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

Valid 00Z Thu May 04 2023 - 00Z Sun May 07 2023

...Western U.S....
Days 1-3...
An anomalously deep mid-to-upper level low is expected to bring
late-season widespread light to moderate, with localized areas of
heavy precipitation including mountain snow, across California
over the next day.  This system, which is currently centered along
the coast of Central California, is expected to continue to drift
south before turning inland ahead of an approaching upstream wave
late today into Thursday.  Increasing onshore flow and dynamics
associated with a well-defined shortwave moving south of the
center are forecast to support heavier precipitation into Southern
California.  Several inches of heavy, wet snow can be expected for
portions of the Transverse Ranges, especially for areas above 5000
ft.  Locally heavy accumulations are also possible farther to the
northwest along the southern into the central Sierra. 

The threat for heavy mountain snow across Southern California is
expected to wane by late Thursday as the low weakens and lifts
northeast into the Great Basin.  As the system tracks northeast,
this will bring generally light snow accumulations to the Nevada
mountains on Thursday.  Then as the low continues northeast,
increasing dynamics and moisture are expected to support
widespread precipitation and locally heavy amounts across the
northern Rockies into eastern Washington and Oregon beginning late
Thursday and continuing into Friday.  Meanwhile, an elongated
negatively-tilted upper trough approaching the coast is expected
to contribute to some heavier precipitation across western
Washington.  However, snow levels are expected to start above 8000
ft across much of the region, limiting the potential for
widespread heavy snow accumulations.

Unsettled weather is likely to persist into the weekend from the
Pacific Northwest and Northern California to the northern Rockies,
with additional moderate to heavy precipitation expected over the
northern Rockies.  However, snow levels are forecast to remain
relatively high, with widespread heavy snow accumulations not
expected. 

The probability of ice accumulations greater than 0.25 inch is
less than 10 percent.

Pereira