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

Valid 00Z Sun May 04 2025 - 00Z Wed May 07 2025


...Sierra Nevada...
Days 1-2...

Digging upper trough just off the CA coast this evening will
deepen and close off as it moves into/through SoCal overnight and
early Sunday. The best dynamics will quickly move into the Interior
West, but light upslope-driven snow will continue over the
central/southern High Sierra with snow levels generally near/over
8000ft. Main upper low will move through AZ into the Four Corners
though trailing streams of vorticity from the north will keep light
snow going over the Sierra until late Monday. WPC probabilities
for at least 8 inches of snow are generally 20-60% for the High
Sierra (above 10,000ft).


...Northern and Central Rockies...
Days 1-3...

Combination of the incoming upper low through the Southwest and a
slowly-moving trough axis in the northern stream pushing its way
across Montana will promote a period of unsettled weather for much
of the Interior West/Rockies through the period, including higher
elevation snow. The upper low is forecast to only move from
central AZ Monday morning to around the Four Corners region by
Tuesday morning then onto the western High Plains by the end of the
period as the upper jet carves out breaks down over the region.
This will favor an influx of moisture from the southeast
(TX/western Gulf) in combination with rotating vort maxes around
the main upper low atop a slowly-moving cold front at the surface
to the north (in addition to the one to the south ahead of the main
upper low). Snow levels will be high in the central/southern
Rockies, generally above the Front Range around 8000-10,000ft
(lowest beneath the cold core upper low) but lower farther north
behind the cold front to around 5000-7000ft across MT into WY. Snow
will be light to moderate and should maximize into the San Juans
via southerly flow but then into the CO Rockies and Sangre de
Cristos as the upper low moves eastward and the flow turns to more
southeasterly to easterly and northeasterly later in the evolution
of the system. Snow to the north will be driven by the surface
front and low-level convergence, but with more limited moisture and
lighter snowfall amounts.

For the three-day period, WPC probabilities for at least 8 inches
of snow are above 50% across the terrain of southern UT into CO
above 10,000ft or so and are at least 40% for at least 18 inches of
snow above 12,000 ft. To the north, probabilities are a bit lower
for at least 8 inches of snow through the period (20-50%), mostly
over the Wind River Range, Uintas, and across SW MT.


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


Fracasso