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FOUS11 KWBC 300733
QPFHSD

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

Valid 12Z Tue Apr 30 2024 - 12Z Fri May 03 2024

...Pacific Northwest and Northern Rockies...
Days 1-3...

Winter maintains its grasp across the Intermountain West as a large
scale trough amplifies over the region. The amplification begins
today as two shortwaves rotate through otherwise confluent mid-
level flow from the Pacific and across the Western CONUS. The lead
shortwave will pivot north into the Northern Plains late tonight,
followed almost immediately by a secondary impulse dropping into
the northern Great Basin on Wednesday. The interaction of these
features will result in gradually lowering heights as an expansive
closed low expands from the Pacific Northwest through the northern
High Plains. Spokes of vorticity rotating around this feature will
provide rounds of enhanced deep layer ascent, aided by periods of
jet-level diffluence as Pacific jet streaks stream overhead.

Much of this forcing will be occurring in a cooling atmosphere in
the wake of a cold front moving through the Central Rockies D1.
This will allow snow levels to fall to around 2000 ft, or even
less, across the Cascades and interior Pacific Northwest, with
slightly higher snow levels of 3000-4000 ft elsewhere. This will
nevertheless be low enough to cause impacts to mountain passes
causing hazardous travel in some areas, although in general,
outside of areas with more intense upslope flow, forcing will be
broad and lack strong focus.

The exception to this is likely to be in the Northern Rockies from
the Lewis Range southward to the Little Belt/Big Belt Ranges, and
possibly as far south as the Big Horns D2-3. In this region, a
surface low dropping south out of Canada will drive a secondary
cold front into the northern High Plains, with post-frontal flow
providing favorable upslope ascent into the eastern facing slopes
of the Rockies. At the same time, a modest extension of a lingering
theta-e ridge will pivot cyclonically to the south, serving as a
residual TROWAL, to enhance both moisture and ascent, and
accompanied by a pivoting axis of deformation. This appears to
align favorably to drive the most intense lift into the DGZ,
coincident with elevated SREF probabilities of the DGZ depth
exceeding 50mb. This will likely support snowfall rates in excess
of 1"/hr, and significant late-season snow accumulations are likely
in these ranges as reflected by WPC probabilities for 6 or more
inches of snow reaching 70-90% D2 near Glacier NP, and 50-70% in
the Little Belt Range. D3 probabilities fall to around 10-30%, but
extend down into the Big Horns as well. Event total snowfall in
excess of 12 inches is likely in some areas.

Elsewhere across the West, WPC probabilities are high for 6+
inches in the Cascades D1 during a period of stronger orographic
ascent on zonal mid-level flow, and moderate for at least 4 inches
of snow D2-D3 in other areas of the Intermountain West including
the Blue Mountains, the Salmon River Range, the Absarokas, and
other terrain around Yellowstone NP.


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

Late D2 into D3, a wave of low pressure is likely to deepen in the
lee of the CO Rockies, with downstream flow drawing moisture
northward from the Gulf of Mexico and rotating cyclonically around
this low back into the Central Rockies. PW anomalies appear
generally modest during this time, but with the cold front to the
southeast of the area, snow levels to fall to around 7000 ft,
and moderate snow accumulations are likely in the Front Range.
This region is favored due to a combination of upslope/isentropic
upglide on easterly flow enhancing ascent and moisture, which will
concurrently overlap with a mid-level RH surge on SW flow emerging
from the base of the longwave trough to the west. The guidance has
backed off on the intensity and coverage of this event tonight, but
WPC probabilities for 4+ inches of snow are still around 40% D2
and 10-30% D3, with 6-8" of event total snow possible in the
highest terrain.


The probability of significant icing is less than 10 percent.

Weiss


$$