Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 PM EST Sat Feb 03 2024

Valid 00Z Sun Feb 04 2024 - 00Z Wed Feb 07 2024

...Rockies... 
Day 1...

The negatively tilted 200-500mb trough oriented over the
Intermountain West and into the southern High Plains continues to
support strong upper level ascent over Rockies this afternoon. The
700mb low over the TX/OK Panhandles will continue to fuel
unusually strong southeasterly 850-700mb flow (NAEFS mean zonal
winds below the 1st climatological percentile) that directs plenty
of 850-700mb moisture flux into the CO/WY Front Range and on north
through central Wyoming and as far north as the Lewis Range.
Snowfall in western Montana is likely to be enhanced by the
development of a surface low over eastern Montana and a cold front
pushing south through western Montana, resulting in additional
upslope flow into the Lewis Range. Snow will gradually taper off
by Sunday morning as the southeasterly 850-700mb moisture fetch
diminishes and the surface low over eastern Montana tracks north
into southern Saskatchewan Sunday afternoon. Latest WPC PWPF shows
high probabilities (>70%) for additional snowfall totals >6" along
the northern Park Range of Colorado on north the ranges of central
Wyoming, eastern Idaho, and the easterly slopes of the Northern
Rockies in western Montana. Localized totals >12" are expected
through Sunday morning. The WSSI sports mostly Minor Impacts in
affected areas, although some Moderate Impacts (which are more
indicative of hazardous travel conditions and travel disruptions)
are also depicted in the highest peaks of the northern and central
Rockies.

...California through the Great Basin...
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**

The West Coast is bracing for a strong Atmospheric River (AR) that
is set to deliver a robust integrated vapor transport (IVT) into
California on Sunday. NAEFS shows the IVT by 12Z Sunday surpassing
the observed maximum IVT values relative to Jan 25 - Feb 19 CFSR
climatology. The culprits responsible for fostering this AR are an
exceptionally deep upper low off the California coast (NAEFS shows
<1st climatological percentile heights between the 1000-500mb
mandatory layers through Sunday afternoon) and a strong ridge over
the eastern tropical Pacific forcing winds within the 850-700mb
layer to surpass the 99th climatological percentiles over
California on Sunday. These winds will be oriented out of the
S-SE, which is ideal for strong upslope enhancement along the
Coastal Ranges, the Transverse and Peninsular Ranges, and into
both the northern California mountains and the Sierra Nevada.

The Sierra Nevada, most notably, are likely to witness
considerable snowfall rates and significant snowfall totals as a
result of this AR. Snow will be measured in feet above 5,000ft
with totals ranging between 4-8 feet by the time snowfall winds
down on Tuesday. Snowfall rates are likely to range between
2-4"/hr Sunday afternoon 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, 700mb winds as strong as
50 kts would add additional downward 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 much of the afternoon and
overnight hours. The WSSI sports Extreme Impacts along the Sierra
Nevada above 6,000ft, as well as along the Inyo Mountains just
southeast of the Sierra Nevada. Snow Amount is the primary driver
in these Extreme Impacts, but Snow Load is also a contributing
factor in these concerning thresholds. Expect some of these areas
to contend with power outages, downed trees, and compromising the
infrastructure of homes, businesses, and buildings.

All these concerns are valid farther north along the Coastal Range
north of Clear Lake and towards the Salmon and Trinity/Shasta
mountains. Anywhere from 2-4ft of snowfall is expected late
tonight and through Monday morning. This AR will produce heavy
snow for much of the West Coast and even into the Intermountain
West. Heavy snow will make its way into the Great Basin and as far
north as the Blue, Sawtooth, Bear River, and Teton mountain ranges
Sunday night and through Monday. WPC PWPF sports high chances
(>70%) for snowfall totals >8" in these mountain ranges through
Monday evening. Farther south, elevations >6,000ft in the
Transverse Ranges and as far east as neighboring 6,000ft+ peaks
north and west of Las Vegas can expect 1-3ft of snowfall through
Tuesday. The WSSI depicts Major to even some isolated Extreme
Impacts in these areas. Heavy snow will eventually work its way
into Mt. Zion and the Uinta of Utah Monday night and into Tuesday
where WPC PWPF sports high chances (>70%) for >6" snowfall totals
through Tuesday 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 into California are
below:

--Life Threatening Flooding and Rainfall
Heavy rain is expected across the San Francisco Bay Area and the
Central coast tonight through Sunday and Southern California
Sunday and Monday. 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 tonight
through Monday with accumulation rates of 2-3â€/hr and wind gusts
up to 60 mph, producing whiteout conditions and near impossible
travel above 5000-6000 feet. Southern California ranges can expect
heavy snow Sunday into Tuesday above about 7000 feet.

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

Mullinax