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

Valid 12Z Fri May 02 2025 - 12Z Mon May 05 2025


...Sierra Nevada...
Days 2-3...

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 into D3,
with the highest amounts expected in the higher terrain of the
southern Sierra where locally 8-10 inches is possible.


...Central Rockies...
Day 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 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 San Juans before the close of D3 /12Z
Tuesday/. 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 as well, but at
this time, for D3, WPC probabilities indicate a 70-90% chance of 6+
inches, focused in the higher elevations of the San Juans.



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

Weiss