Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Thu Nov 03 2022

Valid 12Z Thu Nov 03 2022 - 12Z Sun Nov 06 2022


...Four Corners and Central Rockies...
Days 1-2...

Shortwave troughing digging through the base of an anomalous
longwave trough centered across the Great Basin will amplify into
a an impressive closed low with height anomalies approaching -3
sigma according to NAEFS ensemble tables near the Four Corners by
Friday morning. Although this low is progged to open again quickly
and shift out into the Southern Plains during Friday, a period of
impressive omega will occur across the Four Corners and Central
Rockies associated with this system. Strong height falls,
downstream divergence with accompanying PVA, and increasing upper
diffluence within the left exit region of a 130kt poleward
streaking jet will produce strong UVVs, and lead to lee
cyclogenesis later today across CO, with another wave of low
pressure likely ejecting out of Utah this morning. As the lee low
develops, moist flow from the Gulf of Mexico will begin to rotate
cyclonically around the low and then lift isentropically along the
295-300K surface, with additional lift provided through upslope
flow into the higher terrain aided by moist 305K upglide from the
SW. Although PWs are progged to be just around normal for the time
of year, saturation within the DGZ will help snow growth, and some
overlap of -EPV in a region of theta-e lapse rates less than 0C/km
suggest heavy, potentially convective, snow rates. This is further
reflected by the WPC snow band probabilities suggesting 1-2"/hr
rates across much of CO west of the Front Range and into southern
WY. Snow levels will initially be 6000-8000 ft ahead of the
primary trough axis, but will fall quickly to 3000-4000 ft in
tandem with the more impressive lift, which could result in light
snow out into the High Plains including the I-25 urban corridor.

The heaviest snowfall D1 is likely from the Mogollon Rim northward
into the southern Wasatch, the San Juans, CO Rockies and as far
north as the Laramie range and Cheyenne Ridge, although snow
accumulations could be highly variable due to terrain influence
and the likelihood of scattered banded structures for snowfall.
Total accumulations of snowfall may reach 2 feet in the higher
terrain of the eastern San Juans where orthogonal flow and
persistent isentropic upglide with mixing ratios of 4g/kg should
support prolonged excessive snow rates. As snow levels fall and
these banded structures persist, some moderate to heavy snowfall
could extend as far east as the NE Panhandle, with a few inches of
snowfall likely along the urban I-25 corridor and Palmer Divide as
well noted by WPC probabilities for 2+ inches of 30-60%.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

Impressively confluent mid-level flow angling onshore from the
Pacific will combine with a zonally oriented upper jet streak
reaching 170 kts to drive a robust atmospheric river (AR) onshore
the Pacific Northwest and into the Northern Rockies late this
week. IVT from CW3E is progged by both the GFS and ECMWF to exceed
750 kg/m/s, possibly approaching 1000 kg/m/s, which is nearly 4
sigma above the climo mean according to the NAEFS ensemble tables.
This will support widespread heavy precipitation in the Pacific
Northwest, with moisture spilling over into the Northern Rockies
as well. Dual shortwaves embedded within this AR and confluent
flow will help to drive periods of more intense ascent, the first
moving onshore Friday night with the latter following at the end
of the forecast period. With the AR being of Pacific origin, and
strong WAA preceding the lead shortwave, snow levels will climb to
7000-9000 ft Friday, before falling to 2000-3000 ft by the end of
the period as a cold front gets dragged southward behind a low
pressure moving along the Canadian border. This will leave the
heaviest snow above pass levels through the event, but moderate
accumulations down to the passes are likely during D3.

The SLR forecast is quite challenging with the highly variable
snow levels reflecting a changing thermal structure, but with the
heaviest precipitation occurring within a marginally warmer
column. Strong ascent should overcome some of the warmth, but
still SLRs are likely to be less than the November median from the
Baxter climatology, and may be generally around 8:1 during much of
the heaviest snowfall, which is reflected as well by high
probabilities in the pWSSI due to snow load.

WPC accumulation probabilities are high for 6+ inches D1 in the WA
Cascades with more than 12 inches possible in the highest terrain.
By D2 the highest WPC probabilities for more than 6 inches
continue in the WA Cascades but also spill into the Northern
Rockies, Bitterroots, Tetons, Wind Rivers, and Blue Mountains,
where locally 2 feet of snow is possible. Snow levels crashing on
D3 combined with additional ascent ahead of the next shortwave
should result in heavy snowfall once again noted by WPC
probabilities above 60% in many of the same areas as on D2, but
with the passes also possibly receiving significant snowfall
Saturday and Saturday night.

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

Weiss