Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 PM EDT Wed Apr 16 2025

Valid 00Z Thu Apr 17 2025 - 00Z Sun Apr 20 2025


...Interior Northeast...
Days 1...

Upper low across Quebec will continue moving northeast with the
trailing energy pushing through northern NYS through Northern New
England. Negative vorticity advection in-of NYS will allow for a
quick degrading of the precip field with only some very light snow
shower/flurry activity after 00z. This will lead to little, if any
additional snowfall over the area as we move into the evening.
Remnant snowfall will be ongoing across the northern Green and
White Mtns with the primary areas of focus present within the Jay
Peak and Stowe areas of VT along with the Presidential's in NH.
Additional accums of 1-4" are plausible after 00z in those higher
elevations leading to totals between 4-8" in the Presidential Range
and 6-12" across the Jay Peak/Stowe ski areas of northern VT.
Lighter snow totals will be present in the valleys, but will
maintain little impact. Snow threat will wane between 06-12z in
those affected locations.

Kleebauer


...Northern/Central Rockies...
Days 1-3...

The forecast remains on track with regards to the expected heavy
snow threat across the Northern and Central Rockies into portions
of the Inter-Mountain West thanks to an anomalously evolving height
pattern across the Western CONUS. At the surface, a strong cold
front is analyzed over south-central MT with a forward propagation
running due south along the lee of the Northern Rockies. Recent
RTMA analysis shows 24-hr temp changes >20F from the Canadian
border to not far behind the frontal position. Attendant
height falls within the digging shortwave pattern out of Canada
have assisted in a transition from liquid to fully frozen
hydrometeors in wake of the front with a rooted scattered to
widespread snow setup across the Lewis Range to the higher plains
adjacent. Locally heavy snowfall will continue to impact the
Northern Rockies through the course of the day with the precip
threat scaling back somewhat by early Thursday morning, but still
maintaining some light QPF from remnant light to moderate snow
within the Lewis Range. 12z HREF signals for >1"/hr rates are
high (>70%) within those higher elevations in-of the Northern
Rockies, particularly in the Lewis Range, but also some lower end
probs for similar rates adjacent to the mountains on the eastern
flank. Snowfall totals between 6-12" with locally higher will be
common above 8000ft with 2-6" more likely in the lower valleys and
foothills adjacent.

By this evening, the heavy snow risk shifts into southern MT, with
the heaviest accumulations expected above 4000 feet. Above this
elevation there is a high chance (>70%) of exceeding a foot of
snow, and localized totals could approach two feet in the highest
terrain areas of the northern Absaroka and Big Horns. The forecast
confidence has increased a bit more over the lower elevations of
southeast MT within this forecast cycle. Thermal profiles will be
marginal, but the overnight timing and moderate to heavy rates,
should be enough to change rain to an accumulating snow. Recent
probabilities for at least 4" exceedance have increased towards
the 
40-60% range for Billings and surrounding lower elevation areas,
and the hi-res deterministic outputs have now leaned closer to the
4-6" output for the event entirety. The highest probabilities for
>4" will still be aligned with the higher terrain on the upslope
side of the northerly flow pattern, however there is still some
potential for lower elevation areas within adjacent valleys of
Southern MT to bust high pending on the magnitude and depth of the
CAA regime and forecasted 700mb frontogen intersecting creating a
better ascent pattern within the DGZ. This has been entertained
within some of the hi-res suite, so it's something to continue
monitoring for short term trends.

By Thursday the heavy snow is overspreading WY with the greatest
accumulations again focusing across the favored
higher terrain...such as the Absaroka Range, Bighorn Mountains,
Wind River Range, Laramie Mountains, Medicine Bow Mountains and the
Sierra Madre. All of these areas have at least a 50% chance of
exceeding 12 inches of snow with this system. One of the bigger
changes in the forecast is a greater magnitude of snowfall within
the Wind River Range where probabilities for >18" over the 72 hr
period are upwards of 50-80%, a solid increase compared to the
previous forecast. Totals of 1-2ft with locally higher totals are
expected in those ranges across the state. With the colder air
filtering in, snow is also likely across the lower elevations with
forecast snow amounts remaining steady this forecast cycle across
much of central to southeast WY. The probability of exceeding 4" of
snow has remains elevated into the 50-80% range over much of these
lower elevation areas across this part of the state.

By Thursday night into Friday the snow shifts southward into CO.
With marginal boundary layer temperatures the better accumulations
will likely be confined to elevations above 6000 feet, where
probabilities of exceeding 8" remain over 70%. The setup remains
borderline across the lower elevations within the Front Range,
including the Denver metro as antecedent warmth prior to the
incoming cold along with the primary precip output occurring in the
middle of the day may curb the threat somewhat, limiting totals <4"
for the metro and points east. Nonetheless, WPC probabilities
maintain a signature of a 50-80% chance of exceeding 2" and a
20-50% chance of exceeding 4" over these areas adjacent to the
Rockies. Heavy snow threat will shift south later Friday into
Saturday with a fairly robust snowfall footprint within the Sangre
de Cristos and San Juan Mountains across Southern CO. Probabilities
>70% for at least 8" of snowfall are forecast for the event within
those zones with the San Juans most likely to see totals >12"
during the 36 hr period from Friday evening into Saturday night.


...Northern/Central UT...
Days 1-3...

Elsewhere, high (>70%) probabilities of exceeding 6" of snow
exists across the higher elevations (generally above 8000 feet) of
central and northern UT. This snow will be associated with a
southward dropping cold front Thursday into early Friday, and
should be accompanied by pretty high snowfall rates. Trends for
upwards of foot snow have improved a bit this forecast within the
Monte Cristo and Uinta Mtns. Threats for over 6" also exist across
the Tushar and Escalante Mtns. in south- central UT with slightly
lower probs compared to areas further north (50-70%).

Chenard/Kleebauer