Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Fri May 2 2025

Valid 00Z Sat May 03 2025 - 00Z Tue May 06 2025


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

An anomalous closed mid-level low (700-500mb heights reaching -3
sigma by Sunday aftn) will dive along the CA coast before turning
into the Four Corners states Sunday night. Impressive height falls
and downstream divergence will combine with a sharpening jet streak
to produce deep layer ascent across CA and into the Great Basin
late Saturday through late Sunday. This will result in expanding
coverage of precipitation, with snow likely in the higher terrain
of the Sierra. Initially, snow levels will be quite high, generally
8000-9000 ft, but as the closed low pivots south of the region,
colder air will funnel in and drop snow levels to 7000-7500 ft.
This will keep most of the heavy snow accumulations above the area
Passes, but at least light accumulations are possible even as low
as Donner Pass before precip wanes late Sunday. WPC probabilities
across the Sierra peak around 50% for 6+ inches of snow D2, with
the highest amounts expected in the higher terrain of the southern
Sierra where locally 8-10 inches is possible.


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

A complex mid-level synoptic evolution will occur late in the
forecast period across the Intermountain West as the extremely
amplified flow across the CONUS exhibits minimal change into early
next week. Dual closed lows, one over the Ohio Valley and another
across the Four Corners will meander with little movement, as flow
stalls over the country. The important low for winter purposes will
be the one drifting across the Four Corners, which will shed
multiple lobes of vorticity around it by the end of D2 as nearly a
carousel of shortwaves rotates southward from Canada and towards
Baja and around the primary closed low. The interaction of these
shortwaves with the primary low and secondary developing
subtropical jet streak will result in expanding precipitation, with
snow likely in the higher terrain of the northern/central Rockies
as well as the UT ranges by the close of D3 /00Z Tuesday/. Snow
levels are expected to remain rather high (9000-10000ft) outside of
the northern Rockies by Monday evening, where levels drop to
around 8000ft. More significant precipitation, including more
widespread heavy snow across the Rockies, is possible beyond this
forecast period as a cold front drops through the region. At this
time, primarily for D3, WPC probabilities indicate a 70-90% chance
of 6+ inches, focused in the higher elevations of the San Juans
above 11000ft with low probabilities (10-30%) for 12+ inches.



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

Weiss/Snell