FOUS11 KWBC 220635
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
235 AM EDT Wed Apr 22 2026

Valid 12Z Wed Apr 22 2026 - 12Z Sat Apr 25 2026

...Intermountain West to the Northern High Plains... Days 1-3...

Large mid-level trough/gyre will lift northeast from the Great
Basin into the Northern Rockies while re-amplifying into a closed
low by Thursday morning. This large scale trough will bring
widespread synoptic ascent across the area, while secondary and
even tertiary shortwaves rotating around the larger system bring
additional ascent to locally maximize snowfall across the area.

While confidence is high in widespread snow, especially in the
higher terrain as snow levels begins around 7000-8000 ft before
falling steadily to 4000-5000 ft by Thursday, there is still
significant uncertainty into how the low elevations will fare. In
the higher terrain, especially D1 and D2 from the Blue Mountains of
Oregon through the Sawtooth/Salmon River ranges, east into the
Northern Rockies from near Glacier NP through Yellowstone NP
including the Lewis Range, Little/Big Belts, Absarokas, Tetons,
and Wind Rivers, WPC probabilities are high (>70%) for at least 8
inches of snow, with 1-2 feet possible (30-50%) in the highest
elevations of many of these ranges.

The challenge for this forecast then primarily involves around what
happens in the lower elevations, as well as how ascent responds to
overlapping secondary forcing as a cold front and shortwave dig
southward behind the primary surface low beneath the larger trough.
This may lead to two areas of heavier snowfall.

1) As the primary low deepens over far NE Montana, moisture
wrapping cyclonically around it (low-level flow emerging from the
Gulf) will lift into a TROWAL and pivot southwest back into MT. The
guidance has been insistent in this evolution, but still vary
widely in the intensity and position of this developing
deformation. Should this TROWAL become more intense, as reflected
by the GFS/NAM, but not as much in the ECMWF/CMC, a band of heavy
snowfall into the lower elevations is possible for central and
eastern MT. At this time that is not the likely scenario, but still
worth monitoring as the combination of dynamic cooling and heavy
snow rates could produce a few inches of snow in a short period of
time on Thursday.

2) The secondary shortwave digging out of Canada on the backside of
the larger trough will interact favorable with the low-level
baroclinicity (fgen) as the cold front sinks southward towards
northern WY. Impressive mesoscale ascent through the fgen/height
falls will overlap with intensifying upslope flow in the wake of
this front to create a swath of moderate to heavy snowfall from
Thursday night through Friday night in a relatively narrow
corridor from central ID through eastern WY. While some of this
heavy snow will occur atop areas that receive significant snowfall
from the first impulse, this secondary impulse could result in
snowfall reaching as low as 2500 ft according to the NBM. The exact
placement and intensity of this secondary corridor remains
uncertain as well, but where it does occur, WPC probabilities
indicate a moderate to high risk (50-90% chance) of at least 4
additional inches (or 4 new inches in lower elevations) with heavy
snow rates up to 1"/hr.

The probability for significant freezing rain exceeding 0.1 inches
across the CONUS is less than 10 percent.

Weiss
$$
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 * Origin: Project Scorpio TEST (618:250/6)