Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 PM EDT Mon May 5 2025

Valid 00Z Tue May 06 2025 - 00Z Fri May 09 2025


...Intermountain West...
Days 1-2...

A deep upper low over AZ will slowly wobble eastward over the next
two days as an influx of Gulf moisture wraps in from the
southeast. This will set up a major winter storm for the high
elevations of the central/southern Rockies with snowfall rates of
1-3"/hr at times and total accumulations well over 1-2ft at the
highest peaks. The environment will be mild overall given the late
season timing and focus of colder air well south (central AZ/NM),
meaning that much of the I-25 corridor will remain snow-free
(except for the typical higher elevations along the corridor in the
Palmer Divide and Raton Mesa). The system will become elongated
and stretch eastward tomorrow which will signal the end of modest
snow, but lighter snow will persist into the evening/overnight and
largely end by Thursday morning.

The system will come in two phases for many areas, with the lead-
in WAA-driven lighter snow early D1 (tonight) and the dynamically-
driven heavier snow tomorrow by the afternoon through the evening
as the upper low approaches the central CO/NM border from the WSW.
Models indicate a robust system with strong isentropic ascent at
the 305K level from the Gulf and strong/anomalous/record easterly
flow at 700mb by midday tomorrow (per the CFSR period). As can be
typical with strong systems, dynamical cooling may bring down snow
levels from ~8000-8500ft to 7500ft or so, reaching farther down the
Front Range toward the high elevations of the I-25 corridor before
then rising as the upper low pulls away.

Favored areas for the heaviest snowfall are over the San Juans
this evening/overnight on southerly to southeasterly flow, then
transitioning to the east/upslope side of the CO Rockies and into
the Sangre de Cristos. Total snow may be 2-3ft (perhaps more) at
the highest elevations including Pike's Peak and along the spine of
the Sangre de Cristos. Impacts will be moderate to major per the
WSSI. WPC probabilities for at least 8 inches of snow over the next
two days are >50% above 8500ft or so and >50% above 10,500ft or
so. Depending on the time of day and amount of dynamical cooling,
there are low probabilities (10-40%) of at least 2 inches of snow
as low as 6500ft or so.

Farther north, snow levels will continue to fall behind a cold
front currently through WY with precipitation trailing to its
northwest through Montana, tied to northern stream
troughing/vorticity diving south-southwestward tonight/early
tomorrow. This rain/snow will continue to progress southward, with
snow levels around 6000-8000ft from north to south, favoring snow
over portions of southern MT, the Absarokas, and into the Bighorns
and Wind River Range. There, WPC probabilities of at least 8 inches
of snow are at least 30% (highest in the Absarokas).

The probability of at least 4 inches of snow on day 3 are less
than 10%.

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


Fracasso