Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
213 PM EDT Sat Apr 26 2025

Valid 00Z Sun Apr 27 2025 - 00Z Wed Apr 30 2025


...California and Nevada...
Day 1...

An intense closed mid-level low (500-700mb heights below the 1st
percentile according to NAEFS) will move onshore southern
California just before the start of the forecast period, with
gradual filling/weakening occurring thereafter as it lifts
northeast. As this low moves from CA through the Great Basin, it
will drive deep layer ascent through height falls and downstream
500mb divergence, which will overlap at least modestly with a
strengthening jet streak wrapping around the downstream side of the
trough. Together, this will result in periods of moderate to heavy
precipitation, which will fall as snow in the higher elevations.
While most of the snow accumulations will be above 5500 ft across
the Sierra and into the Ruby Mountains, where WPC probabilities
for 6+ inches reach 70-90%, the anomalously cold pool aloft and
steep lapse rates may allow for at least light accumulations down
below the NBM 10th percentile snow level, or as low as 4000 ft
before the system ejects to the northeast and ends on D2.


...Northern Rockies...
Days 2-3...

The closed low emerging from the Great Basin will pivot into WY/UT
and then begin to fill/open into a trough as it shifts east slowly
into Tuesday. DUring this evolution, the primary trough axis will
become positively tilted and split as northern stream energy ejects
into the Great Lakes while a southern stream shortwave stalls over
the Desert Southwest. This development will result in prolonged
deep layer ascent beneath the slowly advancing trough, with
additional, but weakening, ascent through the LFQ of a downstream
jet streak helping to produce surface cyclogenesis from CO into MN
during D2. As this low tracks northeast, a cold front will dig out
of Canada, leading to enhanced post-frontal ascent into the terrain
and increased fgen in the vicinity of the front. This will work in
tandem with some increased fgen and deformation which develops on
the NW side of the surface low to enhance ascent and increase the
intensity of precipitation from western ND through eastern ID.

The guidance has trended a bit slower and warmer with the
evolution, leading to snow levels and an atmospheric column that
are warmer, at least at precipitation onset around 00Z-06Z Monday.
Thereafter, however, the combination of the cold front and the
increasing ascent through the mesoscale forcing (fgen and
deformation) will help dynamically cool the column and lower snow
levels such that heavy snow rates will accumulate efficiently in
both higher terrain and some of the elevated valleys. There is
still uncertainty into how much snow can accumulate in the lower
levels, especially below around 4500 ft (NBM 10th percentile snow
level). However, it is likely that a lower-than-climo SLR will
result in heavy wet snow that should be impactful to many areas as
snowfall rates potentially reach 1-2"/hr at times, and this is
reflected by WSSI featuring moderate to locally major impacts,
focused in the Absarokas, Big Horns, and in the vicinity of
Yellowstone NP. These areas are also where the highest WPC
probabilities for 6+ inches exist, reaching above 90%, with locally
1-2 feet possible in the Absarokas.


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



Weiss