FOUS11 KWBC 062034
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Fri Jan 06 2023
Valid 00Z Sat Jan 07 2023 - 00Z Tue Jan 10 2023
...Central Rockies...
Day 1...
Exiting shortwave this afternoon/evening will wring out several
more inches of snow to the high peaks of Colorado before ending by
morning as heights continue to rise. This includes the Park Range,
Flat Tops, and Elk Mountains, where WPC probabilities for more
than 4 inches are above 30%.
...Western U.S...
Days 1-3...
Active period will continue for much of the West Coast, focused on
California as a series of Atmospheric River (AR) events marches
on. Through 00Z Tue 1/10, ensembles show increasing probabilities
for at least 250-500 kg/m/s IVT in two waves (Day 1 into early Day
2, then Day 3) with the latter likely stronger than the former.
Per the ESAT table, the NAEFS shows IVT values over the 97th
percentile over much of NorCal into the Sierra (core over the 99th
percentile), closely mimicking the GEFS mean QPF from an M-Climate
perspective. End result will be a continuation of strong moisture
influx into the region with multiple feet of snow likely for the
mountains. With such a strong Pacific flow, snow levels will surge
higher with the core of an AR, but will start near 4000-5000ft
initially before rising to 5000-7000ft with the first AR passage,
then up over 8000ft with the second/stronger AR passage over the
Sierra and NorCal mountains. Farther north, moisture will still
spread over WA/OR but lower in magnitude. Nevertheless, a rather
continued fetch will yield several inches of snow at some pass
levels but much higher totals in the highest terrain of the
Cascades and Olympics. Moisture transport will die off east of the
central ID ranges as the mid-level flow favors more punctuated
ridging. Still, several inches to perhaps a foot are likely over
the Blue Mountains in eastern OR.
To the north, WPC probabilities for at least 4 inches of snow are
above 40% for much of the higher Cascades on Day 1 due to better
forcing, with a lull on Day 2, then another weaker surge on Day 3
with probabilities a bit lower over the Cascades, but higher over
the Blue Mountains. To the south, daily probabilities of at least
a foot of snow are high (>70%) over the high terrain of NorCal Day
1, into the northern Sierra Day 2, then focused on the central
Sierra Day 3. Three-day probabilities of at least two feet are
high (>70%) over the same areas. With QPF forecast to be several
inches (perhaps more than 10 inches at the most favored areas),
even lower SLRs due to the warm air mass (heavy, wet snow) would
yield multiple feet of snow with more beyond the forecast period.
Fracasso
...Key Messages for Early January Atmospheric Rivers...
--Next Atmospheric River to Arrive Tonight: Heavy rain and
mountain snow returns late tonight in northern California,
spreading to central California Saturday.
--Threat of Heavy Rain, Flooding Returns This Weekend: The
cumulative effect of successive heavy rain events will lead to
additional considerable flood impacts this weekend, including
rapid water rises and mudslides, over northern and central
California, and portions of far western Nevada. Minor to isolated
major river flooding is expected through the weekend into next
week, with record river levels possible.
--Dangerous Mountain Travel Resumes Saturday: 1-3 feet of
additional snowfall this weekend, with blowing snow at times, will
lead to dangerous travel above 4000 feet in the mountains of
northern and central California.
--Increasing Risk of Infrastructure Impacts and Avalanches:
Mountain communities will be at increasing risk of infrastructure
impacts and avalanches as snow load continues to increase. By
Sunday, two week snow totals should approach or exceed 100 inches
in many of the higher elevation areas of the Sierra Nevada.
--Stronger Atmospheric River to Arrive Monday: A more intense
atmospheric river will arrive over California and western Nevada
Monday, with additional heavy precipitation and damaging winds
expected to persist through Tuesday. Flash flooding and debris
flows are possible over burn scar areas. Another round of heavy
precipitation is likely later next week.
$$
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