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

Valid 12Z Sat Feb 03 2024 - 12Z Tue Feb 06 2024

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

An anomalous closed low with heights fall to nearly -4 sigma
according to NAEFS will drift across the Southern Plains Saturday
into Sunday as an extremely convoluted but amplified mid-level
pattern evolves. This closed low will shear out to the southeast
through Sunday but maintain a large area of lowered heights from
the Southern Plains northwest through the Pacific Northwest. Above
this closed low, a 150+kt Pacific jet streak will arc from west to
east across the southern tier of the CONUS, leaving favorable LFQ
diffluence for ascent atop the greatest height falls and PVA. This
will lead to surface cyclogenesis in the lee of the Rockies, with
this surface low amplifying Saturday before slowly advecting ESE
while beginning to fill. However, downstream of this low,
increasing moist isentropic ascent from the Gulf of Mexico will
surge a theta-e ridge northward, which will expand a shield of
precipitation from the Southern Plains through the Northern
Rockies.

The airmass is generally of Pacific origin so marginally cold,
which suggests most of the snow will fall in the terrain from the
Sangre de Cristos northward through the CO Rockies, across the NW
WY ranges, and continuing north to near Glacier NP. In these
ranges, the broad synoptic ascent will be enhanced by easterly
flow which will act to both drive upslope ascent while also advect
higher moisture westward into the terrain. Additionally, an
elongated surface trough axis could help enhance convergence to
further produce lift across the area. WPC probabilities are high
(70-90%+) in focused higher terrain from the Northern Rockies
through the Absarokas, the NW WY ranges including the Wind Rivers,
and down into much of the CO Rockies and Sangre de Cristos.
Locally, more than 1 foot is possible in any of these higher
terrain features.

More uncertainty exists east of the Front Range of CO as guidance
still indicates a deformation axis will develop on the backside of
the departing low Saturday evening/night. This should enhance
snowfall east of the Front Range, but the continued marginal
thermal structure may keep significant accumulations confined to
areas like the Raton Mesa and Palmer Divide, sparing the I-25
urban corridor. However, the models have become increasingly
aggressive with this deformation, and most guidance suggests a
potential overlap of negative theta-e lapse rates collocated with
-EPV within this axis, suggesting convective snow rates are
possible across the Palmer Divide and Raton Mesa and east into the
High Plains. Confidence is low, and snowfall will likely vary
considerably in intensity, accumulating only when dynamic cooling
can offset the marginal thermal structure, but WPC probabilities
for more than 2 inches have increased along the I-25 urban
corridor, and WSE plumes indicate the potential, if these bands
develop, for significantly higher especially from Denver southward.


...California through the Great Basin...
Days 1-3...

A deepening closed low off the CA coast will spin almost in place
Sunday before elongating and shearing out to the northeast before
advecting onshore as an open trough Monday. To the south of this
feature, impressively pinched mid-level flow will result in robust
moisture confluence as an atmospheric river (AR) pivoting onshore
into CA with GEFS and ECENS probabilities for IVT exceeding 750
kg/ms reaching the coast late D1 into D2. The confluent mid-level
flow beneath a potent subtropical jet streak will allow some of
this moisture to spill over the Sierra and push well inland as
reflected by modest IVT probabilities for 250 kg/ms reaching into
the Great Basin. This impressive and long-duration moisture plume
will be acted upon by increasingly impressive lift as lobes of
vorticity embedded within the flow pivot eastward to combine with
intense upslope flow into the terrain features on W/SW winds
between 850-500mb, and a strong low pressure moves onshore near
the CA/OR coast. The guidance has shifted a bit northward today,
but the overall evolution remains unchanged with a long duration
of impressive overlap of ascent and moisture resulting in
exceptional precipitation in CA late D1 through D2, with still
significant precip spilling over into the Great Basin and Central
Rockies by D3.

The heaviest snow is likely in the Sierra this period which will
experience the most intense moisture flux, which will result in
intense snowfall due to the extreme upslope flow likely on the
orthogonal mid-level flow. Snow levels will initially be rather
low, 2500-3500ft, but will climb steadily within the most potent
WAA to 5000-6000 ft, possibly above 7000 ft across southern parts
of CA. The warming snow levels will also result in lowering SLR,
and it is likely the snow will gradually become wetter and heavier
as the event unfolds. This will result in major impacts due to
snow load, snow rates, and snow amounts, especially in the Sierra
where WPC probabilities for more than 6 inches are above 70% each
day of the 3-day period, and event total snowfall will almost
certainly exceed 4 feet in many areas above 6000 ft. This includes
some of the significant passes including Donner Pass and Echo
Summit where major travel impacts are expected.

Elsewhere across CA and spreading into the Great Basin, especially
on D2 and D3, WPC probabilities for more than 6 inches exceed 70%
in the Shasta/Trinity/Siskiyou region, across many of the
central/southern NV mountain ranges, and even as far south as the
San Gabriels/San Bernadino ranges. Later in the forecast period,
moderate WPC probabilities for more than 6 inches spread as far
east as the Uintas and Tetons. Total snow in some of the higher
terrain of several of these ranges could exceed 2 feet.

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:

--Strong Atmospheric River into California
The next Atmospheric River (AR) event is expected to impact
California tonight through Tuesday. This will spread heavy
precipitation across the area, leading to rapidly deteriorating
travel conditions.

-Heavy Mountain Snow and Strong Winds
Heavy snow will spread across the Sierra Nevada beginning late
tonight, and will accumulate to several feet above 5000 ft
elevation. Snowfall rates in excess of 2"/hr combined with wind
gusts up to 60 mph will produce near impossible travel due to
whiteout conditions. Power outages and downed trees are possible.

--Excessive Rainfall and Flooding
Heavy rainfall will impact lower elevations and coastal areas,
with significant rainfall totals from central to southern
California including the Transverse and Peninsular Ranges. This
should result in considerable flash, urban, and small stream
flooding with debris flows and mudslides. River flooding is also
likely to occur.

--Strong Winds and High Surf
Powerful, damaging onshore winds are expected over the central
California coast and Coastal Ranges, with potentially damaging
high surf.


Weiss