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

Valid 12Z Sun Feb 04 2024 - 12Z Wed Feb 07 2024

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

**Strong atmospheric river to produce very heavy snowfall in
northern California mountain ranges, the Sierra Nevada, and
highest elevations of the Transverse Ranges through Tuesday**

A closed low just offshore the CA coast will slowly weaken into
Monday while rotating nearly in place before opening to a longwave
trough and eventually advecting onshore by Tuesday morning. As
this longwave trough pivots eastward, spokes of vorticity rotating
through the flow will generate waves of ascent across the Pacific
Coast and into the inter-mountain west, with a final more potent
lobe surging the southern end of this trough into a more
pronounced southern stream wave moving into the Desert Southwest
late in the period. The resulting broad trough which will
encompass much of the West this period will result in periods of
height falls/PVA, which will combine with periods of enhanced
diffluence within the LFQ of an undulating subtropical jet streak
to produce broad ascent through the period.

S/SE and downstream of this trough evolution, impressively
confluent flow at 850-500mb will surge a potent atmospheric river
(AR) as moisture crosses from the tropical Pacific and advects
into CA D1. IVT probabilities from both the ECENS and GEFS are
above 90% for 500 kg/ms D1, with impressive probabilities for 150
kg/ms of IVT extending well inland, indicating the strength of
this flow. This IVT is progged to approach +8 sigma according to
NAEFS in CA D1, and while the core of this AR will weaken with
time as it shunts southeast through Tuesday, it will remain as
high as +4 sigma, surging excessive moisture across a large part
of the west indicated by PW anomalies of +1 to +3 sigma
encompassing much of the West. Additionally, at least two surface
lows beneath the strongest mid-level height anomalies, 1 moving
into northern CA late D1 into D2, with a second wave moving from
southern CA through the Great Basin into D3, will further enhance
omega in their vicinity. The overlap of this large scale ascent
into this extreme column moisture will result in waves of heavy
precipitation expanding west to east through the forecast period.

Since this airmass will be sourced from the tropical Pacific, the
WAA accompanying this AR will warm snow levels steadily. Early D1,
snow levels across the West will be generally 2000-4000 ft, but
will rise quickly especially in the Sierra beneath the core of the
AR and highest IVT to above 6000 ft by tonight. Snow levels will
cool subtly across CA/OR back to 3000-5000 ft through the rest of
the forecast period, but will rise more generally across the rest
of the Intermountain West to 5000-7000 ft through Tuesday. This
suggests that the heaviest snow accumulations will be in the
higher terrain, with the most significant snowfall expected in the
Sierra where WPC probabilities for more than 12 inches are high
both D1 and D2, and it is likely that above 6000 ft some areas
will receive more than 6 feet of snow during the next 3 days.
Heavy snow reaching multiple feet is also likely in the
Shasta/Trinity/Siskiyou regions of northern CA, and in the highest
terrain of the Transverse Ranges as well.

Elsewhere across the West, WPC probabilities for more than 6
inches of snow are high (>70%) D1 across many of the ranges of NV,
with moderate probabilities extending into the higher terrain of
the Cascades and parts of the Salmon River/Sawtooth Ranges. These
probabilities extend farther northeast on D2, and while they
continue to be high across the higher elevations of NV, they also
reach as far northeast as the Absarokas, Tetons, and Wind Rivers,
with lower probabilities occurring in northern UT. During D3, the
secondary low shifts towards the Southwest, enhancing snowfall
across the Four Corners while remaining in the Great Basin. WPC
probabilities for more than 6 inches on D3 are above 80% for the
Mogollon Rim, the San Juans, southern Wasatch, and even back
towards Mt. Charleston in NV. 3-day snowfall of 1-3 feet, with
locally higher amounts, is likely in the higher terrain of much of
this area.


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 into California are
below:

--Life Threatening Flooding and Rainfall
Heavy rain is expected from the San Francisco Bay Area and the
Central coast through Southern California today and Monday.
Rainfall amounts of 2-5", with locally 6-10" in parts of Southern
California, are likely. This will result in considerable flash,
urban, and small stream flooding with debris flows and mudslides.
River flooding is also likely.

--Heavy Mountain Snow and Whiteouts
Heavy, wet snow will spread across the Sierra Nevada this morning
and continue through Tuesday morning. Snowfall rates at times will
reach 2-3"/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 today into Tuesday above about 7000 feet
producing similar impacts.

--Damaging Winds and High Surf
Powerful, damaging onshore winds will impact northern and central
California today, shifting to southern California for tonight.
Power outages and downed trees are possible. Potentially damaging
high surf is expected for much of the California coast through
Monday.


Weiss