Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EDT Wed May 03 2023

Valid 12Z Wed May 03 2023 - 12Z Sat May 06 2023

 
...Western United States...
Days 1-3... 

An anomalously strong closed low just off the CA coast will slowly
pivot southward as the omega block to the east persists. This
closed low will begin to advect eastward, finally, Thursday night,
leading to ridge breakdown across the Plains and allowing features
to begin to advect into the westerlies once again. As this closed
low pivots east and then northeast into the Great Basin Friday, it
will interact with other shortwaves rotating around the large
western trough, resulting in a deep but open longwave trough
centered just inland from the Pacific Coast by Saturday morning.

This evolution will drive widespread moderate synoptic ascent
across much of the West, expanding from primarily just CA D1 into
the Great Basin and Pacific Northwest by D3. Moisture will spread
across the West downstream of this closed low, eventually pivoting
well northward into the Northern Rockies and Pacific Northwest
late Friday. PW anomalies are progged to reach as high as +2
standard deviations by D3, but more generally 0.5 to 1 sigma above
the climo mean. This increased moisture in the presence of the
broad large scale ascent will drive expanding precipitation across
much of the West. As the trough expands across the West, snow
levels will fall to generally 3000-5000 ft, allowing for at least
some accumulations to pass levels despite overall a marginal
thermal structure.

WPC probabilities for more than 4 inches of snow on D1 are
minimal, but expand significantly D2 across the Sierra and into
some of the Great Basin ranges in NV, reaching 30-50%, with some
low probabilities even in the transverse ranges of Southern CA.
During D3, the overlap of moisture into cold enough air for snow
expands all the way to the Canadian border, with WPC probabilities
for more than 4 inches reaching as high as 50% in the Sierra, as
well as the higher portions of the Sawtooth/Salmon River ranges,
and into the WA Cascades. 
 
The probability of significant icing greater than 0.25" is less
than 10 percent. 
 
Weiss