Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Mon Oct 31 2022

Valid 00Z Tue Nov 01 2022 - 00Z Fri Nov 04 2022

...Western U.S....
Days 1-3...

Confluent mid-level flow across the Pacific Ocean will spread
inland to the Pacific Northwest and sharpen with some increasing
WAA ahead of a shortwave trough which will lift onto the WA/OR
coasts Tuesday morning. This shortwave will gradually deepen into
a full latitude trough across the Great Basin by Thursday, with a
leading spoke of vorticity shedding into the Northern Rockies by
Wednesday morning. This will result in widespread snowfall across
much of the western CONUS, with two areas of heaviest snowfall
likely.

The first is from the OR Cascades into the Northern Rockies D1
into D2 associated with the leading shortwave. This shortwave will
drive height falls and PVA in tandem with the favorable diffluent
RRQ of a departing but strengthening upper jet streak to produce
robust deep layer ascent. At the same time, strong WAA will
enhance lift, but also drive snow levels as high as 7000 ft before
a developing surface low moves northeast to advect the cold front
eastward lowering snow levels in its wake. Along this front, an
impressive axis of 700-600mb fgen will develop and may linger for
a time as the low moves slowly due to the shortwave closing off.
This fgen appears to be collocated with the deepening DGZ, which
could result in heavy snowfall rates progged by the WPC snow band
tool of 1+"/hr at times in the vicinity of the Northern Rockies
and southward towards the northern Snake River Valley.
Additionally, heavy snow is likely in the OR Cascades where the
most impressive moisture anomalies overlap with orthogonal mid
level flow to drive better upslope ascent. WPC probabilities have
increased for 6+ inches, especially on D2, and above 3000-4000 ft
for the OR Cascades, Blue Mountains of OR, Bitterroots and
Northern Rockies, where 1-day snowfall of 1-2 ft is possible.
Lighter snowfall of more than 4 inches is possible into the WA
Cascades and ranges of NW WY D1-2.

As the trough amplifies further Wednesday and Thursday into a full
latitude trough, a second shortwave will move out of the Pacific
into CA and merge with the deepening trough to enhance ascent
across the Great Basin and into the Four Corners area as the
trough closes off late D3. Snow levels will rapidly drop as this
trough axis swings eastward, and pronounced ascent will develop
downstream in response to potent height falls, WAA, and secondary
jet development downwind of the primary trough axis. Together
these will produce widespread precipitation, with a surface low
developing in the lee of the CO Rockies late in the period. Snow
levels fall to 3000-4000 ft in the Great Basin, UT, and AZ, and to
around 6000 ft in CO/NM. Above these levels, the anomalous PWs
driven by the increasingly meridional flow out of Mexico will
support widespread snowfall, but the heaviest amounts are likely
D1-2 in the Sierra due to onshore flow/orographic enhancement, and
then D3 from the Wasatch into the Uintas, San Juans, and CO
Rockies, especially where southerly flow impinges orthogonally
into the terrain. WPC probabilities for more than 6 inches are
30-50% in the Sierra D2, with event total snowfall above 10 inches
possible in the higher terrain. Farther to the east, WPC
probabilities D3 reach 50-70% across much of the terrain of UT and
into CO/WY, with more than 12 inches of snow likely in the San
Juans as snowfall rates are likely to be intense beneath
steepening lapse rates within a moist DGZ.

The probability of significant icing greater than 0.25" is less
than 10 percent.

Weiss