Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Wed Feb 07 2024

Valid 12Z Wed Feb 07 2024 - 12Z Sat Feb 10 2024

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

The longwave trough over the western U.S. will continue to support
additional rounds of heavy mountain snow along many mountain
ranges stretching from the Coastal Ranges of the West Coast to the
Front Range of the Rockies. The upper level disturbance
responsible for the powerful atmospheric river that caused
numerous significant impacts in California will finally be making
its way out of the Four Corners region later today. As of this
morning, however, the Four Corners region remain directly beneath
the divergent left-exit region of a 150kt 250mb jet streak while
modest 500-700mb moisture flux embedded within southwesterly flow
aloft continues to flow over the southern and central Rockies.
Sufficiently cold temperatures and vertical ascent aloft, combined
with favorable upslope flow along orthogonally oriented mountain
ranges will support additional heavy snow in these ranges today.
WPC PWPF shows high chances (>70%) for >8" of snowfall today and
into Wednesday night along ranges that include the Mogollon Rim,
Zion National Park, the Wasatch, the San Juans, Sangre De Cristo,
and the peaks of northwestern Colorado. The Probabilistic WSSI
(WSSI-P) continue to show moderate to high chances (50-80%) for
Moderate impacts through Wednesday night most notably in the
>10,000ft peaks of the an Juans, along the Mogollon Rim, over the
Gila Mountains of eastern Arizona, and near Zion National Park in
southwest Utah.

As this initial vort max ejects into the Great Plains Wednesday
night, another vigorous upper level vort max will dive south
through northern California Wednesday afternoon and into the
Desert Southwest by Thursday morning. This disturbance will once
again bring another surge in 850-700mb moisture into California
with 500mb vorticity advection and upslope flow via southwesterly
850-500mb winds helping to produce additional heavy mountain snow
along the Sierra Nevada this afternoon and into Wednesday night.
WPC PWPF shows high chances (>70%) for snowfall >6" along the
Sierra Nevada and into the 7,000ft+ elevations of the Transverse
Ranges. The upper low over the Lower Colorado River Valley will
direct 700mb moisture flux into the Mogollon Rim bringing
additional heavy snow to northern Arizona, as well as to southwest
Utah, and the western Colorado Rockies. WPC PWPF shows high
chances (>70%) for additional snowfall >6" along the Mogollon Rim
and the western Colorado Rockies on Thursday.

Lastly, the Cascade Range will witness a period of lighter
snowfall today as an upper level disturbance meanders its way over
Washington. Snow will pick up in intensity Thursday into Friday
morning as a more potent vort max dives southeastward from British
Columbia. The nose of a 110kt 250mb jet streak and a resurgence of
700mb moisture flux being directed at the Pacific Northwest via
northwesterly flow will spur periods of heavy snow along the
Cascade Range and even as far inland as Oregon's Blue Mountains.
WPC PWPF shows low-to-moderate chances (20-40%) for >4" of
snowfall on Thursday and into early Friday morning. Lower chances
(10-30%) for >4" of snow are depicted across the Blue mountains of
northeast Oregon late Thursday through Friday morning.

...Northern High Plains and Northern Rockies...
Days 1-3...

The aforementioned left-exit region that fostered heavy snow over
the Four Corners region will give rise to a developing surface low
over eastern Wyoming this morning. Throughout the day on
Wednesday, southeasterly 850-700mb moisture will be directed at
the northern High Plains and Rockies, leading to a growing
precipitation shield across Montana, northern Wyoming, and the
eastern Dakotas by Wednesday night. Temperatures will be plenty
cold enough to support snow from as far west as northern Idaho and
the Lewis Range on east to the central Montana. By Wednesday
afternoon, the 700mb low is likely to consolidate itself over
eastern North Dakota, placing a TROWAL on its western flank over
Montana. It is here beneath the TROWAL and where also some upslope
enhancement is likely to occur where the heaviest snowfall
transpires. The 00Z HREF showed the potential for >1"/hr snowfall
rates today in the Little Belt and Big Snowy Mountains of central
Montana, while farther east and closer to the strengthening 850mb
front, heavy snow bands may set of over east-central Montana
midday and into the afternoon hours. Over far eastern Montana and
into North Dakota, the warm nose of 850-700mb WAA will likely
result in a burgeoning warm nose with >0C at some heights within
that layer. With surface temperatures remaining sub-freezing,
these areas will initially start off as an icy wintry mix
Wednesday morning and persist into Wednesday afternoon. It is
worth noting WPC PWPF shows no signal for >0.1" of ice
accumulations, but there are moderate chances (40-60%) for >0.01",
implying a light glaze of ice is still possible and could cause
slick spots on roads.

By 00-03Z Thursday, the strengthening surface low will track from
central South Dakota into eastern North Dakotas, causing winds
within the 850-700mb layer that were originally out of the SE to
flip more out of the NE-ENE. This will allow for the atmospheric
column to cool below freezing and allow for snow to become the
primary precipitation type. Snow is likely to fall heavily at
times in eastern Montana and western North Dakota where hourly
snowfall rates could approach 1"/hr. As the storm occludes
Thursday morning and the best forcing tracks north, snow rates
will gradually taper off Thursday morning but occasionally gusty
winds will persist as well. While not extreme by northern High
Plains standards, this could still contribute to reduced
visibilites and drifting snow on roads. The storm will lift into
northern Minnesota by Thursday afternoon with snow falling across
northern North Dakota. Snow is possible across the Red River of
the North Thursday night and into northern Minnesota by early
Friday morning. Snow should finally come to an end by Friday
afternoon as the storm tracks northeast into the heart of Ontario.

WPC PWPF shows moderate-to-high chances (50-70%) for snowfall >6"
across much of the Northern Rockies, the Little Belt and Big Snowy
Mountains of central Montana, and the Tetons of western Wyoming.
Farther east, confidence is a little more elevated to
moderate-to-high levels (50-70%) for >4" across the eastern half
of Montana and northwest North Dakota. The WSSI sports Minor
Impacts for most of these areas mentioned, although localized
Moderate Impacts which focus on potential closures and disruptions
to infrastructure are possible.

...Central Rockies & Central High Plains...
Day 3...

The upper level trough barreling through the Four Corners region
Thursday evening will foster strong vertical ascent aloft via
positive vorticity advection (PVA) and lee cyclogenesis over
eastern Colorado late Thursday night and into Friday. Guidance
remains at odds over the evolution, strength, and speed of this
developing surface low. The CMC/GEFS ensembles are more bullish on
snowfall totals >4" along the Front Range and near the Denver
metro area on Friday, while the ECMWF ensembles are on the lighter
side. The ECMWF deterministic did, however, sport a fairly potent
upper trough and track that would support periods of snow in
central Colorado on Friday. Latest WPC PWPF shows generally low
chances (10-30%) for snowfall >4" along the Palmer Divide and
including the Denver metro, but probabilities were closer to
moderate (30-50%) along the Front Range. While details and totals
are still subject to change, ensemble guidance does suggest the
potential for accumulating snowfall that does have low-to-moderate
chances (30-50%) of producing Minor Impacts along the I-25
corridor from the the Denver metro area on south over the Palmer
Divide.

Mullinax