Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
304 PM EST Thu Jan 05 2023

Valid 00Z Fri Jan 06 2023 - 00Z Mon Jan 09 2023

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

Periods of snow look to continue through this evening in the
Sierra Nevada as one final 500mb shortwave trough traverses the
heart of California. Latest WPC PWPF does still indicate some
40-50% probabilities for >6" of snowfall from the Shasta on south
through the Sierra Nevada tonight. Meanwhile, the southern portion
of the upper trough that slammed into the West Coast last night
tracks into the Four Corners region overnight. The PVA associated
with the trough, as well as the left-exit region of a 130 knot jet
streak situated over UT and CO, fosters divergent flow atop the
atmosphere. With a stream of 700mb moisture flux also accompanying
the shortwave trough, this provides sufficient lift and moisture
for heavy snow to fall in the Wasatch and western CO Rockies. The
Wasatch have the best odds for >8" of snowfall tonight through
Friday afternoon with WPC PWPF showing moderate chances (40-60%
chances) for snowfall >8". The peaks east of Salt Lake City and
Provo have the best odds of seeing snowfall totals >12" (30-50%
chance). It is the Wasatch Range that the WSSI suggests could
witness "moderate" impacts, which means the likelihood of
experiencing hazardous travel conditions and potential closures.
Farther east, the western CO Rockies may see some localized
amounts exceed 12", but confidence is higher in totals >8" in
areas such as the Elk Mountains and even as far north as the
Ferris Mountains of southern WY.

Meanwhile, along the West Coast, there will be a brief reprieve in
the action as a very brief bubble of high pressure orients itself
from the Columbia Basin on south through Nevada and into southern
California. However, this will not last long as another strong
storm system in the northeast Pacific directs its cold front east
towards the Pacific Northwest coast by Friday afternoon. The upper
low aloft delivers additional rounds of moisture to northern
California and both western Oregon and Washington late Friday and
into early Saturday morning. This is due to another atmospheric
river directing anomalous 850-700mb moisture flux at these
regions. In fact, NAEFS does depict an IVT of 400-500 kg/m/s along
the northern and central California coast, which is above the 90th
climatological percentile. The mountain range with the best odds
of seeing "moderate" impacts, according to the experimental PWSSI,
is the Shasta where probabilities are up to 60% between 12Z Fri -
12Z Sat.

As this storm system weakens late Saturday, the next Pacific
disturbance embedded within the active and robust Pacific jet
stream ushers in yet another atmospheric river Saturday night into
Sunday. NAEFS showed this atmospheric river, yet again, with IVT
values above the 90th climatological percentile. One other factor
is wind speeds are likely to be stronger than the atmospheric
river Friday morning as 850mb winds over the Sacramento Valley are
>97.5 climatological percentile around 06Z Sunday. Unlike late
Friday into Saturday morning where the moisture axis was placed
farther north, the best axis of 850-700mb moisture is pushed back
south into California and portions of southern Oregon. Periods of
heavy snow will return to the Sierra Nevada on Saturday night and
continue through Sunday. The experimental PWSSI does indicate
40-60% probabilities for "major" impacts from the Shasta/Trinity
mountains to the Sierra Nevada. Snow will once again be measured
in feet in these ranges, with 1-2 feet of snow expected from most
areas above 6,000' in elevation. Travel conditions will be
dangerous on roads with whiteout conditions likely in these
ranges. In addition, strong winds aloft could result in power
outages from the Coastal Range to the Shasta and northern Sierra
Nevada.

...Key Messages for Early January Atmospheric Rivers...

--Strong Atmospheric River to Subside Tonight: A powerful
atmospheric river over California will subside tonight. Lingering
heavy rainfall could still produce flooding over coastal
California and favored upslope areas.

--Next Atmospheric River to Arrive Late Friday: The next
atmospheric river will arrive late Friday and Friday Night over
northern and central California.

--Threat of Heavy Rain, Flooding Returns Saturday: The cumulative
effect of many rounds of heavy rainfall will lead to additional
considerable flood impacts this weekend, including rapid water
rises and mudslides, over northern and central California. Flash
flooding and debris flows are possible over burn scar areas.

--Dangerous Mountain Travel Resumes Saturday: 1-2 feet of
additional snowfall this weekend will lead to dangerous travel
above 5000 feet in the mountains of northern and central
California.

--Stronger Atmospheric River to Arrive Monday: A more intense
atmospheric river will arrive over California Monday, with
additional heavy precipitation and gusty winds expected.


Mullinax