloft to generate a ribbon of precipitation from
the Colorado Rockies to the Nebraska Panhandle. Further enhancing
the strong vertical ascent aloft is the region's placement beneath
the diffluent right-entrance region of a 250mb jet streak which
combined with falling freezing levels will allow for precipitation
to fall in the form of snow in the Colorado Rockies and both the
Medicine Bow and Park Ranges of southern Wyoming. Snow will fall
heaviest between 21Z Wed - 09Z Thursday and quickly conclude by
early Thursday morning as the area of low pressure in lee of the
Central Rockies races east into the Central Plains. As is the case
this time of year, the heaviest totals will be confined to the
highest elevations, particularly above 9,000ft. WPC PWPF shows low
chances (10-30%) for snowfall amounts >8", but it does show
moderate-to-high chances (50-70%) for >4" of snowfall. Overall,
impacts will generally top out on the minor-side (some winter
driving conditions, use caution while driving) as depicted by the
WSSI-P which shows low chances (10-30%) for Minor Impacts late
Wednesday and through early Thursday morning. Attention then
shifts
to a fast moving upper level disturbance that will produce
additional mountain snow in similar mountains ranges and elevations
Friday morning and into Friday afternoon. WPC PWPF continues to
show low chances (10-30%) for snowfall totals >8", and the WSSI-P
shows low chances (10-30%) of Minor Impacts along the Front Range
of the Central Rockies through Friday afternoon.

The probability of significant icing is less than 10 percent.

Mullinax