Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Mon Oct 11 2021

Valid 12Z Mon Oct 11 2021 - 12Z Thu Oct 14 2021

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

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

A digging trough moving into the Great Basin and its attendant
cold front will continue pushing southward and eastward today. An
upper low will form tonight as the trough deepens through northern
Arizona and by Tuesday morning heights at 500/700mb are progged to
be near the minimum of the CFSR climatology (1979-2009) for this
time of year. The cold front, bent across the Rockies, will
promote surface convergence and upslope flow which will foster
heavy snow over southern Montana and northern Wyoming tonight and
heavy snow along the Wasatch in UT tonight into Tuesday as the
front pushes through. Concurrently, the northern stream jet will
continue diving through California while the subtropical jet over
AZ/NM buckles northward, allowing phasing to occur and help spur
cyclogenesis over eastern Utah into Tuesday before reforming over
eastern Colorado. Surface low pressure will lift northeastward as
the front occludes over the Plains Tuesday afternoon. Strong upper
diffluence, surface convergence/upslope, and lower-level
frontogenesis will promote robust snowfall over the Bighorn
Mountains as moisture from the east gets wrapped into the
occluding system, aiding in TROWAL development and some intense
snowfall rates. Models continue to advertise an anomalous QPF
event just to the east with day 2 QPF near MAX values per the GEFS
ensemble mean relative to its reforecast via a healthy integrated
water vapor transport from the south. As the cyclone lifts out of
Colorado, heavy snow could occur in the deformation axis day 3
over western South Dakota early Wednesday as precipitation starts
to lift out. Probabilities of at least 4 inches of snowfall are
confined to the Black Hills and surrounding areas, generally 40 to
70%. Totals could exceed two feet over the Bighorns with a notable
shadow effect to the west in the Bighorn Basin. The 00Z guidance
shifted a bit farther west with its surface low track out of
Colorado and will continue to wobble around as overall the
consensus has been fairly stable, but mesoscale processes will
become more dominant as the system wraps up overnight Tuesday.

Day 1 WPC probabilities of at least 12" of snowfall are 30-80%
across southwestern Montana to northern Yellowstone in WY
including the northern Absarokas, Beartooth, and the Bighorns.
Snow levels will fall to the valley floors over most of Wyoming by
Tuesday with mesoscale banding/dynamical cooling aiding snowfall
production farther east. There, spread in the guidance was
maximized and uncertainty highest. For day 2, probabilities of at
least 12" of snowfall decrease along the Bighorns but are still
10-60%. Probabilities of at least 4" of snowfall are at least 20%
over much of central WY, stretching down into CO and back to UT.

There, the cold front will drive upslope-enhanced snowfall across
eastern Nevada ranges and 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 6-12" and locally even a bit more over the
southern Wasatch. Day 2 WPC probabilities of at least 4" of
snowfall are 30-80% over UT and up to the Wind River Range of WY.
Over Colorado, moist SSW flow into the San Juans will yield
several inches to near a foot with snow levels around 9000ft but
lowering as the QPF exits. Probabilities of exceeding 4 inches of
snow were generally above 40% in the higher terrain.

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

Fracasso