Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 AM EDT Thu Oct 07 2021

Valid 12Z Thu Oct 07 2021 - 12Z Sun Oct 10 2021

...Intermountain West to the North-Central Rockies...

Days 2-3...

Western third of the CONUS will see a multi-day period of
reinforcing/punctuated troughing well ahead of a much deeper upper
low over Alaska. At the start of day 2, 12Z 8 Oct., one upper
shortwave (briefly closed low) will be on approach into central CA
with an additional digging shortwave to its north just west of
Washington and weaker/lead shortwaves streaming northeastward in
broad southwest flow aloft across the Rockies. 120-135kt upper jet
across CA into southern NV will translate eastward Fri-Sat and
provide broad-scale ascent in the left exit region. Precipitable
water values currently in the +2 to +3 sigma range will decrease
as the lead trough lifts through UT/WY at the end of day 2 into
day 3, but remain above normal even into early Sunday morning.
Temperatures will cool with time but snow levels will start high
(10-11k ft) Friday before lowering as the second/trailing upper
low swings through AZ late Sat into early Sun. Largest spread in
the ensembles with respect to 700mb temperatures was in the warm
sector along and east of the ensemble mean 4C isotherm (Four
Corners region eastward) but with a secondary max in spread around
0C from southern UT into WY early Sat as a second wave of
rain/snow moves across the area. Day 2 WPC probabilities for >4"
snow were 40-60% over the Uintas and 40-70% over the Wind River
Range in WY, with lower probabilities over other higher elevations
(Tetons and Absarokas). Day 3 probabilities for >4" snow drop off
to <30% over northern WY as most of the moisture moves into the
northern Plains.


For Days 1-3, the probabilities for significant icing (0.25 inch
or more) are less than 10 percent.


Fracasso