Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
442 PM EDT Sun Oct 10 2021

Valid 00Z Mon Oct 11 2021 - 00Z Thu Oct 14 2021

...First potent winter storm to bring heavy snow to portions of
the Great Basin and north-central Rockies Monday into Wednesday...

...Great Basin, Northern and Central Rockies...
Days 1-3...

A reinforcing shortwave trough that amplifies as it digs south
from the Pacific Northwest coast tonight will close off into an
upper low over southern Nevada and track across northern Arizona
Monday night before ejecting northeast to the northern Great
Plains by Wednesday morning. 12Z guidance remains in good
agreement with overall track and depth with the GFS a bit
faster/farther north with the track over the Plains on Wednesday.
The 00Z NAEFS data in the Ensemble Situational Awareness Table
continue to have the 500 and 700mb heights near the minimum of the
CFSR climatology (1979-2009) for this time of year Tuesday morning
over southern NV and then along the UT/AZ border. The northern
stream jet shifting through California and the subtropical jet
over AZ/NM will phase on Monday night and help spur cyclogenesis
over eastern Utah Monday night then over eastern Colorado on
Tuesday with surface low pressure deepening through the day as it
lifts northeastward and the front occludes. The leading cold
front, coupled with surface convergence and upslope will promote
heavy snow over southern Montana and northern Wyoming
Monday/Monday night and heavy snow along the Wasatch in UT Monday
night into Tuesday. On the northwest side of the developing low
pressure over Colorado on Tuesday, strong upper diffluence will
promote robust snowfall over the Bighorn Mountains in upslope flow
as moisture from the east gets wrapped into the occluding system,
aiding in TROWAL development and some intense snowfall rates. This
heavy snow looks to spill onto the plains east of the Bighorns
(and west of the Black Hills) Tuesday into Tuesday night. Totals
could exceed two feet over the Bighorns with a notable shadow
effect to the west in the Bighorn Basin.

Day 1.5 WPC probabilities of at least 12" of snowfall are 30-70%
across southwestern Montana to northern Yellowstone in WY
including the Absarokas, and the Bighorns. Snow levels will fall
to the valley floors over most of Wyoming by Tuesday with
mesoscale banding/dynamical cooling needed to produce heavy
accumulations that day over far eastern WY.

Farther south, the cold front will drive upslope-enhanced snowfall
across eastern Nevada ranges and across Utah as the upper low
swings just south of the region. Though duration will be somewhat
limited, impressive dynamics of the system will make good use of
the available moisture against the mainly north-south ranges via
upslope flow, with higher mountain tops and western upslope
elevations seeing 6" or more as day 1.5 WPC probabilities of
30-60% over eastern NV increase to 30-80% over UT and up to the
Wind River Range of WY for Day 2. A foot or more for is possible
for the southern Wasatch. Day 2 WPC probabilities for at 6" or
more 30-80% for the San Juans of CO. Then Day 2.5 probabilities
for 6" or more of 30-80% expand across terrain in south-central WY
and north-central CO.

The probability for significant icing (0.25 inches or more) is
less than 10 percent Days 1-3.

Jackson