Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 PM EDT Sat Nov 04 2023

Valid 00Z Sun Nov 05 2023 - 00Z Wed Nov 08 2023

...Northwest, California to the Northern Rockies...
Days 1-3...

A series of upper level shortwaves and their associated surface
fronts cross the region, producing periods of unsettled weather,
including mountain snow through the early part of the week. 

The leading shortwave will continue to move onshore through the
evening and advance progressively east into the northern Rockies
tomorrow.  Few impacts are expected in the Northwest with this
first system as snow levels are expected to remain high until much
of the precipitation has already fallen.  While favorable forcing
ahead of 130+ kt upper jet is expected to enhance ascent farther
east across portions of the northern Rockies, snow levels are
forecast to be high as well, confining any significant
accumulations mostly to the high peaks of the northwestern Wyoming
ranges.

The next system will begin to impact the Northwest tomorrow and
begin to carve out a broader and more amplified trough that will
move across the West as upstream energy moves onshore.  While
mountain snow coverage is expected to become more widespread
Sunday night into Monday, impacts will remain limited. In the
Cascades, snow levels are forecast to remain above pass level
until at least Monday night.  However, some of the higher Sierra
passes may see an inch or two of snow late Monday, with additional
accumulations expected overnight into Tuesday.

As the upper trough continues to amplify and snow levels drop, the
threat for accumulating snow at the pass level is expected to
increase over Cascades.  WPC probabilities show at least a
moderate risk (40 percent or greater) chance for accumulations of
4 inches or more for the major passes Monday night into Tuesday. 
Farther west, probabilities for accumulations of 2 inches or more
are moderate to high (70 percent or greater) along the northern
Idaho-Montana border passes, Marias Pass in northwestern Montana,
and for many of the northwestern Wyoming passes.

Pereira