Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
231 AM EDT Fri Apr 18 2025

Valid 12Z Fri Apr 18 2025 - 12Z Mon Apr 21 2025


...Central/Southern Rockies & Adjacent High Plains...
Days 1-2...

Significant mid-April winter storm continues to slide southward
throughout southern WY, the CO Rockies and northern NM Mts. Heavy
snow is forecast for much of the higher elevations and the
immediately adjacent foothills.

The period begins this morning with a positively tilted longwave
trough stretching from south-central Canada with its core centered
over Utah. This feature will deepen as it digs south today,
potentially closing off over the Four Corners into early Saturday.
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 until exiting
late-Saturday.

As this trough digs south and strengthens, 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 continue early D1 across southern WY and expand
across much of 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, reaching the northern TX
Panhandle this morning, 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 southern
WY through NM/AZ.

The heaviest additional snowfall is expected in the higher terrain
of CO northern NM 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 fall 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 remains 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 a bit farther east as reflected by a
large difference in the 25th-75th percentile (3-4 inches)
according to the 01Z NBM. With impressive ascent likely even into
the High Plains, it seems plausible that at least periods of heavy
snow will impact the urban corridor, 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 southeast WY and
the CO foothills.

The heaviest snowfall is likely 12Z Friday through 18Z 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 06Z Sunday near and just north of the
upper low, 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 a complete end by the start of D3.

For days 1-2, WPC probabilities are high (>70%) for more than 8
inches across much of the CO Rockies, including the Front Range,
Park Range, Sawatch and San Juans, as well as the Sangre de
Cristos into NM. More than 12 inches and locally as high as 2 ft of
snow is possible above 9,000-10,000 ft in the southern Sawatch and
northern Sangre de Cristos in CO. Outside of the Palmer Divide and
Raton Mesa region, WPC probabilities for greater than 4 inches
reach 30-50% (higher west and lower east) along the I-25 corridor
through the Denver metro.

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 D1, 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%.


Snell/Weiss