Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 PM EST Tue Dec 05 2023

Valid 00Z Wed Dec 06 2023 - 00Z Sat Dec 09 2023

...Central and Southern Appalachians...
Day 1...

A weakening clipper-like low pressure will dissipate as it moves
over the Central Appalachians tonight, and as secondary low
pressure development occurs east of NC in response to a potent
shortwave rotating through the base of an eastern CONUS longwave
trough. This trough axis will gradually shift eastward as it gets
pulled offshore by this impulse, leaving pronounced NW flow in its
wake. This NW flow will demarcate the leading edge of CAA, within
which lapse rates will steepen and forcing for ascent will
actually increase due to the intensifying upslope component of the
flow. Initially, this upslope flow will exist within still
saturated low-mid levels as noted on model cross sections and
regional soundings, with favorable omega being driven into the
lowering DGZ on the CAA. This should result in at least periods of
moderate to heavy snow, especially before 12Z Wednesday which is
when the DGZ rapidly dries. Snow levels will initially be around
2000 ft in WV, and 4000 ft farther south in NC, but will fall
through D1 to as low as 500 ft in WV and 1500 ft in NC. This
indicates that the heaviest snowfall accumulations will be above
the former levels, with only light accumulations possible at the
lower elevations. WPC probabilities for more than 4 inches of snow
are around 50% in the higher terrain of WV, and 30% in the highest
terrain of NC.


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

The western CONUS will become quite active through the remainder
of the week as onshore Pacific flow persists, within which
multiple shortwave troughs will traverse to drive rounds of
precipitation. On D1, the tail of an impressive AR (peak IVT
around 850 kg/ms) will sag southward to focus primarily into
Oregon with CW3E probabilities for IVT above 250kg/ms exceeding
90%. This will wane rapidly during D1 however as the best moisture
flux penetrates well inland and weakens. However, a potent
shortwave and accompanying vorticity lobe will swing southward
near the OR/CA border, moving onshore Wednesday aftn, bringing
height falls and PVA sufficient for robust synoptic ascent in the
still moist atmospheric column. This will also drive a surface
cold front eastward into the Great Basin, providing additional
ascent through low-level convergence while also lowering snow
levels from extremely high, around 9000 ft early D1, to 3000-5000
ft late. This will allow precipitation to change to snow through
the day, but accumulations will likely remain confined to only the
highest elevations, and above pass levels. WPC probabilities on D1
reach 30-40% for more than 4 inches, but only across the highest
peaks from the Shasta/Trinity region northward along the Cascades.

During D2 the mid-level flow remains fairly zonal in response to
the flattening of the trough as paired shortwaves move through the
flow. The lead shortwave, which was moving onshore CA/OR to finish
D1 will remain progressive and shift all the way into the Northern
Plains during D2, while a secondary impulse follows immediately in
its wake to reach the northern Great Basin by Thursday. At the
same time, a strengthening Pacific jet streak will arc eastward
and impinge into the coast, driving additional ascent through LFQ
diffluence while also increasing column moisture noted by a surge
in mid/upper level RH. The lead shortwave will also help drive the
first cold front well to the east, with a reinforcing front
sagging into the West by the end of D2. This second shortwave will
quickly become the dominant feature however, and by D3 it sags
southward into the Central Rockies while amplifying into a closed
low, and causing more NW mid-level flow across much of the West as
the resultant longwave trough amplifies. Considerable ascent will
spread eastward during this time, aided by an impressive jet
streak along the leading edge of the next AR progged to move
onshore just beyond this forecast period. However, the widespread
synoptic lift and near-normal moisture should cause widespread
mountain snows from the Cascades through the Central Rockies, with
snow levels falling steadily to as low as 1500 ft in the northern
Cascades and Northern Rockies by Friday night, and as low as 3000
ft elsewhere.

WPC probabilities D2 for snowfall exceeding 6 inches rise above
50% across much of the WA and OR Cascades and into the northern CA
ranges, with high probabilities also extending into the Northern
Rockies near Glacier NP, the Salmon River/Sawtooth mountains of
ID, and along the Grand Tetons. Locally more than 12 inches is
likely in the Cascades, and the WSSI-P indicates a high risk for
at least moderate impacts due to snow amounts in this region,
including hazardous travel at Santiam, Willamette, and Marias
passes on D2. During D3 the heavy snow spreads to encompass even
more of the western terrain, but with more diffuse ascent the
maximum snowfall amounts may wane compared to D2. Still, WPC
probabilities Thursday night into Friday for more than 6 inches of
snow are generally 30-50% in much of the terrain from the
Cascades, through the Northern Rockies, into the NW WY ranges, the
Wasatch of Utah, and into much of the CO Rockies, where low
pressure development late may enhance snowfall in this latter
terrain, and also result in some light accumulations along the
I-25 corridor and into the Palmer Divide.


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

A shortwave moving out of the Northern Rockies will lift northeast
around a downstream ridge and into Manitoba/Ontario D3 while
deepening into a closed low. This will promote low-level
cyclogenesis, and a surface low is likely to develop over ND
Thursday night and track into Canada during Friday. As this low
pulls away, at least modest theta-e advection will surge moisture
cyclonically into the low, with an impressive deformation axis
possibly developing to the NW. This deformation will likely pivot
southward on Friday, bringing some enhanced ascent in the still
moistening column across ND. Although this low is likely to be
transient and progressive to the east, and forecast soundings
suggest a less than ideal environment for heavy snowfall due to an
elevated DGZ and gusty winds beneath it, above-climo SLR could
still accumulate efficiently and WPC probabilities for more than 4
inches have increased to 10-20% in northern ND. Despite the modest
snowfall expected, gusty winds could still result in blowing snow
impacts, especially in the fluffy SLR environment, reflected by
the recent WSSI.


Weiss