Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 AM EDT Sat Apr 19 2025

Valid 12Z Sat Apr 19 2025 - 12Z Tue Apr 22 2025


...Southern Rockies...
Day 1...

Significant mid-April winter storm continues for one additional day
across the Rockies of southern CO and northern NM.

As of early this morning, the amplified but positively tilted
trough is clearly evident on GOES-E WV and IR imagery, centered
across northern Arizona. This trough will continue to amplify, and
as the northern portion of this trough ejects to the east, the
southern portion will close off across the Four Corners. This
amplification will result in pronounced downstream synoptic lift,
as mid-level divergence intensifies and at least peripherally
overlaps with the LFQ of a poleward arcing jet streak into the
Central Plains. At the same time, the cold front being driven south
by the amplified trough will dig into Texas and reach the Rio
Grande Valley by this afternoon, leaving impressive 850-600mb fgen
in its wake tilted north into NM and CO. This even more enhanced
ascent will overlap with upslope flow on post-frontal E/NE winds,
to drive intense snow rates across the Sangre de Cristos and San
Juans through the first half of D1.

This deep layer ascent (and additional mesoscale dynamics through
fgen/upslope) will occur within a moistening column as 700-300mb
flow remains out of the SW downstream of the primary trough axis.
Although PWs progged by NAEFS will be around normal values, this
will be sufficient to be wrung out as heavy snow, with rates
possibly (30-60% chance) exceeding 1"/hr, especially in the
Sangres and San Juans. Snow levels will begin to slowly rise today
as the cold airmass once in place begins to modify, rising from
around 5,000 ft to 7,000 ft.

For the tail end of this event, the heaviest snow is likely across
the Sangre de Cristos and the San Juans, where WPC probabilities
D1 are high (>70%) for more than 8 inches of snow, and locally over
1 foot is possible despite rapidly waning coverage and intensity
of snow after 00Z Sunday.


...Washington Cascades & Northern Rockies...
Days 2-3...

Shortwave entering the Northwest Sunday evening and crossing over
the northern Rockies on Monday will be accompanied by favorable
upper jet dynamics, with the LFQ situated over the region.
Associated height falls will also be coincident with lowering snow
levels, starting out around 3,500 ft in the WA Cascades before
crashing to 2,500 ft briefly. For the northern Rockies, snow levels
around 6,000 ft will fall to around 4,000 ft in northwest MT and
5,000 ft near Yellowstone. The heaviest snowfall is expected across
the northern WA Cascades and parts of western MT into Yellowstone
Country, where 6-12 inches are possible in the higher peaks. WPC
probabilities are high (>70%) through D3 for at least 6 inches of
snow across parts of the WA Cascades and the western/south-central
MT ranges above 7,000 ft.


The probability of at least 0.10" of freezing rain across CONUS is
less than 10%.


Snell