Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EDT Thu Oct 22 2020

Valid 00Z Fri Oct 23 2020 - 00Z Mon Oct 26 2020

...Northern and Central Rockies through the Central Plains...
Days 1-3...

A potent storm system will begin to take shape late tonight as it
dives into the Pacific Northwest. A shortwave embedded within the
broad trough/cyclonic flow will dive towards the Northern Rockies
through Saturday and then elongated/shear to both the SW and NE as
the mid-level trough becomes strongly positively tilted. At the
same time, a potent Pacific jet streak will stream eastward,
before splitting off to the east downstream of the amplifying
trough into Sunday. This leaves the favorable diffluent LFQ and
divergence maxima atop the Pacific NW early, with a coupling of
jet streaks developing the latter half of the period. The height
falls, coupled jet streaks, and shortwave together will drive
surface low development which will drop to the southeast through
the High Plains of CO Sunday.

Deep layer ascent is expected to be robust, and will only be
enhanced by mesoscale features. Upslope enhancement into the
eastern terrain is likely behind a cold front diving through the
Plains and banking against the mountains. Additionally, some
enhanced fgen is likely beneath the jet streaks, and mid-level
ascent will additionally maximize in the vicinity of the 700mb low
tracking SE across the Northern Rockies. All of this will occur in
a region of high PW, +1.5 standard deviations above the climo
mean, as a modest atmospheric river (IVT 500kg/m/s) transports
moisture well inland.

This together suggests widespread heavy snowfall beginning early
Friday in the WA Cascades, and spreading east/south through the
weekend. The guidance is in very good agreement in placement and
timing, and the biggest factor to snow amounts appears to be snow
levels, as moisture and ascent will be in place. Snow levels
initially may be 3000-4000 ft in the terrain of the Cascades and
Northern Rockies, but should fall steadily as the event unfolds. A
higher snow level is likely at precip onset across WY/CO, but
again, these are expected to drop quickly as precipitation begins
and the trough digs southward. The colder temperatures at higher
elevations will lead to higher SLRs, and the heaviest snowfall is
likely above 4000 ft across the ranges. WPC probabilities are
moderate to high for 12 inches in the WA Cascades, Bitterroots,
Northern Rockies, Absarokas, Tetons, and into the Central Rockies
west of the Front Range by Sunday. Some areas may exceed 18" of
snowfall. At lower elevations including the valleys in these
ranges, as well as the High Plains of eastern MT, WY, and into the
I-25 corridor of CO, WPC probabilities for 6" are moderate,
although a relative minimum may occur across WY where the jet
streaks split. Further east into the Plains, including
ND/SD/NE/MN, confidence is lower here in the evolution, but WPC
probabilities indicate a moderate chance for a swath of 4"
spreading eastward beneath the leading upper jet streak.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.


Weiss