Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
224 AM EST Thu Feb 01 2024

Valid 12Z Thu Feb 01 2024 - 12Z Sun Feb 04 2024

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

An extremely active pattern will resume across the West late week
and into the weekend as an increasingly amplified mid-level flow
pattern develops across the CONUS leaving a large trough and
onshore Pacific flow across the West.

Thursday morning will feature a short-wavelength mid-level flow
with a sharp ridge blossoming over the Plains and a deepening
longwave trough approaching the Pacific Coast. The core of this
trough will be a closed low off the British Columbia coast which
is progged to dive southward towards Oregon by Saturday, but the
accompanying longwave trough will shift onshore becoming
negatively tilted in response to shortwave energy shedding around
it and into the Four Corners region. The pattern becomes quite
complex thereafter, with multiple spokes of vorticity amplifying
into several closed lows oriented sharply WNW to ESE. This will
result in widespread and broad, but overall modest ascent across
much of the West, with steep lapse rates beneath the trough and
waves of PVA driving locally enhance omega. Into this large scale
gyre, an impressive Pacific subtropical jet streak reaching
160+kts will surge into CA Friday and then extend into the
Southern Plains by Sunday, surging moisture into the region and
enhancing ascent within its LFQ. This will result in a pronounced
AR surging eastward, with IVT probabilities from the GEFS and
ECENS reaching above 90% for 250kg/ms all the way to the
Intermountain West, resulting in PW anomalies reaching +2 to +3
sigma according to NAEFS. The most significant overlap of ascent
and moisture will gradually shift east into the weekend.

With the air across the West being generally Pacifically sourced,
it will be relatively warm, noted by snow levels that will be
generally 5000-7000 ft within the highest PW anomalies, but
cooling gradually to 2000-4000 ft by the end of the period.
However, much of the precipitation will have ended by the time
snow levels collapse, so impactful snow into the lower elevations
is not likely for most of the region. The exception may be D3, in
response to lee cyclogenesis which should occur across CO Saturday
morning and then shift east into the Southern/Central Plains by
Sunday. Increasing moist advection downstream of this low will
surge moisture northward along an inverted surface trough, with
the accompanying theta-e ridge lifting all the way into the
Northern Rockies, and it is D3 when the most widespread and
potentially lower-elevation snow is expected, but the heaviest
accumulations are still expected to be in the higher terrain,
especially on the upslope side of the ranges.

On D1, WPC probabilities for more than 6 inches of snow are
highest in the Sierra where they exceed 80%, and locally more than
2 feet is possible in the higher terrain. Heavy snow exceeding 6
inches is likely (70%+) across much of the terrain of the Great
Basin and into the Four Corners, including the southern Wasatch,
San Juans, and San Bernadinos. During D2, the heavy snow spread
across a larger portion of the Intermountain West, with WPC
probabilities for more than 6 inches exceeding 40% from the
Sawtooth/Salmon River Ranges southward through the Uintas and
Wasatch, into the White Mountains of AZ, and covering the San
Juans/most of the CO Rockies. Additional heavy snow is also
expected in the Sierra D2, but with the heaviest snow expected
north of D1's greatest accumulations. Locally up to 1 foot is
likely in the higher terrain of many of these ranges.

During D3 the precipitation shield becomes more focused along the
inverted trough and stationary front, with moisture riding up from
a surface low in the lee of the CO Rockies. This should spread
heavy snow in a NW to SE oriented axis from the Northern Rockies
through the Absarokas, NW WY ranges, into the CO Rockies and as
far south as the Sangre de Cristos. Snow levels will generally be
4000-6000 ft, and WPC probabilities for more than 6 inches are
above 50% across most of this area. The heaviest snow is likely in
the Front Range and Park Range of CO D3, where WPC probabilities
for 12+ inches are as high as 40%. There is still considerable
uncertainty for the I-25 urban corridor as a deformation axis may
try to pivot eastward Saturday to enhance snowfall into the High
Plains, but a dry slot in the area combined with the marginal
thermal structure at this time supports only limited snowfall east
of the higher terrain.


...Northern Maine...
Day 1...

A potent but filling shortwave will race ESE from Ontario to
across northern Maine and then become absorbed into a long wave
trough as it ejects into the Canadian Maritimes by D2. Height
falls downstream of this shortwave overlapping the diffluent LFQ
of a modest jet streak to the south will result in a clipper-type
surface low moving across the region with an area of moderate
precipitation. Although the column will be marginally cold, there
will be sufficient moisture and ascent to produce a narrow
corridor of heavy snow, generally focused across far northern
Maine, with some upslope snow developing behind the associated
cold front as far west as the Adirondacks. Still, accumulations
exceeding 4 inches will likely be confined to just northern Maine,
where WPC probabilities are as high as 50%.

The probability of significant icing greater than 0.10" is less
than 10% for days 1-3.

Weiss