Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Fri Nov 25 2022

Valid 00Z Sat Nov 26 2022 - 00Z Tue Nov 29 2022

...New Mexico to West Texas...
Day 1..

Deformation band in the vicinity of an impressive closed upper low
over West Texas will pivot slowly northeastward through Saturday
aftn before ejecting away from the region. While low-level
thermals will continue to be marginal outside of the terrain,
impressive dynamic cooling is anticipated through intense ascent,
resulting in periods of heavy snowfall from the Sacramento
Mountains southeastward through the Davis Mountains and into the
Trans-Pecos/Permian Basin. Moist isentropic upglide into the
region will provide plentiful moisture for precipitation, while
the developing TROWAL helps drive higher instability from east to
west. This suggests snowfall rates of 1-2"/hr are likely which is
reflected by HREF probabilities, which will overcome the marginal
surface temps to rapidly accumulate. The heaviest snowfall is
likely in the Sacramento, Guadalupe, and Davis mountains where
upslope enhancement will occur, with a secondary maxima likely
over southeast NM where the deformation band pivots the longest.
WPC probabilities indicate just a low-risk for an additional 4+
inches, highest across the terrain, but locally higher amounts are
possible before the system ejects northeast late D1.


...Pacific Northwest and northern Rockies...
Days 2-3...

Multiple shortwaves embedded within an amplifying trough across
the Northwest will lead to an expansion in coverage of
precipitation, combined with lowering snow levels, and heavy snow
is likely to spread across much of the region by early next week.
The first, weaker, shortwave is progged to move across WA this
evening and then race quickly in otherwise fast zonal flow into
the Northern Rockies before weakening as it continues to dig
southeast towards the Four Corners on Sunday night. Moisture
associated with this is modest noted by PW anomalies just slightly
above normal for late November, but height falls and accompanying
weak upper diffluence will provide enough ascent to wring out
precipitation as snowfall, generally above 4000 ft. Overall
forcing on D1 is transient, but WPC probabilities for more than 4
inches reach 20-40% for the Bitterroots, Northern Rockies, and as
far southeast as the Big Horn range.

Increasingly confluent mid-level flow will then begin to angle
into the Pacific Northwest, within which a strung out lobe of
vorticity will begin to advect southeastward as the leading edge
of the amplifying trough. This will combine with a re-energizing
Pacific jet streak to produce impressive ascent trough LFQ
diffluence, height falls, and PVA stretching across much of the
area as far east as MT and into northern CA. Increasing mid-level
RH will spread across the Pacific Northwest, and although PW
anomalies are progged to be minimal, persistent and at times
intense forcing through the synoptic flow and periods of
impressive upslope enhancement will result in snowfall that will
be heavy across much of the terrain, while at the same time snow
levels drop to below 1000 ft in the far NW, and 1500-3000 ft
elsewhere. While the most significant accumulations will be in the
higher terrain, these falling snow levels will result in heavy
snow even down to pass level creating notable travel impacts,
although at this time snow accumulations are expected to remain
out of the lowlands. WPC probabilities are moderate to high for
more than 6 inches on D2 for the WA Cascades, the Bitterroots, and
Northern Rockies, with locally more than 18 inches possible,
especially in the WA Cascades. By D3, heavy snow spreads south and
east, and WPC probabilities for 6+ inches exceed 60% for the WA/OR
Cascades, the Blue Mountains, and into the Sawtooth and ranges of
NW WY including the Tetons. With snow levels collapsing, WPC
probabilities indicate a high chance for more than 6 inches at
Stevens, Snoqualmie, and Santiam Passes with locally 12 inches
possible. Lighter but still significant snowfall is likely farther
east including Deadman and Lookout Passes. The latest pWSSI
indicates a high potential for at least moderate impacts to travel
across these passes Sunday into Monday.  

The probability of significant icing exceeding 0.1 inch is less
than 10%.

Weiss