Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EST Sun Feb 04 2024

Valid 00Z Mon Feb 05 2024 - 00Z Thu Feb 08 2024

...The West...
Days 1-3...

**Strong atmospheric river to generate very heavy snowfall in
California mountain ranges, the central Great Basin, and highest
elevations of the Central and Southern Rockies through mid-week**

The impressive atmospheric river (AR) directing a robust >750
kg/m/s IVT (peaking above the max observed IVT in the CFSR
climatology along the south-central California coast at 18Z today
according to the ECMWF ENS) is responsible for the copious amounts
of precipitation streaming into the Golden State this afternoon
and even protruding into portions of southern Oregon and the Great
Basin. There is no shortage of vertical ascent over California and
the Great Basin thanks to not only their favorable position
beneath the diffluent left-exit region of a 140kt 250mb jet
streak, but the orthogonally-oriented SErly mean layer flow aimed
at the northern California mountains, the Coastal Ranges, the
Transverse Ranges, and the Sierra Nevada. The 12Z HREF sported
high chances (>70%) for 3"/hr snowfall rates in parts of the
Salmon/Trinity mountains, the southern and central Sierra Nevada,
and in the >7,000ft elevations of the Transverse Ranges this
evening and into the overnight hours. The exceptionally high
moisture content is a concern with regards to snow load, making
downed trees and power lines a concern. Compounding upon the snow
load concerns are 700mb winds as strong as 50 kts would allow for
55+ mph wind gusts, adding additional force on top of snow covered
trees, power lines, and buildings. These winds will also make for
whiteout conditions, potentially even blizzard conditions given
these strong winds will persist for the remainder of the afternoon
and into tonight. Expect some of these areas to contend with power
outages, downed trees, and compromising the infrastructure of
homes, businesses, and buildings through Monday.

Moderate-to-heavy snow will persist across many of the California
mountain ranges above 6,000ft on Monday as the upper trough off
the California coast steadily amplifies over the subtropical East
Pacific and continues to direct a steady barrage of 850-500mb
moisture flux into the Southwest. Heavy snow will stick around in
the Sierra Nevada and >6,000ft peaks of the Transverse Ranges, as
well as into the parts of south-central Nevada. As the trough
slowly moves east Monday night, strong positive vorticity
advection and the steady stream of Pacific moisture will advance
east into the Four Corners states. Strong upslope ascent via
orthogonally-oriented SErly flow aloft sets the stage for heavy
snow along the Mogollon Rim and Gila Mountains of Arizona, around
Mt. Zion National Park in southwest Utah, and in the San Juans of
southwest Colorado. Latest 48-hr WPC PWPF sports high chances
(>70%) for snowfall totals >12" in portions of these mountain
ranges Tuesday and into Wednesday. The Probabilistic WSSI (WSSI-P)
sports moderate chances (40-60) for Moderate Impacts in these
areas, which would suggest the potential for hazards driving
conditions and possible closures and disruptions to
infrastructure. Farther north, portions of the Uinta, Wasatch,
Sawtooth, Tetons, and Absaroka can also expect some heavy snowfall
as moisture from the AR advances into the heart of the
Intermountain West. Snow will pick up in intensity over the
interior Northwest on Monday with the heaviest snowfall in Utah
holding off until Tuesday night and into Wednesday. WPC PWPF's
48-hour probabilities depict high chances (>70%) for >12" of
snowfall in portions of these ranges through Wednesday afternoon.

The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.


Key Messages for the Feb 4-7 Atmospheric River are below:

--Life Threatening Flooding and Rainfall
Heavy rain is expected into Monday for much of California,
continuing for Southern California through Monday night. Rainfall
amounts of 3-6â€, with locally 8-14†in parts of Southern
California, are forecast. Locally catastrophic flash and urban
flooding impacts are possible across Los Angeles. Small stream and
river flooding with debris flows and mudslides are expected.

--Heavy Mountain Snow and Whiteouts
Heavy, wet snow will continue across the Sierra Nevada through
Monday night. Snowfall rates at times will reach 2-3â€/hr,
accumulating to multiple feet. These rates combined with wind
gusts up to 60 mph will produce whiteout conditions and near
impossible travel above 5000 feet. Southern California ranges can
expect heavy snow through Tuesday above 7000 feet causing similar
impacts.

--Damaging Winds and High Surf
Powerful, damaging onshore winds shift to southern California
tonight. Further power outages and downed trees are likely.
Potentially damaging high surf is expected for the Central and
Southern California coasts through Monday.

Mullinax