Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
127 PM EST Sat Feb 04 2023

Valid 00Z Sun Feb 05 2023 - 00Z Wed Feb 08 2023


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

Amplified longwave trough approaching the Pacific coast will move
onshore Sunday morning and then become elongated as vorticity lobe
amplifies through the base of the trough into the Southwest, while
a northern stream impulses races forward towards the Northern
Plains Monday morning. This will result in a stretched out
positively tilted longwave trough arcing from the Great Lakes
through the Southwest to start D3 /Monday night/ with impressive
vorticity energy rotating into the Four Corners at the Upper
Midwest late in the forecast period. This extended region of
height falls will result in pronounced deep layer ascent, aided by
Pacific jet energy surging onshore as the 130kt core of this jet
streak advects zonally into the Four Corners and Central Plains.
The resultant UVVs will act upon an increasingly moist environment
as the downstream confluent flow beneath the upper jet will drive
IVT of more than 250 kg/m/s according to CW3E onshore, providing
an environment favorable for heavy snow. Snow levels will be
modest through the weekend and into early next week thanks to this
Pacific sourced airmass, remaining around 4000 ft except in CA
where the initial surge of WAA will drive snow levels to around
7000 ft Sunday. However, a cold front pushing eastward beneath the
primary trough axis will cause a brief drop in slow levels to
2000-4000 ft across much of the west before recovering once again
late in the forecast period, although most of the snow should
occur before the frontal passage.

On D1, the heaviest snow is likely across the Sierra where
orthogonal flow and steep lapse rates will combine to produce
impressive snow rates, which could reach 3"/hr at times as
reflected by the WPC prototype snow band tool, and with SLRs
likely around or just below the Baxter median for February, at
least to start, this will result in an impactful heavy and wet
snow as shown by high probabilities in the pWSSI for major impacts
due to snow load and snow rate. WPC probabilities for more than 6
inches of snow are high for the length of the Sierra, the
Shasta/Trinity/Siskiyou region, Oregon Cascades, and as far east
as the Salmon River range. Maximum snowfall of more than 2 feet is
likely in the Sierra. During D2 the forcing begins to shear to the
east and become less intense, but the expansion of overlap between
moisture and ascent will spread heavy snow with larger terrain
coverage. WPC probabilities for more than 6 inches are 20-40% for
parts of the Cascades, Bitterroots, Wasatch, and CO Rockies, with
again significant impacts likely in the terrain of the Wasatch due
to heavy snow rates. During D3, a secondary surge of moisture with
another shortwave approaching the Pacific Northwest will spread
heavy snow into the WA Cascades once again, with moderate
accumulations potentially falling across both Stevens and
Snoqualmie Passes.

There is some potential that the closed low moving through the
Desert Southwest combined with the RRQ of the downstream jet
streak that develops D3 could result in an area of low pressure
strengthening over TX and spreading snow back into the High Plains
of NM and the Panhandle of TX. The guidance is split on this
evolution with the ECMWF and its ensembles being more aggressive
than the other models. At this time confidence is low and WPC
probabilities for even 1 inch of snow are less than 10%, slightly
higher into the Sacramento Mountains.

The probability of significant icing is less than 10 percent.

Weiss