Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Tue Nov 01 2022

Valid 12Z Tue Nov 01 2022 - 12Z Fri Nov 04 2022

...Western U.S....
Days 1-3...

Upper trough off the British Columbia coast will sharpen and dig
into and through the Western CONUS over the next few days. With
multiple vorticity centers passing through the longer wavelength
trough, several areas will see enhanced snow rates and
accumulations via lower/mid-column frontogenesis and strong lift
through the DGZ, in addition to upslope enhancement. By late
Wednesday into Thursday, one vort max will lift through western MT
while the western side of the upper trough will carry another axis
of vorticity southward and southeastward into the Desert
Southwest, strengthening the baroclinic zone over the Four Corners
as the upper jet intensifies to over 130kts. Mid-level heights
will plunge to around -3 sigma as a surge of moisture lifts
into/through AZ/NM/CO as the system steadily moves eastward into
Friday morning. Also by then, the next atmospheric river will
point into Washington with a somewhat narrow axis of PW values >1"
(or +2 sigma) but IVT values likely over 700 kg/m/s.

Day 1 will see the lead-in shortwave move into NorCal before
lifting into Idaho by early Wednesday. Strong frontogenesis lying
at the nose of the 0.50" PW plume (though decreasing) will
capitalize on upslope enhancement across the Sierra and northern
CA ranges where several inches are likely especially above 6000ft.
Rates may likely exceed 1"/hr in northern areas but perhaps 2"/hr
over the central Sierra above 7000ft. Snow levels will drop in the
wake of the cold front, with some light snow possible down to
about 3500ft. This shortwave will also enhance snowfall over
northern ID and northwestern MT tonight into early Wednesday where
WPC probabilities for at least 8 inches are above 50%, including
the Oregon Cascades and Blue Mountains.

As the upper trough deepens/sharpens into Day 3, broad
southwesterly flow and WAA will drive widespread QPF over the
Great Basin/central Rockies with lowering snow levels (from 5000
to 3000-4000 ft in the Great Basin and western UT/AZ and from 8000
to around 6000 ft in CO/NM). Above these levels, the anomalous PWs
driven by the increasingly meridional flow out of Mexico will
support widespread snowfall, with the heaviest amounts from the
Wasatch and southern UT ranges into the Uintas. The CO Rockies and
especially the San Juans will also see modest to heavier snow,
especially where southerly flow impinges orthogonally into the
terrain. WPC probabilities for at least 8 inches are above 50%
over the higher terrain in UT/CO during late day 2 into day 3.
Lastly, the Wyoming ranges will see at least several inches of
snow in between the better forcing but enhanced via upslope into
the Wind River Range and Bighorn Mountains.

The probability of significant icing greater than 0.25" is less
than 10 percent.

Fracasso