Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 AM EDT Sat May 3 2025

Valid 12Z Sat May 03 2025 - 12Z Tue May 06 2025


...Sierra Nevada...
Days 1-3...

Light to at times moderate precipitation will persist across the
Sierra and Great Basin through Sunday as an anomalous closed low
drifts across CA and into the Four Corners region. Although this
feature will be impressive, the best ascent through height falls
and the overlapping of a subtropical jet streak rotating through
the base will occur east into a warmer column. This suggests that
the only significant accumulating snow will be across the Sierra,
generally above 7000 ft, with the heaviest accumulations in the
high southern Sierra. In this area, WPC probabilities indicate a
low to medium chance (30%) for 6+ inches both D1 and D2.


...Northern and Central Rockies...
Days 2-3...

The closed low drifting across the Desert Southwest Sunday will
lift slowly northeast into the Four Corners and then pause nearly
in place through the remainder of the forecast period. This slow
evolution is due to a highly amplified pattern across the CONUS
with dual closed lows bookending an impressive omega block across
the center of the Nation. With little impetus for features to move,
this will result in an extending period of unsettled weather
across the Rockies as height falls in the vicinity of the closed
low, modest diffluence within the LFQ of a modest but favorably
oriented upper jet streak, and a southeast advancing cold front
interact to provide broad and long-lasting lift to the region.

As this pattern evolves across the West, moisture will steadily
increase as a surge of theta-e lifts out of the Gulf and ascends
isentropically along the 305-310K surfaces as far north as CO and
UT as it wraps cyclonically around the low and into the Great
Basin. This will be most impressive late Monday into Tuesday thanks
to pinched and nearly unidirectional southerly flow through the
column helping to advance moisture return. Forcing impinging into
the moistening column will lead to rapid expansion of
precipitation, with much of the Intermountain West experiencing
rain and high-elevation snow beginning 00Z Monday, with periods of
heavy precipitation rates likely.

Snow levels during precipitation expansion will be quite high,
generally 9000-11000 ft across much of the Rockies. The exception
will be in the vicinity of a cold front dragging into ID/MT late
D2 into D3, behind which snow levels will crash to 4000-5500 ft.
These lowest snow levels are progged by the NBM to remain generally
north of Wyoming, but farther south into the Central and Southern
Rockies, snow levels will also fall beneath the core of the upper
low, reaching to 7000-8000 ft. These lowering snow levels will help
expand the wintry precipitation areas, with periods of heavy snow
becoming more widespread late D3, especially from ID through WY
where fgen along the front will drive more pronounced mesoscale
lift. It is across these areas, generally from Glacier NP southward
through the Absarokas, towards Yellowstone NP, and into the Wind
River range where snowfall will be heavy, as reflected by WPC
probabilities that reach as high as 50% for 6+ inches on D3.
Additionally, heavy snow accumulations exceeding 6" are likely D2
(70-90%) and possible D3 (30-50%)in the higher terrain of the San
Juans where storm-total snowfall of 10-15" is possible.

The probability of significant freezing rain across the CONUS is
less than 10%.

Weiss