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FOUS11 KWBC 250722
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EDT Thu Apr 25 2024

Valid 12Z Thu Apr 25 2024 - 12Z Sun Apr 28 2024


...The West...
Days 1-3...


Dual amplifying shortwaves within an increasingly anomalous trough
developing across the Western CONUS will result in widespread
late-season heavy snow across much of the terrain.

The first of these will be a southern stream impulse lifting into
the southern CA coast to start the period /Thursday morning/. This
feature will lift quickly northeast through the Four Corners
before closing off over the Central Rockies/Central High Plains
Friday morning. This strengthening low will spawn a surface low in
the lee of the Rockies with precipitation spreading into the High
Plains, but the column appears too warm for any wintry
precipitation with this first wave. However, the synoptic evolution
of this first impulse will cause large scale height falls across
the West, noted by NAEFS 700-500mb height anomalies reaching -1
sigma across much of the area by Friday aftn.

As the longwave trough amplifies behind the first shortwave, a more
impressive feature will dig from the Pacific Northwest towards the
southern Great Basin Friday aftn, and this will also close off at
500mb over the Four Corners during Saturday. This will additional
lower heights within the trough over the West, while producing
enhanced synoptic lift through height falls, PVA as lobes of
vorticity shed around it, and increasing upper diffluence as the
subtropical jet arcs poleward leaving the favorable LFQ overhead.
As this feature lifts again into the Central Rockies/Central
Plains, following a similar path to the first, it will tap into
impressive moisture noted by PW anomalies reaching +1 to +2 sigma
across the Central Rockies, resulting in expanding precipitation,
with heavy snow likely shifting eastward from the Northern Rockies
and northern Great Basin into the Central Rockies.

At the same time, dual surface fronts will be digging through the
Northern High Plains and into the Central Rockies, providing
additional ascent through upslope flow and periods of enhanced
fgen, potentially overlapping a deformation axis in the Central
Rockies, to produce heavier snowfall rates that may reach 1"/hr at
times. Snow levels will remain generally elevated, primarily above
7000-8000 ft, but with steepened lapse rates beneath the cold
trough and some areas of enhanced ascent, snow is possible into
much lower elevations.

For D1, the focus of the heaviest snow will be in the OR Cascades
where WPC probabilities for more than 6 inches reach 60-80%.
However, much of the period will be dominated by snow across the
Intermountain West D2 and D3. During this time, WPC probabilities
reach above 80% for 6+ inches D2 in the Absarokas, Wind Rivers,
Uintas, Wasatch, and into the Front Range, then focusing in the
Front Range, San Juans, Park Range, and Mosquito Range D3. While
there is some uncertainty into how far west into the Front Range
the heaviest snow will fall, locally 2-3 feet appears likely in the
higher peaks.

The probability of significant icing is less than 10 percent.


Weiss


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