Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EDT Sat Nov 05 2022

Valid 00Z Sun Nov 06 2022 - 00Z Wed Nov 09 2022

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

The pattern over the western U.S. will be driven by an amplifying
upper trough diving south from British Columbia tonight to the
Pacific Northwest on Sunday. It will eventually end up as far
south as California by Monday and Tuesday. For tonight, the IVT
responsible for delivering the steady onslaught of rich Pacific
moisture remains above the 99th climatological percentile across
the Great Basin this evening. The anomalous moisture aloft
combined with upslope flow in topographically-favored terrain
(Wasatch, Uinta, Sierra Madre, Medicine Bow Ranges) is a favorable
setup for periods of heavy snow above 7,000 ft. Closer to the West
Coast, the upper trough dives south through the Pacific Northwest
on Sunday with dramatic height falls occurring from Central
California to the interior Northwest. Come 00Z Monday, 500-700mb
heights are 3-4 sigma below normal according to NAEFS over western
Oregon. At the base of the trough, a steady barrage of 850-700mb
moisture flux within strong WSW flow will be directed at the
Sierra Nevada. In addition, a strong cold front plunging south
will cause snow levels to plummet and SLRs to rise. These factors,
combined with the strong upslope enhancement, allowing for heavy
snow from the Shasta and Trinity ranges Sunday evening to the
Sierra Nevada Sunday night and into Monday.

Anomalously cold mid-level temps overtake northern and central
California by Monday. NAEFS standardized anomalies for 500-700mb
temps are as cold as 2-3 sigma below normal there, with values
closer to ~2 sigma in Oregon and Idaho. Plus, moisture-rich WSW
flow embedded within a favorable upsloping regime and
synoptically-forced PVA will foster periods of heavy snow from the
Sierra Nevada and the central Great Basin to the northern Rockies.
Latest WPC probabilities are as high as 80-90% for snowfall
amounts >12" in the Sierra Nevada. In fact, probabilities for >18"
of snowfall are as high as 60-70% going from late Monday into
Tuesday. The shear amount of snowfall is the primary driver in the
WPC experimental pWSSI, which is suggesting a 50-60% chance for
"Major" impacts. By 00Z Tuesday, there is an impressive fetch of
700mb moisture stretching from southern California all the way to
the Tetons. This moisture feed is responsible for continuous
rounds of snowfall as far north as the Absaroka and Wind River
Ranges, where WPC probabilities show 60-80% chances for snowfall
totals >8".

By Tuesday, another shortwave trough diving south within the large
longwave trough on the West Coast supplies yet another round of
PVA to California and the Intermountain West. At 250mb, the
divergent right-entrance region of a ~130 knot jet streak will be
positioned over Utah while a second jet streak on the backside of
the upper trough places its divergent left-exit region over
southern California. Meanwhile, high pressure over southwest
Canada will continue to anchor a dome of sub-freezing temperatures
over the northern Rockies and High Plains. Strong frontogenetical
forcing combined with a steady diet of 700mb moisture flux into
the Wasatch gives the range an ideal setup for heavy snowfall
rates. WPC probabilities for >8" of snow are highest in central
Nevada where they are as high as 50-60% in the tallest elevations.
Farther south, the conveyor belt of 850mb moisture flux continues
to pump rich Pacific moisture into the Transverse Range and the
Great Basin. WPC probabilities do depict some of the higher
terrain of the Transverse range with as much as 20-40% chance for
>4" of snowfall. There does remain a wide range of snowfall
possibilities here, largely driven by the amount of precipitation
and how cold the mid-levels can get to support even heavier
snowfall totals.

...Northern High Plains...
Days 2-3...

A little farther east, the northern High Plains can also expect
impactful wintry weather. The same upper trough digging south
along the coast if the Pacific Northwest will provide a steady
stream of PVA over the region, prompting supportive upper level
divergence overhead. Meanwhile, strong high pressure over the
Middle Mississippi and Ohio Valleys will work in tandem with the
deepening low pressure system in the Northwest to create a strong
SE 850mb jet over eastern Montana and the western Dakotas by late
morning Monday. In fact, by 18Z Monday according to NAEFS, 850mb
winds (50-60 kts) are as high as +3-4 sigma just north of the
Black Hills. Embedded within this 850mb jet is a surge in 850mb
moisture flux, which with resulting isentropic glide means periods
of snow should develop. It begins in lee of the Rockies Monday
morning where, not only is 850mb frontogenesis taking shape, but
easterly surface winds result in upslope enhancement. As the
850-700mb frontogenesis strengthens, heavier banding will ensue
over north-central Montana Monday evening. Latest WPC
probabilities show a show 40-70% chance for snowfall totals >4" in
north-central Montana on Monday. Latest experimental pWSSI
indicates a >60% chance for "Minor" impacts on Monday, but current
"Moderate" impact probabilities are ~5-10% on average.

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

Mullinax