FOUS11 KWBC 150727
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 AM EST Sun Dec 15 2024

Valid 12Z Sun Dec 15 2024 - 12Z Wed Dec 18 2024

...Pacific Northwest through the Northern Rockies... Days 1-3...

An active period of weather will bring periods of of heavy snow to
much of the region through the middle of next week.

The period begins with a broad longwave mid-level trough centered
over the Rockies. Within this trough, a modest shortwave impulse
and accompanying vorticity maxima will be shedding northeast into
the Northern Rockies, interacting with modest downstream moisture
reflected by near-normal PWs according to NAEFS. This will spread a
swath of precipitation across ID/WY/MT the first half of D1 before
an approaching shortwave ridge brings an end to the precipitation.
Most of this precipitation will be light, with the exception likely
in the vicinity of NW WY/Tetons where upslope flow and some
enhanced fgen will drive heavier rates and greater snowfall
accumulations. WPC probabilities D1 for more than 8 inches of snow
are high, above 70%, in the Tetons and adjacent Absarokas to the north.

Behind this first shortwave, the aforementioned shortwave ridging
will cause a brief respite to precipitation in the Pacific
Northwest, but this will come to an end by the start of D2 as
another amplified closed low drifts eastward across the Pacific
Ocean. Downstream of this low, mid-level divergence will approach
the coast Monday morning, with periodic PVA through shedding
vorticity lobes helping to enhance ascent. The downstream SW/WAA
ahead of this feature will surge moisture back onshore as well,
reflected by IVT approaching the 97th percentile as both GEFS and
ECENS probabilities for IVT of 500 kg/m/s reach 20-30%. This will
spread heavier precipitation back onshore the Pacific Northwest,
with snow falling above generally 3000 ft in the Cascades, and
spilling into the interior Northwest where snow levels will be even
lower, around 1500 ft. This will result in moderate to heavy snow
from the Shasta/Siskiyou/Klamath region of CA northward along the
Cascades and as far inland as the Blue Mountains and Salmon River
range. Across these areas, WPC probabilities are moderate to high
(50-70%) for at least 8 inches of snow, and locally more than 12
inches is likely (>70%) in the highest terrain of the OR Cascades
and near Mt. Shasta.

During D3, a warm front just offshore will lift northward, and the
accompanying precipitation will follow on enhanced WAA lifting
across OR/ID/WA. The intensity of this precipitation should
generally be lighter in the snow areas than on D2, but additional
accumulations exceeding 6 inches are possible (30-50%), highest in
the WA Cascades.

...Central Appalachians... Day 1...

Weakening surface low pressure moving across the OH VLY will spread
downstream moisture northward into the Central Appalachians and
Mid-Atlantic States. This moisture will surge northward on
intensifying 295K isentropic ascent, lifting atop a retreating but
still expansive high pressure centered over New England and its
resultant wedge east of the Appalachians. As the high retreats and
the surface low weakens while moving eastward, low-level flow will
veer to become primarily S/SE, not ideal for locking in any low-
level cold air. This suggests that while precipitation will begin
wintry (snow/sleet/freezing rain) in most areas from the highest
elevations of SC northward, it will quickly turn to rain outside of
the higher terrain. Even in the Mid-Atlantic and Northeast as far
southeast as the I-95 corridor, precipitation may start as a brief
period of sleet or snow before turning to rain. This will limit
wintry accumulation in many areas.

However, in the higher terrain of WV, through the MD Panhandle, and
into the Laurel Highlands, a prolonged period of freezing rain is
likely, which will accrete to moderate to substantial ice amounts
through Sunday night. Some modest conditional instability reflected
by theta-e lapse rates around 0C/km will support at times briefly
heavy rain rates, somewhat limiting the accretion potential
(especially without any dry-bulbing affects), but prolonged
duration of freezing rain has still bumped WPC probabilities to
moderate (30-50%) for 0.25" centered near the MD Panhandle, with
high probabilities (>70%) for at least 0.01" stretched from NC
northward along the Appalachians into Upstate NY.

...Northern Plains... Days 1-2...

A wave of low pressure moving from MT through MN Sunday into Monday
will deepen in response to a mid-level shortwave closing off
overhead, collocated with the LFQ of a modest but poleward arcing
jet streak. As this low slowly intensifies, the downstream plume of rich
theta-e air spreading northward from the Gulf of Mexico will begin
to wrap cyclonically around the low, potentially supporting modest
TROWAL development pivoting into far northern ND and northern MN
Sunday night. The leading WAA ahead of this developing low will
likely result in some modest freezing rain across far western ND
and eastern MT early D1 due to lack of saturation within the DGZ,
but WPC probabilities on D1 are above 70% for at least some icing,
with a narrow channel of 10-30% near the ND/MT border for 0.1 inches.

More impressive is likely to be the snowfall on the northern edge
of this system as it progresses east, with snow becoming more
intense Monday from eastern ND through northern MN. Here, elevated
instability beneath the TROWAL combined with modest deformation
could result in banded snow structures, offsetting otherwise modest
omega present within the column. There is still some uncertainty
into how much snow may result as precipitation expands and
intensifies Monday, but WPC probabilities have increased and are
50-70% from northern ND across into northern MN for 2+ inches, and
the WSE plumes do suggest at least a low-end potential for 4-5
inches in some areas. Regardless of the intensity and amounts of
snow, hazardous travel is likely as fluffy SLR snow combines with
gusty winds to cause snow covered roads with restricted visibility,
but WSSI-P for moderate impacts remains quite low for this area
suggesting primarily minor impacts.

Weiss

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