FOUS11 KWBC 130751
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Wed Nov 13 2024

Valid 12Z Wed Nov 13 2024 - 12Z Sat Nov 16 2024

...Western U.S...
Days 1-3...

The weather pattern over the next few days will be driven largely
by a longwave upper level trough with a pair of upper level
disturbances tracking into the West Coast. The first storm system
is already impacting the Pacific Northwest with an IVT up to 750
kg/m/s (topping the 97.5 climatological percentile) escorting
anomalous moisture as far south as northern California and as far
east as the Lewis Range in Montana. Elevations above 5,000ft,
whether it be in the Cascades, Olympics, northern Sierra Nevada, or
inland through the Blue, Sawtooth, Bitterroots, and Lewis Range,
will be favored for witnessing accumulating snowfall that could
result in treacherous travel conditions. The WSSI does depict some
localized areas of Minor to Moderate Impacts in the >5,000ft
elevations of the Olympics, Cascades, Blue, and Shasta Mountains
today.

Onshore flow into the Pacific Northwest will keep high elevation
snow in the forecast through Thursday in the Northwest. By
Thursday night, the next Pacific storm system will track farther
south into California with 500mb heights that below the 2.5
climatological percentile on NAEFS. Heights will be low enough
Thursday night and into Friday morning that snow levels in the
Sierra Nevada will be as low as 4,000ft (5,000ft in the San Gabriel
and San Bernadino of southern California). The heaviest snowfall
will be observed above 6,000ft in the central Sierra Nevada. WPC
PWPF shows moderate-to-high chances (50-70%) for snowfall totals
>4" in the central Sierra Nevada between Thursday evening and
Friday morning. By Friday afternoon, the upper trough will track
into the heart of the Intermountain West with strong upper level
ascent over the Northern Rockies. WPC PWPF shows moderate chances
(40-60%) for snowfall totals >4" in the >7,000ft ranges of the
Absaroka.

...Northern New England...
Day 3...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
plume of low level moisture back towards northern New England on
Friday. The air-mass over northern New England (particularly
Maine) will be sufficiently cold and dry enough for wet-bulb
temperatures to remain below freezing within the boundary layer.
Strong easterly mid-level flow with air-mass origins over the
Atlantic will lead to warm air-advection that causes a >0C nose
between 850-750mb Friday morning. This is a setup that will likely
result in periods of sleet/freezing rain over northern Maine and
possibly as far west as northern New Hampshire. WPC PWPF does show
low chances (10-30%) for ice accumulations >0.1" in parts of the
North Woods and Central Highlands of northern Maine. One notable
location with those potential ice accumulations includes Baxter
State Park. These icy conditions could result in Minor Impacts
according to the Probabilistic WSSI with low-moderate chances
(30-50%) through Friday afternoon. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions.

Mullinax


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