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

Valid 12Z Sun Oct 10 2021 - 12Z Wed Oct 13 2021

...Pacific Northwest...
Day 1...

A shortwave will dig into Washington today and split its
northeastern part into Montana as its southwestern part continues
through Oregon. Surface cold front will continue southeastward,
lowering snow levels to around 3000ft in tandem with decreasing
QPF. Sharpening of the trough aloft will help to limit moisture
but will still be capitalized in the highest elevations of the
Cascades. WPC probabilities of at least 4 inches of snow are
modest (10-30%) except for around Mt. Rainier and areas above
Snoqualmie Pass where they lied near 70%.


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

Robust and dynamic snow event will unfold over the region starting
Monday and continuing into Wednesday.

Sharpening/deepening upper trough moving into the Great Basin
early Monday will close off an upper low near Las Vegas and turn
ENE across the Four Corners region Tuesday before moving into
western Nebraska by early Wednesday. Models and ensembles remained
in good agreement overall with its track and depth with some
lingering speed differences as it turns the corner across the
Divide. Per the NAEFS Ensemble Situational Awareness Table, both
500 and 700mb heights are forecast to be near or at the minimum of
the CFSR climatology (1979-2009) for this time of year Tuesday
morning along the UT/AZ border. Northern stream jet through
California and subtropical jet across northern Mexico will phase
on Tuesday and help spur cyclogenesis out of Utah into eastern
Colorado with surface low pressure deepening through the day as it
lifts northeastward and the front occludes. Lead cold front
coupled with surface convergence and upslope will promote heavy
snow over southern Montana and northern Wyoming Monday 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. Totals may be
well over a foot in favored areas but with significant shadowing
in the Bighorn Basin. WPC probabilities of at least 4 inches of
snowfall are 10-40% on Sunday across southwestern Montana but then
increase and expand eastward Monday-Tuesday (large area of >80%).
Probabilities of exceeding 12 inches are 20-60% over mostly higher
elevations in the Absarokas and Bighorns. Snow levels will fall to
the valley floors nearly everywhere by early Wednesday over
Wyoming.

Farther south, cold front will drive upslope-enhanced snowfall
across eastern Nevada into 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 north-south ranges via upslope flow,
with higher mountain tops and western upslope elevations seeing
several inches to perhaps locally more than ten inches of snow.
WPC probabilities for at least 4 inches are in the 10-50% range
overall on Monday. As the system lifts across Colorado on Tuesday,
San Juans may see the higher totals in the region, where WPC
probabilities of at least 4 inches lie close to 70%.

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

Fracasso