FOUS11 KWBC 180826
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 AM EST Mon Nov 18 2024

Valid 12Z Mon Nov 18 2024 - 12Z Thu Nov 21 2024

...Western U.S...

Days 1-3...

**Multiple days of heavy snow to continue in the mountains of the
 Pacific Northwest through mid-week**

An anomalously deep 200-500mb mean trough over the Northwest will
keep snow levels lower than usual as low-mid level westerlies
deliver a persistent fetch of Pacific moisture into the the
Olympics, Cascades, and as far inland as the Northern Rockies
today. Snow levels will be as low as 1,000ft in some parts of the
northern WA Cascades, but 1,500-2,000ft will be more of the norm
throughout the remainder of the Northwest. Through Tuesday morning,
WPC PWPF shows high chances (>70%) for snowfall >6 inches in the
Olympics, WA/OR Cascades above 3,000ft, the peaks of the Lewis
Range, and in the Tetons. Expect hazardous travel conditions in
these areas with roads along complex terrain more difficult to navigate.

The longwave trough parked to the north off the coast of
British Columbia will dive south Monday night and phase with
another potent vorticity maximum to produce a powerful sub 950mb
low off the Pacific Northwest coast Tuesday afternoon. This storm
system will direct a robust atmospheric river (topping 1,000 kg/m/s
at its peak, or above the 99.5 climatological percentile per
NAEFS) at the northern CA and OR coast. This atmospheric river
will accompany strong warm air advection that will force snow level
to rise 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 associated
with the storm cause tree damage and power outages Tuesday night
and Wednesday. This same plume of rich Pacific moisture will spread
well inland with heavy snow set to occur in the Blue, Boise,
Sawtooth, northeast Washington, and Bitterroot mountains. While
snow gradually lessens in the Northern Rockies and Cascades
Wednesday night, heavy snow will continue in the Shasta and
northern Sierra Nevada of California. It is noteworthy that snow
levels will gradually rise across northern California Wednesday
night, ultimately reaching as high as 8,000ft by early Thursday morning.

In summary, there is no shortage of snow in the forecast for the
Pacific Northwest, Northern Rockies, and northern California. Snow
will be measured in feet (1-3ft in most cases) along the Cascades
above 3,000ft, the Salmon/Shasta/Siskiyou of northern California above
4,000ft, and well inland through the Blue, Sawtooth, Boise,
Bitterroot, Tetons, and Lewis Mountain Ranges. The latest WSSI
still depicts Moderate to Major Impacts in the Cascades, Olympics,
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. The WSSI also now shows
some instances for Extreme Impacts in the Salmon/Shasta of northern
California where several feet of heavy/wet snow combined with
strong winds will exacerbate the potential for power outages and tree damage.

...Northern Plains...
Days 2-3...

Guidance is coming into better agreement on the track of a highly
anomalous area of low pressure (NAEFS and ECMWF shows MSLP values
below the 0.5 climatological percentile over MN midday Tuesday). Snow
is expected to fall across much of northern North Dakota starting
Tuesday morning with the heaviest snowfall occurring Tuesday
evening across much of northern North Dakota and even into parts
of northwest Minnesota. Winds of 30-40 mph would result in whiteout
conditions for areas like central and eastern ND would also
experience their first round of accumulating snowfall this season
in some areas. Snow intensity will gradually diminish through
Wednesday morning, but blustery winds throughout the Red River of
the North may result in reduced visibilities that make for
hazardous travel conditions. WPC's WSSI-P shows moderate chances
(40-60%) for Moderate Impacts (disruptions to daily life, including
hazardous driving conditions and some closures) across central
North Dakota. The highest confidence in those impacts are in north-
central North Dakota. Tuesday's PWPF for >6" is 40-70% over north-
central North Dakota to the west of the Red River Basin and north of I-94.


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


Mullinax

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