FOUS11 KWBC 132008
QPFHSD

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

Valid 00Z Thu Nov 14 2024 - 00Z Sun Nov 17 2024

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

A positively tilted longwave trough will move into the West Coast
Thursday. The trough will become increasingly elongated as the
upper level flow along the Canadian border attempts (and well
eventually succeed) at returning to zonal flow. Meanwhile the
southwestern side of the trough will slow down to a crawl over
southern California by Saturday. As the trough elongates, a strong
jet streak on the west side of the trough will gradually weaken as
it orients in an unusual northeast to southwesterly flow. Meanwhile
the jet streak ahead of the trough will gradually strengthen,
eventually speeding up to peak over 150 kts from the Four Corners
to the Upper Midwest.

The cold air associated with the trough will support low snow
levels over much of the Pacific Northwest, and extending into
central California by Friday. Meanwhile all of the shortwave energy
supporting the longwave trough will locally enhance lift with the
various lows that move into the coast or develop over the Rockies.

The strongest of these lows won't come anywhere near the U.S., but
rather will stay over the coast of British Columbia. The associated
cold frontal precipitation will have a long fetch of Pacific
moisture with it, which will advect northeastward into the Pacific
Northwest. Snow has been ongoing in the Washington and Oregon
Cascades, and will move inland with the longwave trough to put a
fresh blanket of snow over the mountains of Idaho and Montana,
primarily in the Day 1/Tonight-Thursday time frame. The heaviest
snow totals will likely be in the Washington and Oregon Cascades
going forward, but amounts over a foot are probable for far
northern Idaho and into the Bitterroots of Montana, as well as the
Sawtooth of central Idaho. As the parent low north of Vancouver
Island weakens, so too will the attendant front, which will allow
for snow rates to diminish by Thursday night.

Further south, a new low will form over Nevada Thursday night as a
kink in the jet stream develops as a highly energetic shortwave
rounds the base of the longwave trough. This will shift the axis of
heaviest precipitation south into California, Nevada, and
southeastern Idaho. By this point the heaviest snow totals will be
into the Sierras of California as storm total snow into Friday
night will approach a foot for the highest elevations. While truly
heavy snow will likely only be relegated to the favored
upslope/west facing slopes of the Cascades on Thursday and the
Sierras on Friday, snow spreading well inland may still have
localized impacts as cooler air into the Pacific Northwest allows
snow levels to drop enough to potentially cause a few inches to
accumulate in some of the more populated valleys of Nevada and into
Idaho and over to Yellowstone N.P. by Friday night and lingering
into Saturday.


...Northern Maine...
Day 2...

A retrograding upper level and surface low will move westward out
of the Canadian Maritimes towards Maine Thursday night. The low's
occluded front will spread precipitation into Maine starting at the
eastern tip of the state late Thursday night, then overspreading
much of northern Maine through Friday morning. Since this
precipitation will move in during the coldest time of day, ground
temperatures will support a period of frozen precipitation as the
front moves westward. There is good agreement in the guidance that
there will be abundant warm air aloft, which will support mostly
freezing rain with the initial (and likely heaviest) round of
precipitation. Thus, ice accumulations were nudged upward just a
bit, though most areas of northern Maine will see less than a tenth
of an inch of ice accumulation. The freezing rain will change over
to plain rain with solar heating. The low will stop retrograding
during the day Friday and gradually pull away, resulting in rain
shower activity Friday afternoon.

Wegman



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