Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 PM EDT Sun May 4 2025

Valid 00Z Mon May 05 2025 - 00Z Thu May 08 2025


...Intermountain West...
Days 1-3...

The very blocky upper pattern (typical for May during our
atmospheric spindown into summer) is manifesting itself as two
closed lows over the CONUS, the western one of which will be
responsible for a wintry thump of snow over the Rockies. As of
Sunday evening, the upper low will move from the CA deserts/CO
River Valley northeastward through AZ on Monday then into northern
NM Tuesday before finally moving into the Plains Wednesday. The
southern track of the upper system will keep the largest negative
anomalies (height and temperatures) over northern Mexico and
astride the southern US border, meaning snow levels will be on the
higher side over the Rockies (9000ft +/- 500ft). Later, colder air
will eventually advect in from the northeast (modestly) which
should decrease snow levels a bit to around 8000ft (and a bit lower
at times). Significant snow is expected for the mountains in CO
and northern NM with moderate to major impacts near/above the
treeline.

The storm system will progress in phases, with the initial WAA-
driven snow first on southerly flow D1 to be followed by jet/PVA-
driven snow as the upper low approaches northern NM D2 as the flow
turns to more easterly, maximizing upslope potential on the east
side of the Rockies. Finally by D3, as the system unwinds/extends,
and modest snow to start will gradually wind down but not fully end
until just after the end of this D3. Areas in favor of the most
snow include San Juans D1, Sangre de Cristos northward to the
higher elevations of the Front Range D2-3. There, snowfall rates of
1-3"/hr are possible (>50% probs for 2"/hr in the HREF) D2. WPC
probabilities for at least 8 inches of snow over the next three
days are highest (>50%) above 10,000ft with >50% probs for at least
18 inches above 12,000ft. As some colder air filters in early
Wednesday, some snow may fall into the higher elevations of the
I-25 corridor (Palmer Divide and Raton Mesa along the CO/NM
border).

Farther north, an incoming cold front will aid in producing some
mountain snow for the Uintas, Wind River Range, Absarokas and
Bighorns, and southwestern Montana where snow levels will be a bit
lower (6000-8000ft). There, WPC probabilities for at least 8 inches
of snow Days 1-2 are moderate to high (50-80%).

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


Fracasso