Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 PM EDT Thu Apr 17 2025

Valid 00Z Fri Apr 18 2025 - 00Z Mon Apr 21 2025


...Central and Southern Rockies, Adjacent High Plains...
Days 1-3...

Significant mid-April winter storm expands and intensifies this
evening, bringing heavy snow to a large portion of the Central and
Southern Rockies. Uncertainty continues into how much of the High
Plains will be impacted.

The period begins /00Z tonight/ with a positively tilted but
amplifying trough with its core centered over Utah. This feature
will deepen as it digs south tonight into Friday morning,
potentially closing off over the Four Corners Friday evening, with
the resultant trough amplifying into full latitude as it digs from
the Northern Plains into the Desert Southwest. The northern portion
of this trough will likely split off to the east by Saturday
afternoon while the southern /closed/ portion stalls over the Four
Corners. This evolution will result in widespread and impressive
synoptic ascent across much of the Intermountain West, leading to
periods of heavy precipitation, likely falling as snow, across much
of the region.

As this trough digs south, areas of locally enhanced ascent will
develop to force impressive snowfall rates and amounts. East of the
closing mid-level low, robust mid-level deformation will develop
across WY and CO, which will be aided by at least periphery LFQ
diffluence as a jet streak pivots around the base of this trough
and lifts meridionally into the Central Plains. At the same time, a
strong cold front will race southward, from northern CO 00Z Friday
to the Texas Panhandle 00Z Saturday, while banking westward against
the Front Range and Sangre de Cristos. SW flow between 700-300mb
will help enhance moisture from the Pacific and cause additional
isentropic ascent, further enhancing the coverage of heavy snow
from WY through NM/AZ.

The heaviest snow is expected in the higher terrain of CO and WY as
snow levels crash rapidly behind the cold front. Initially, snow
levels will be as high as 8000-9000 ft ahead of the front, but will
crash to below 3000 ft immediately in its wake. In addition to the
lowering snow levels, this front will cause post-frontal NE flow to
enhance upslope ascent into the terrain, as well as lead to
increasing low to mid level frontogenesis, especially in the
850-600mb layer. For the upslope flow, cross-sections indicate a
developing robust barrier jet east of the Front Range and Sangre de
Cristos, with omega overlapping some folding of theta-e surfaces in
response to this jet forcing. This suggests enhanced snowfall rates
and local maxima of snowfall amounts in upslope regions.
Additionally, the impressive fgen will help drive strong ascent
despite a lack of a TROWAL, as the cold conveyor becomes the
dominant mechanism for heavy snow. Both the WPC prototype snowband
tool and the WPC HREF probabilities suggest this will result in at
least 1"/hr snowfall rates in these regions, despite SLRs that will
likely be less than climo, with a heavier than typical snowfall
leading to substantial travel and even some infrastructure impacts
as reflected by the WSSI-P.

The more challenging aspect of this forecast is what will happen in
the I-25 urban corridor and eastward into the High Plains/lower
elevations. While it is likely that heavy snow will accumulate
across both the Palmer Divide and Raton Mesa, there is considerable
uncertainty farther east as reflected by a large spread in the
10th-90th percentile (6-8 inches) according to DESI. With
impressive ascent likely even into the High Plains, it seems
plausible that at least periods of heavy snow will impact the urban
corridor and out into the High Plains (especially the Pine Ridge
area of western Nebraska), but farther south into CO and NM the
displaced jet streak may limit the eastward advance of heavy snow.
NBM v5.0 probabilities in the lower elevations are much lower than
then NBM v4.2 probabilities, but are matched well with the
calibrated probabilities, so this suggests that the risk for heavy
snow farther east is lower than the NBM would suggest, but still a
plowable snow is probably across much of eastern WY and CO.

The heaviest snowfall is likely 00Z Friday through 12Z Saturday
before forcing begins to become more diffuse and pushes off to the
southeast. Periods of moderate to heavy snow may linger across the
Sangre de Cristos through 12Z Sunday, however with weakening
forcing, this may end as a period of rain and snow mixed, even at
some of the higher elevations, before coming to an end during D3.

On D1, WPC probabilities are high (>70%) for more than 6 inches
across much of the higher terrain of WY and CO, including the Wind
Rivers, Laramie Range, Front Range, Park Range, and San Juans.
Locally more than 12 inches of snow is possible in the higher
elevations, especially across the Wind Rivers and Laramies. By D2
as things shift south, WPC probabilities above 70% for 6+ inches
pivot into the southern Front Range and along the Sangre de
Cristos, while continuing across the San Juans as well. 2-day
snowfall in some of these areas will also likely exceed 12 inches,
with locally as much as 2 feet in the San Juans. By D3, much of the
precip winds down, but an additional 2-4" is possible (30-50%)
across the Sangre de Cristos of New Mexico.

Although slightly more removed from the worst of the winter
weather, the upper trough pivoting across the Four Corners will
also result in periods of heavy snow in the Wasatch, along the
Mogollon Rim, and into the White Mountains late D1 into D2, and
here WPC probabilities reach as high as 50% for 6+ inches of snow.


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


Weiss