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

Valid 12Z Thu Apr 17 2025 - 12Z Sun Apr 20 2025


...Northern/Central Rockies & Adjacent Central High Plains...
Days 1-3...

Ongoing moderate to heavy snow across parts of the northern
Rockies this morning will slide and expand southward over the next
few days. This leads to a decent footprint of heavy spring snowfall
for the Rockies and into adjacent sections of the central High
Plains. The latest WSSI highlights moderate to locally major
impacts associated mainly with snow amount and snow load across
much of the higher elevations, with minor impacts expanding into
parts
of eastern WY and the CO Foothills. The driving force behind this
event is a positively- tilted trough and embedded mid- level lows
churning at its base as it swings from the Great Basin on Day 1 to
the southern Rockies on Day 3, while also tilting more neutral to
negative with time. This trough will aid in producing ample upper
diffluence near and north of a 120kt jet streak extending from the
subtropical Pacific. Meanwhile, an associated cold front will also
surge southward over the next few days, with much colder air in its
wake and favorable upslope into the northern and eastern facing
slopes. Snow levels south of this front will begin above 8,000 ft,
but quickly crash below 5,000 ft and equate to accumulating
snowfall into the High Plains of WY, SD, NE, and CO.

For Day 1 (ending 12Z Fri), much of the focus for heaviest snow
will be centered over WY as the best upslope component and 700mb
convergence exists. This includes the potential for 1-2"/hr rates
as depicted by the 00Z HREF. Ranges such as the Absarokas,
Bighorns, Wind River, Rattlesnake and Black Hills, as well as the
Laramie and Shirley Mts are expected to see over a foot of snow
(>70% probabilities), with some of the higher north- northeast
facing peaks seeing 1-2+ feet. Additionally, lower elevations that
aren't shadowed can also expect anywhere from 3 to 6 inches of
snow.

By Day 2 (ending 12Z Sat) the cold front and precipitation shield
quickly pushes south into CO while gradually waning in WY. The
greatest forcing and mid-level convergence focuses on the CO
Rockies, including the Palmer Divide and as far east as the Urban
Corridor, where more uncertainty exists regarding how much
accumulation occurs given the mid-April sun angle on Friday. As the
base of the trough approaches the Four Corners and eventually the
central/southern High Plains by the end of the period, lingering
snowfall exists over southern CO/northern NM and specifically for
the San Juans and Sangre de Cristo Mts. Total snowfall amounts
throughout the entire event are expected to exceed 1 foot (>70%)
across the CO Rockies above 10,000ft, with over 8 inches possible
(30-60%) along the Palmer Divide. Probabilities of 50-80% for
snowfall totals above 4 inches extend into the High Plains just
east of the Denver metro.


...Utah & Arizona...
Days 1-2...

Elsewhere, high (>70%) probabilities of exceeding 6" of snow
exists across the higher elevations (generally above 8000 feet) of
UT and some of the higher terrain of AZ. This snow will be
associated with a southward dropping cold front today into early
Friday, and could be accompanied by pretty high snowfall rates
given the potent upper low crossing directly overhead and
increasing lapse rates over 7-8C/km. Over a foot of snow is
possible (50-70%) across the highest peaks of the UT ranges, with
probabilities for over 8 inches around 30-50% for the Kaibab
Plateau, as well as the White, Carrizo, and Chuska Mts of AZ and
northwest NM.


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


Snell