Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Tue Feb 06 2024

Valid 12Z Tue Feb 06 2024 - 12Z Fri Feb 09 2024

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

The upper trough responsible for the significant atmospheric river
(AR) producing heavy rain and snow across much of the West will
further amplify west of Southern CA into Tuesday morning before
finally advecting onshore late Tuesday into Wednesday. Downstream
of this feature, impressive mid-level divergence will occur from
the Desert Southwest into the Central Rockies, which will be
paired with waves of PVA and subtle height falls through shortwave
impulses rotating northeast to drive pronounced ascent through
Wednesday. Additionally, a strengthening subtropical jet streak
intensifying above 150kts will begin to arc poleward due to the
amplification of the upper level pattern, resulting in enhanced
LFQ diffluence atop the region of greatest mid-level divergence.
This will lead to impressive omega through a deep layer of the
atmospheric column shifting from Southern CA D1 into the Four
Corners and Central Rockies D2.

This impressive and still intensifying lift will impinge into an
airmass loaded with 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 
into Wednesday. Exceptionally rich 500mb and 700mb moisture will
stream over the region and, combined with strong upsloping SWrly
flow, will result in heavy snowfall above generally 6000-7000 ft
from the southern Sierra and Transverse Ranges of CA through the
Four Corners, then slowly lift farther northeast through D2 and
into D3. An additional, although less intense, wave of moisture
and accompanying ascent will shift again to the Pacific Coast D3
as a wave of low pressure downstream of a southward dropping jet
streak approaches CA.

WPC probabilities both D1 and D2 focus across the terrain of the
Four Corners, exceeding 80% for 6+ inches both days along the
Mogollon Rim, the White Mountains of AZ, the eastern Great Basin
Ranges, and into the Wasatch, Uintas, San Juans, Sangre de
Cristos, and CO Rockies. Snowfall rates in the favored upslope
terrain regions at times will likely exceed 2"/hr as reflected by
the WPC prototype snowband tool, and 2-day snowfall accumulations
could exceed 3-4 feet in some of the higher terrain. This will
create considerable impacts through the forecast period.
Additional heavy snow D1-2 is likely in the southern Sierra,
Transverse Ranges, and as far north as the Absarokas and NW WY
ranges, but amounts and impacts should be somewhat less than the
aforementioned ranges. However, in the Sierra, this will be in
addition to the several feet of snow which has already fallen, so
travel will remain extremely challenging.

During D3, the next, but less impressive, wave of moisture will
surge onshore CA and into the West, bringing additional moderate
to heavy snow to the region. WPC probabilities for more than 6
inches on D3 reach above 50% once again in the Sierra and along
the Mogollon Rim, with lighter amounts stretching into the
Cascades and ranges of the Great Basin.

...Northern High Plains and Northern Plains...
Days 2-3...

A surface low is likely to develop in the lee of the Colorado
Rockies Wednesday afternoon in response to pressure falls driven
by a shortwave lifting northeast along the eastern periphery of a
longwave trough expanding across the Intermountain West,
overlapped with the diffluent LFQ of a potent subtropical jet
streak arcing poleward. The shortwave will lift northward and
close off near the Northern Plains, and while this should support
some intensification of the surface low, deep layer ascent may
become somewhat muted as the upper jet pivots farther east to
separate from the better mid-level ascent. This suggests the low
may only slowly deepen before occluding. However, impressive moist
isentropic ascent out of the Gulf of Mexico should surge
northward, lifting the theta-e ridge into a TROWAL which will then
rotate cyclonically around the low and into the Dakotas/eastern MT
to enhance moisture and modest elevated instability. This could
somewhat offset the displacement of the synoptic lift, resulting
in an expanding precipitation shield from WY/MT Wednesday night
into the Dakotas on Thursday. Where the WAA associated with the
isentropic ascent lifts into the colder air NW of the low but
beneath the TROWAL, this will likely manifest as a large area of
light to moderate snowfall, with some enhancement in rates likely
as a deformation axis pivots eastward during D3.

Moisture is expected to be more than sufficient for heavy
precipitation as reflected by NAEFS PW anomalies reaching as high
as +3 sigma in the Dakotas, and evaluation of regional forecast
soundings indicate the column should be sufficiently cold for snow
for the High Plains and, cool with time into the eastern Dakotas.
However, the DGZ depth appears modest and elevated, which is also
reflected by SPC SREF probabilities for DGZ depth exceeding 50mb
peaking at just 10-30%. With the potential forcing being offset,
this could manifest as more widespread light to moderate snow
rather than expansive heavy snow, which is reflected by WPC
probabilities for 4+ inches that peak around 40-50% from eastern
MT through north-central ND.


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:

--Continued California Rainfall and Flash Flooding
Occasional heavy rain will continue today across Southern
California and the Transverse Ranges. Additional rainfall of 1-3"
atop fully saturated grounds will create new, or enhance ongoing,
flash flooding. Considerable impacts will persist today, with
river flooding, debris flows, and mudslides expected.

--Flash Flooding Risk expands to the Southwest
Heavy rain will spread into the Southwest today producing 1-2"
with locally higher amounts. This will likely produce instances of
flash flooding along creeks and washes, potentially resulting in
dangerous travel and road closures.

--Heavy Mountain Snow and Gusty Winds
Heavy snow will expand northeast from the Transverse Ranges to the
Great Basin and Four Corners today. Snowfall rates of 2+"/hr are
possible (30-50%) above 6000 ft along the Mogollon Rim, and into
the Wasatch and San Juans. This snowfall will combine with winds
gusting above 40 mph to produce significantly reduced visibility
and very difficult travel.


Weiss/Mullinax