Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EDT Tue May 6 2025

Valid 12Z Tue May 06 2025 - 12Z Fri May 09 2025


...Central and Southern Rockies...
Days 1-2...

Pinwheeling upper lows within a larger gyre centered over the Four
Corners will continue to produce significant winter weather in the
higher elevations of the Rockies through Wednesday morning.

The first, and less impressive, area of snow is expected to drift
south across WY today as a cold front driven southward by a
vorticity center drops through the area. Height falls and
convergence along the front will produce ascent, with post-frontal
upslope aiding ascent. PWs this far north will be somewhat less
impressive than points farther south, but an impressive stream of
moisture emerging from the Gulf will lead to PW anomalies above the
90th percentile into WY on D1. With snow levels fall from as high
as 9000 ft near Cheyenne to around 5000 ft around Yellowstone,
significant accumulations are likely in the favored upslope
terrain, especially above 7000 ft, before precip wanes after 00Z
Wednesday. Additional snow after 12Z Tuesday (beginning of D1) will
be modest, but WPC probabilities suggest a low-risk (10-30%) for an
additional 2-4 inches of snow.

Farther south, a more significant snowfall is expected from the
Front Range through the Sangre de Cristos, potentially including
portions of the I-25 urban corridor. The larger upper low swinging
across NM will pivot slowly eastward leading to height falls to
drive synoptic lift, while moisture surging northward from the Gulf
pivots NW as a theta-e ridge into the region. Low-level flow will
become increasingly from the east, leading to additional ascent
through upslope flow and isentropic ascent as weak high pressure
builds down the terrain. This will induce impressive lift from the
Sangres northward into the Front Range, which could produce
snowfall rates of 1-3"/hr at times as reflected by the WPC
prototype snowband tool. These rates will be confined to the
highest terrain, generally above 8500 ft, but folding theta-e
surfaces within the terrain and in the presence of the
strengthening barrier jet and accompanying ascent will likely lead
to dynamic cooling to allow snow accumulations falling to as low as
7000 ft, below the NBM 10th percentile. While the heaviest snow
accumulations will remain above 9,000 ft, light but impactful snow
is possible into the Foothills including the Palmer Divide and
Raton Mesa, with hazardous travel possible along portions of I-25.

As far as accumulations, this does appear to be a significant event
and the latest EFI suggests a higher than 95% probability of an
extreme event with SoT exceeding 2 in the Sangre de Cristos. In
this area, heavy snow accumulating to above 8 inches is likely
(>70%) from the Front Ranges through the southern Sangre de
Cristos, and including portions of the San Juans and eastern CO
Rockies. 1-2 feet of snow is likely above 9,000 feet, with 3 feet
or more expected in the highest peaks such as Pike's Peak. Farther
east, light snow of a few inches remains possible, especially
across the Palmer Divide and Raton Mesa, impacting the portion of
I-25 that traverses those crests. Most of this snow occurs D1, as
by D2 only lingering probabilities for 4+ inches continue across
the San Juans.


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


Weiss