Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EST Sat Dec 03 2022

Valid 12Z Sat Dec 03 2022 - 12Z Tue Dec 06 2022

...California, Great Basin, and Northern/Central Rockies...

Days 1-3...
An upper low off the Oregon coast this morning will drift quite
slowly southward and southeastward over the next three days into
northern California by late Monday into early Tuesday. To the
south, a frontal boundary over southern CA will carry a wave of
low pressure inland toward and into the Sierra today with yet
another round of heavy snow for the mountains. With precipitable
water values around +2 to +3 sigma across the Central Valley
lifting into the Sierra, and flow out of the southwest, orographic
enhancement will support heavy snowfall rates 1-3"/hr this morning
as the axis lifts northward. Snowfall on Day 1 alone may be in
excess of a couple feet over the central and southern Sierra above
7000-8000ft. Modest snows are likely for the northern mountain
ranges (Klamath and Shasta Siskiyous).  Farther east, mid-level
wave streaking across the Wasatch/Rockies will promote a few
inches and locally more to higher elevations, where WPC
probabilities for at least 4 inches of snow are generally 10-50%.

On Day 2, the upper low off the coast will creep closer as the
moisture plume lifts northeastward through the Great Basin to the
Divide. Modest upper divergence via a jet streak will couple with
lower-level convergence and some orographic enhancement over
central Idaho into southwestern Montana to yield at least modest
snow totals there but higher amounts likely over the western
Wyoming ranges as a vort max spoke from the Pacific upper low
splits off and traverses the region. WPC probabilities for at
least 6 inches of snow are highest over western Wyoming in the
Tetons but lower over Idaho into Montana. Still, 1"/hr rates are
possible. Back into California, moisture associated with the upper
low will rotate into northern areas, yielding another day of snow
for the central Sierra northward where more than 8 inches is
likely in higher elevations as snow levels will be around 4000ft
in northern areas.

For Day 3, the upper low will open into a positively-tilted trough
over northern CA and the upper dynamics will weaken. As a result,
lighter snow will be more widespread from the Sierra eastward to
the Rockies as well as across Montana, aided by a frontal boundary
and some upslope flow. Highest totals will likely be over the
Colorado Rockies in the broad SW mid-level flow beneath the 130kt
jet. There, WPC probabilities for at least 6 inches are moderate
(>40%) with lower probabilities elsewhere.


For Days 1-3, the probability of significant icing exceeding 0.10
inch is less than 10%.


Fracasso