Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Mon Apr 24 2023

Valid 00Z Tue Apr 25 2023 - 00Z Fri Apr 28 2023

...Northern/central Rockies...
Days 1-2...

A late season winter storm is set to unfold in the northern and
central Rockies, with the latter of the two Rockies' regions
seeing the heaviest snowfall. The culprit is potent 500mb low
embedded within a generally larger 250-500mb upper low diving
south through northern Utah tonight and into western Colorado by
Tuesday afternoon. When looking for anomalous temperatures or
winds, there is not a strong signal present in the situational
awareness table. At 500mb, heights are below the 10th
climatological percentile Monday night and into Tuesday night over
the Four Corners region. What this event has going for it is
strong synoptic scale support, ample low-mid level moisture, and
favorable upslope flow into the Colorado Rockies.

Focusing on the synoptic scale forcing, the track of the 250-500mb
mean layer low is ideal for positioning the strongest divergence
aloft over the central Rockies. The divergent left-exit region of
the the mean 250-500mb jet will be oriented over Wyoming, northern
Utah, and northwest Colorado tonight, with the strongest
ageostrophic wind divergence occurring over Colorado on Tuesday.
Mean 700-300mb Q-vector convergence will only increase Tuesday
afternoon and evening, which coincides when the heaviest snowfall
rates are expected. Next, a deepening area of low pressure in lee
of the Sangre De Cristo in northern New Mexico will create a
strengthening easterly flow response on the northern flank of the
circulation. This will allow for the mean 700-500mb low to tap
into rich 850-700mb moisture flux and direct it at the Colorado
Rockies, providing no shortage of moisture for an atmosphere where
strong ascent aloft is present. Lastly, aforementioned easterly
flow will be a favorable upslope component to further enhance
vertical velocities and dynamically cool the column over the Front
Range and as far west as the San Juan and Sawatch Range. While the
upper low is not exceptionally deep or cold aloft, the combination
of intense vertical velocities on the synoptic and mesoscale, as
well as plenty of moisture aloft, is a classic setup for
significant late season snowfall in the Colorado Rockies.

Snow will start out initially in the northern Rockies tonight and
continue to develop over the Colorado Rockies on Tuesday. Snowfall
totals ranging between 6-12" are forecast in the Absaroka, Wind
River, Wasatch, and Uinta ranges tonight and into Tuesday morning.
Snow will gradually wind down by early Wednesday morning over the
Rockies of Colorado and far northern New Mexico, but not before it
produces 1-2 feet of snow in elevations above 8,500 feet. Some
areas above 10,000 feet of could make a run at 3 feet of snow. The
WSSI shows Major impacts for locations near and above 10,000, and
Moderate impacts as low as 8,500 feet. Snow Amount and Snow Load
are the primary drivers in the WSSI, with some of the highest
elevations (above 11,000 feet), also having some Minor to Moderate
impacts from Blowing Snow. Expect difficult to even impossible
travel for roads above 8,000 feet Tuesday afternoon and likely
into Wednesday. Minor impacts will stretch as far east as the
Palmer Divide and far northern New Mexico, including the Sangre De
Cristo, Park Plateau, and Raton Mesa.

The probability of significant icing greater than 0.25" is less
than 10 percent.

Mullinax