Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Sat Oct 09 2021

Valid 12Z Sat Oct 09 2021 - 12Z Tue Oct 12 2021

Day 1...
...Wyoming...
Exiting shortwave through the area will spread some accumulating
snow over parts of the Tetons and Absaroka Range but mostly over
the Bighorn Mountains as the moisture pulls away later tonight
into early Sunday. WPC probabilities of at least 4 inches of snow
exceed 60-70% over the Bighorns, especially over higher peaks
above 9000 ft. 

Day 2...
...Northern Cascades...
A northern stream trough will start to dig southward through the
Pacific Northwest downwind of a building ridge and
arcing/anticyclonically-curved 160-170kt jet, driving a strong
cold front southeastward on Sunday. With the frontal passage, snow
levels are expected to lower to about 3000ft as 700mb temps crash
to about -12C by early Monday. Moisture will be limiting, and
probabilities of at least 4 inches are only around 10-30% for the
higher peaks of the Cascades, with spotty areas over central Idaho
and southwestern Montana as well.

Day 3...
...Great Basin to Northern Rockies... 
Upper trough will continue to dig south-southeastward Monday and
close off near the NV/AZ/UT intersection by early Tuesday. 500 and
700mb heights may be as much as 4 sigma below average over
northern Arizona, which would be near the lowest observed heights
for this time of year per the CFSR climo. Right entrance region of
the northern stream jet will lift through the area, promoting
broad-scale lift coupled with low-to-mid level frontogenesis near
the surface cold front. Snow will spread across the Great Basin
where probabilities of at least 4 inches of snow are around 20-50%
over higher elevations (Shell Creek and Snake Ranges near the UT
border as well as over the Uintas). Best ingredients come together
over southwestern Montana into Wyoming where surface convergence
and higher QPF via higher precipitable water values, coupled with
lowering SLRs, will result in much higher snowfall totals in
excess of 6-12" in the terrain. Here, WPC probabilities are
greater than 70% for accumulations of 4-inches or more and exceed
40% for accumulations of 8-inches or more.
 
For Days 1-3, the probabilities for significant icing (0.25 inch
or more) are less than 10 percent. 

Fracasso