FOUS11 KWBC 172042
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 PM EST Sun Nov 17 2024

Valid 00Z Mon Nov 18 2024 - 00Z Thu Nov 21 2024

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

Active winter pattern for the Northwest at least through the next
week.

A leading wave over the PacNW coast this afternoon will track east
over the northern Rockies through Monday before rapidly developing
into a deep low over North Dakota on Tuesday. A deep low currently
over the Gulf of Alaska is rounded by a powerful shortwave Monday
night allowing for particularly rapid development to a surface low
well off Vancouver Island on Tuesday. This low then stalls/becomes
a gyre off the PacNW coast through at least Friday which will
result in prolonged moisture-potent onshore flow into the PacNW
starting later Tuesday.

The cold front from the leading wave currently crossing the
Cascades crosses the northern Rockies tonight with low levels
plummeting by this evening to 2,000ft in the WA Cascades and
2,500ft in the OR Cascades. These snow levels then gradually drop
an additional 500ft through Monday with moderate precip rates
persisting from continued onshore flow.

Snow levels fall over the northern Rockies from 5000-7000ft now to
2000-2500ft overnight. The unusually cold air-mass is in place at
the same time as persistent onshore flow, courtesy of the longwave
trough to the north, strikes mountain ranges such as the Cascades
at an orthogonal angle to maximize the snowfall potential through
Monday. Day 1 PWPF for >6" are categorical for the Cascades above
about 3000ft and the northern Rockies from the Tetons to the
Bitterroots above about 4000ft.

Following a brief lull in minor ridging Monday night, a rapidly
strengthening storm system (the 12Z GFS/ECMWF both have a sub 950mb
low by 21Z Tuesday!) will direct a stronger atmospheric river at
the Pacific Northwest with not only exceptional moisture advection
(>750 kg/m/s IVT on NAEFS would be above the 97.5 climatological
percentile Tuesday afternoon), but intense winds along the
mountainous terrain. Warm air advection will raise snow levels
to above 2,500ft in the WA Cascades, around 4,000ft in the OR
Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern is for a heavier/wet snow type to
rapidly accumulate, while powerful wind gusts potential lead to
tree damage and power outages Tuesday night and Wednesday.

All told, there is no shortage of snow in the forecast for the
Pacific Northwest and Northern Rockies. Snow is likely to be
measured in feet along the Cascades above 3,000ft, the
Salmon/Shasta/Siskiyou of northern California, and well inland
through the Blue, Sawtooth, Boise, Bitterroot, Tetons, and Lewis
Mountain Ranges. The latest WSSI shows Moderate to Major Impacts in
the Cascades and Bitterroots with impacts including not just
dangerous travel conditions (including notable Cascade passes), but
very heavy/wet snow and strong winds by Tuesday night that could
result in tree damage and power outages in some areas.

As of now Day 3 PWPF for >6" are categorical in the Shasta/Siskiyou
of CA (which is the center of the prolonged moisture plume, but
where topographical lift should keep snow levels from rising too
far above 4000ft) and the WA Cascades where a burst of snow is
expected from the arrival of the occluded low.


...Northern Plains...
Days 2/3...

A powerful late November cyclone will track from the Southern
Plains tonight to Minnesota Monday night. The 12Z ECMWF is not as
far east as previous runs and a bit more in line with the 12Z GFS
in the track north over MN. This exceptionally strong storm system
tracking over Minnesota coupled with high pressure building in
over the Northern Rockies will make for a very windy setup over the
Northern Plains. Snow is expected to fall across much of North
Dakota and winds of 30-40 mph would result in whiteout conditions
for areas like central and eastern ND would experience their first
round of accumulating snowfall this season. The Day 2.5 PWPF for
>6" is 20-70% over much of ND west of the Red River Basin and north
of I-94.


The probability of significant ice across the CONUS is less than 10
percent.


Jackson


$$

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