Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Mon Feb 05 2024

Valid 00Z Tue Feb 06 2024 - 00Z Fri Feb 09 2024

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

The upper trough responsible for the significant atmospheric river
(AR) producing heavy rain, heavy mountain snow, high surf, and
strong wind gusts will continue to deepen west of Baja California
tonight and into Tuesday morning. Positive vorticity advection and
ongoing topographically-forced ascent will continue to support
periods of heavy snow above 6,000ft along the southern Sierra
Nevada, the Transverse Ranges, and into the heart of the Great
Basin. WPC PWPF shows moderate-to-high chances (50-70%) for >12"
of snowfall through Tuesday in the southern Sierra Nevada and
Transverse Ranges above 7,000ft. Probabilities are high (>70%) for
similar totals above 7,000ft in central Nevada. By 12Z Tuesday,
NAEFS indicated that 200mb heights well west of Baja that are
likely to be the lowest observed 200mb heights relative to the
CFSR climatology period (1979-2009). In addition, strong vertical
ascent aloft will come via the left-exit region of a 150kt+ 250mb
jet streak moving overhead Tuesday afternoon and into Tuesday
night. What this effectively does, when working in tandem with
ridging over the Southern Plains, is tap into rich subtropical
moisture from the East Pacific and, via strong SWrly low, direct a
>300 kg/m/s IVT (above the 97.5 climatological percentile
according to NAEFS) through Arizona and into the heart of the Four
Corners region Tuesday morning. Exceptionally rich 500mb and 700mb
moisture will stream over the region and, combined with strong
upsloping SWrly flow, will result in heavy snowfall from the
Mogollon Rim and north of Zion National Park to the Uinta of
northern Utah and San Juans of southern Colorado/northern New
Mexico. The latest 12Z HREF shows snowfall rates of 2-3"/hr are
possible along the Mogollon Rim and San Juans Tuesday afternoon
and into Tuesday night.

Latest 48-hr WPC PWPF sports high chances (>70%) for snowfall
totals >18" north of Zion National Park, along the Mogollon Rim
above 7,000ft, the Gila Mountains above 8,000ft, and in the San
Juans above 9,000ft. The WSSI shows Major impacts in these ranges
Tuesday and into Wednesday, suggesting considerable disruptions to
daily life and dangerous to even impossible travel in these areas.
Farther north, WPC PWPF shows high chances (>70%) for snowfall
totals >12" along the Uinta above 9,000ft and similar
probabilities for >8" of snow in the Sawtooth and Absaroka. The
WSSI depicts Minor to in some cases Moderate Impacts in these
mountain ranges late Tuesday into Wednesday.

...Northern High Plains...
Day 3...

By 06Z Wednesday, the aforementioned left-exit region that
fostered heavy snow over the Four Corners region will help give
rise to a developing surface low in lee of the Rockies over
northeast Colorado. Throughout the day on Wednesday, southeasterly
850-700mb moisture will be directed at the northern High Plains
and lead to a growing precipitation shield across Montana,
northern Wyoming, and the eastern Dakotas by Wednesday night.
Sampled model sounding Wednesday night into Thursday morning would
be sufficiently cold enough to support snow, but guidance differs
on whether the heavy snow expands farther west into central
Montana or as far east as central North Dakota. Latest ensemble
guidance continues to show a high amount of spread in the
evolution and placement of an 850mb low as it tracks into the
Dakotas Wednesday night and into the day on Thursday. This is
critical in being able to key in on where the heaviest axis of
potential snowfall sets up and how long it is anchored over an
area. The 00Z ECMWF EFI was showing its highest EFI values
(0.8-0.9) along the ND/MT border, which aligned closest to the
northwest flank (and beneath the TROWAL) of the ECMWF ENS mean
850mb low position Wednesday night into Thursday morning. This
area is highlighted as having the best odds of >4" snowfall totals
according to WPC PWPF where low-to-moderate chances (20-50%) are
present through Thursday evening. WPC PWPF even depicts mountain
ranges (such as the Big Horns and Absaroka) as having
moderate-to-high chances (50-80%) for >4" of snow Wednesday night
and into Thursday.

While details on exactly which areas see the heaviest snowfall
remain unclear, the evolution of the storm with a tightening
pressure gradient as the storm deepens will result in periods of
moderate-to-heavy snow that include gusty winds. The Probabilistic
WSSI (WSSI-P) shows moderate chances (40-60%) for Minor Impacts
over eastern Montana and western North Dakota. Snow Amount is the
primary driver in these impacts, but there are some low chances
(10-20%) of Minor Impacts due to Blowing Snow over these regions
as well. Interests in these areas should monitor the forecast
closely over the next 48 hours as there are likely to be
detrimental impacts to travel in parts of the region starting
Wednesday evening and continuing through Thursday.

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 will continue today for much of California with rain
lingering over Southern California into Tuesday. The forecast
calls for an additional rainfall of 1-3" with locally 3-6"
possible in parts of Southern California and the Transverse
Ranges. Considerable flash and urban flooding impacts are likely
to continue across Los Angeles. Small stream and river flooding
with debris flows and mudslides are also expected.

--Heavy Mountain Snow and Whiteouts
Heavy, wet snow will continue in the Northern California mountain
ranges and along the Sierra Nevada today and into tonight.
Snowfall rates between 2-3"/hr are possible with an additional
1-2ft of snow expected. Heavy snowfall rates combined with wind
gusts near 50 mph will cause whiteout conditions and near
impossible travel above 5,000ft. The Transverse ranges above
7,000ft can expect heavy snow through Tuesday causing similar
impacts. Heavy snow is expected in portions of the Intermountain
West through mid-week.

--Gusty Winds and High Surf
Winds will diminish throughout the day, but gusts near 50 mph are
still possible along Northern Californiaâ€s mountain ranges
today. Additional tree damage and power outages are possible. High
surf is likely to persist along the Southern California coast
through this evening.

Mullinax