Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 PM EST Wed Nov 23 2022

Valid 00Z Thu Nov 24 2022 - 00Z Sun Nov 27 2022

...Central Rockies...
Day 1...

The amplifying northern stream shortwave across MT this morning
will close off and drop almost due southward as mid-level flow
becomes quite amplified. This closed low will deepen as it shifts
into the Southern High Plains, leaving strung out vorticity across
the Rockies overlapped with modest LFQ diffluence as an upstream
jet streak sharpens and digs southward in concert with the
mid-level energy. This trough will also drive a cold front
eastward through the middle of the country, with NE flow
developing around a large high pressure in its wake. This will
drive significant upslope flow in the still moist airmass across
the Central Rockies, leading to some enhanced snowfall in the
terrain from the WY Front Range southward as far as the Raton
Mesa. WPC probabilities are high for more than 4 inches from the
Big Horns into the Laramies and CO Rockies, but are most
aggressive in the northern Sangre de Cristos and along the Raton
Mesa where locally 10+ inches of snow is likely.


...Texas Panhandle to Southeast New Mexico...
Days 2/3...

Shortwave over WY this evening will rapidly deepen into an
anomalously strong closed low near the TX/NM border Thursday
evening, before slowly spinning to the east by Saturday morning.
The guidance has trended stronger, slower, and southwest today,
and NAEFS ensemble tables suggests 500mb heights falling towards
-3 standard deviations across West Texas. At the same time,
upstream and downstream upper level jet streaks will strengthen,
resulting in robust deep layer ascent spreading across the
Southern High Plains. Downstream of this feature, impressive warm
and moist advection will drive intense 290-295K isentropic upglide
across Texas and into New Mexico, while the resultant theta-e
ridge lifts within the WCB to a TROWAL. Mixing ratios within the
best isentropic lift will reach 4-6 g/kg, suggesting a very moist
environment supporting widespread and heavy precipitation. The
TROWAL aloft will help drive at least modest instability as well,
reflected by pockets of theta-e lapse rates of <0C/km indicating
CSI potential. While the low-level thermal structure is modest,
especially across TX, intense ascent should produce heavy
precipitation rates which will dynamically cool the column to
result in bands of heavy snow. The heaviest snow is likely in NM
which has marginally cooler low-levels, but in TX, especially in
the Permian Basin and Trans-Pecos region, p-type may shift at
times from rain to snow depending on precip intensity.

With this slow moving storm system, there are likely to be two
snowfall maxima. The first is along the upwind side of the
Sacramento, Guadalupe, and Davis mountains where flow will upslope
efficiently to enhance ascent and wring out the impressive
moisture. The heaviest snowfall is likely Friday and Friday night,
when WPC probabilities for more than 4 inches are as high as 70%,
with probabilities for 8+" reaching 30%. It is possible that local
maxima above 12 inches will occur. Farther to the east across far
SE New Mexico, the environment will support an impressive and
pivoting deformation band of snowfall with rates exceeding 1"/hr
despite low SLRs yielding a heavy and wet snow. WPC probabilities
for 4+ inches and 8+ inches reach 70% and 30%, respectively, with
again local maxima above 12 inches possible where this band pivots
the longest. A broader area of more than 4 inches is also possible
covering much of the Permian basin and into the southern Texas
Panhandle. Difficult travel is likely across much of this area as
heavy snow rates result in snow covered and slippery roads at
times.


...Pacific Northwest...
Days 2-3...

Zonal mid-level flow will sharpen ahead of a shortwave trough
progged to move onshore Washington/Oregon early D3 /Friday night/.
Modest WAA ahead of the trough axis will advect moisture onshore,
and combine with the RRQ of a departing upper jet streak to drive
deep layer ascent to expand precipitation across the region. The
flow will remain fast so total forcing is expected to be
relatively transient, but where upslope flow can locally drive
stronger UVVs, heavy snow is likely. Snow levels will climb to
around 6000 ft into the Cascades within the most impressive WAA,
but should remain generally 2000-3000 ft from the Okanogan
Highlands into the Northern Rockies. WPC probabilities are 20-30%
on D2, and 30-50% on D3, for more than 4 inches of snow along the
Washington Cascades. For D3, moderate snow is likely to spread
eastward into the Bitterroots and Northern Rockies where WPC
probabilities for 4+ inches reach 20-40%.

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

Weiss