Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Sun Oct 17 2021

Valid 12Z Sun Oct 17 2021 - 12Z Wed Oct 20 2021

...Pacific Northwest and Northern California to the North-central
Rockies...
Days 1-3...
A well-defined shortwave moving through the base of a broader
scale trough currently centered off of Pacific Northwest coast
will continue to dig south, with models showing an upper low
developing as it moves across northern California into Nevada
Monday morning.  Widespread precipitation is expected to spread
across western Washington and Oregon and northern California
Sunday into early Monday.  Snow levels beginning around 6000-7000
ft on Sunday are forecast to drop below 5000 ft Monday morning. 
With the latest run, WPC probabilities for accumulations of 4
inches or more have increased across portions of the southern
Cascades and northern Sierra Nevada.  For the Day 1 period --
ending 12Z Monday -- probabilities of 40-70 percent for
accumulations of 4 inches or more now cover a larger extent of the
northern Sierra Nevada.  WPC probabilities continue to indicate
that locally heavy amounts are also likely for portions of the
Warner Mountains in northeastern California Sunday night into
Monday morning.

For Monday into early Tuesday, models have remained consistent in
showing the upper low moving east across northern Nevada and Utah
to the western Wyoming-Colorado border.  Favorable upper jet
forcing is expected to help support organized precipitation,
including high elevation snow, from northern Nevada and southern
Idaho into the north-central Rockies.  Areas impacted are forecast
to include the Independence and Ruby mountains in northeastern
Nevada -- where probabilities for accumulations of 4 inches have
increased with the latest run as snow levels drop to around 5000
ft on Monday.  Probabilities for accumulations of 4 inches or more
have increased for portions of the Uintas in northern Utah as
well.  Probabilities for accumulations of 4 inches or more remain
above 40 percent for parts of the Tetons and Absaroka ranges, with
some greater than 70 percent probabilities noted for portions of
the Wind River Range.  Snow levels are expected to drop to around
6500 ft across much of western Wyoming Tuesday morning.

On Tuesday, the upper low is forecast to continue east along the
Wyoming-Colorado border into the High Plains by late in the day. 
Favorable upper forcing will continue to support organized
precipitation along and northeast of the low track.  The greater
potential for heavy snow accumulations is expected to shift east
into central and eastern Wyoming.  For the Day 3 period -- ending
12Z Wednesday -- WPC probabilities are greater than 50 percent for
accumulations of 8 inches or more for portions of the Laramie
Mountains.  Significant accumulations are possible for parts of
the Bighorn Mountains as well -- with probabilities for 4 inches
or more above 50 percent.   

For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 10 percent.

Pereira