087
FOUS11 KWBC 042001
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 PM EDT Sat May 4 2024

Valid 00Z Sun May 05 2024 - 00Z Wed May 08 2024

...Cascades through Sierra Nevada, Intermountain West, and Rockies...
Days 1-3...

A large vertically stacked low is moving ashore near the
California-Oregon border this afternoon. The storm is characterized
by low heights indicative of an unseasonably cold air mass. As of
this writing, snow levels are falling through the 5,000s ft above
sea level, resulting in snow ongoing across much of the Sierra
Nevada range of California. The peak precipitation rates over the
next 3 days are happening now into the Sierra Nevada in association
with the strong cold front demarcating the leading edge of the much
colder air mass.

The surface low will rapidly weaken as it moves inland and is
sheared apart by the mountains. Thus, while overall precipitation
rates will diminish, the upper level forcing will continue pressing
eastward, allowing the associated cold air to do the same. Thus,
snow will continue to move inland into the Intermountain West
through Day 2/Monday, with nearly all mountain ranges in the
western states outside of Arizona and New Mexico picking up at
least some snow. The greatest snowfall amounts will be in the
Sierra Nevada broadly, but over the next 3 days, the highest peaks
of the Cascades down to the Sierras could easily top 4 feet of new
snow, again with the heaviest snow ongoing through tonight before
gradually tapering to more of a long-duration light snow.

Atmospheric moisture amounts are about 3 sigma above normal across
the Central Valley this afternoon. While these levels will taper
as the storm moves into the Intermountain West, the unseasonably
cold air to follow keeps the snow going through the mountains and
passes, causing travel delays with major impacts nearly certain in
the probabilistic winter storm severity index across the Sierras
with potential for near major impacts by Tuesday into the Little
Belt and Highwood Mountains of central Montana.

The upper level low will track eastward and open up into a
negatively tilted trough over Colorado and Wyoming by early Monday
morning. The vigorous energy associated therewith will allow the
development of a new surface low over the High Plains of eastern
Montana Sunday night. The low will quickly intensify as the upper
level energy wraps back on itself along the ND/MT border Monday.
This will allow the comma-head region of the low to draw in Gulf
moisture as an extended LLJ reaches from the waterlogged upper
Texas Coast all the way into the Canadian Prairies. Due to it being
almost mid-May by this point, most areas will get rain, but the
higher elevations of Montana and the Bighorns of Wyoming will see
upwards of a foot of new snow on Monday as a result.

The probability of significant icing is less than 10 percent.

Wegman


$$