FOUS11 KWBC 190943
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EST Tue Nov 19 2024

Valid 12Z Tue Nov 19 2024 - 12Z Fri Nov 22 2024

...Pacific and Interior Northwest... Days 1-3...

***Impressive and long-lasting atmospheric river event to bring
 heavy snow to the Northwest***

This morning, a large ridge of high pressure building over Alaska
will help to force the longwave trough that has been parked off the
British Columbia coast for the last 24 hours southward into the
heart of the Northwest Pacific. Rounding the base of the longwave
trough is a potent 500mb vort max that will phase and lead to the
rapid cyclogenesis of a sub 950mb low off the Pacific Northwest
coast by Tuesday afternoon. The NAEFS SAT page is littered with
atmospheric parameters (wind speeds, heights, MSLP, IVT) that are
approaching or reaching maximum (or minimum) climatological 
percentiles in the CFSR climatology. Perhaps the most notable 
parameters are the IVT (topping 1,000 kg/m/s at its peak off the 
northern California coast) and 850-700mb winds that are above the 
97.5 climatological percentiles Tuesday night into Wednesday 
morning. This same moisture plume will spill over the West Coast
mountain ranges and into the Northern Rockies 

These two parameters are vital in producing what should be prolific
snowfall rates in the Olympics, Cascades, Siskiyou, Salmon, and 
Shasta Mountains Tuesday night. WPC's Snowband Probability Tracker 
(SPT) utilizing the HREF shows the potential for 2-3"/hr snowfall 
rates as the heaviest precipitation arrives Tuesday evening with 
some potential for localized 4"/hr rates. Snow levels will begin 
the forecast period quite low; only around 200 ft east of the 
Cascades to around 2000ft in northern CA. However, the exceptional 
WAA accompanying the AR will raise snow levels steadily D2 and D3, 
reaching as high as 4000ft in northern WA and the Northern Rockies,
and to 8000ft across northern CA. Despite this rise in snow 
levels, hazardous snowfall impacts are likely at many passes before
p-type changes over to rain in the Olympics and Cascades first 
(both via WAA and dry slotting aloft), then over the northern 
California ranges Wednesday evening. This coincides with both a 
thermodynamic profile that will lead to snowfall being a heavy/wet 
consistency that when paired with strong wind gusts will cause 
stress to the trees and power lines.

WPC PWPF shows high chances (>70%) for snowfall totals >12 inches
along the spine of the WA/OR Cascades, as well as the Blue,
Sawtooth, and Bitterroots Mountains that stretch as far north as
the Idaho Panhandle. In the Salmon/Siskiyou/Shasta and Olympics,
WPC PWPF shows high chances (>70%) for >30" with the highest
elevations seeing anywhere from 3-5 feet in some cases. The impacts
will be exceptional in northern California where the WSSI shows
Extreme criteria being met in portions of the
Salmon/Siskiyou/Shasta. The Snow Amount and Snow Load components
are the primary drivers in these impacts that are likely to include
extremely dangerous to impossible driving conditions, as well as
extensive closures and both tree damage and power outages. Major
Impacts are highlighted along the spine of the Cascades above
5,000ft and the Olympics above 3,000ft. 

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

The much anticipated storm system responsible for today's winter
storm across puritans of the Northern Plains is tracking north
through the Upper Midwest this morning with a comma-head shaped
precipitation shield wrapping around the northern and western 
flanks of the storm. As the morning unfolds, temperatures will 
continue to fall across North Dakota and allow for lingering areas 
of rain to changeover to snow later this morning. The heaviest 
period of snow arrives this afternoon and into tonight as the 
TROWAL on the backside of the storm pivots through with a
deformation axis capable of generating 1-2"/hr snowfall rates 
across north-central North Dakota. On top of the heavy snowfall 
rates, whipping wind gusts topping 40 mph in many cases means 
significant visibility reductions that will lead to whiteout 
conditions Tuesday evening. 

As a complex 500mb evolution unfolds over Minnesota Tuesday night
and into Wednesday morning, an enormous upper level low gyre will
still have a saturated ribbon of 700-300mb moisture over the
Northern Plains that will spill east over the Red River of the
North and as far south as northeast South Dakota. WPC PWPF does
show low-to-moderate chances (30-50%) for snowfall totals >6" 
east of the Red River in northwest Minnesota and in northeast South
Dakota. Periods of snow will persist throughout the day on 
Wednesday but coverage and intensity will gradually dissipate 
throughout the day until the snow finally comes to an end overnight
Wednesday. When all is said and done, portions of north-central 
North Dakota are likely looking at 6-12" of snowfall with the 
heaviest amounts likely to occur closest to the north-central North
Dakota/Canada border. The latest WSSI shows a fairly large 
footprint for Moderate impacts over most of central North Dakota 
and northeast South Dakota, which suggests there are likely to be 
some disruptions to daily life such as hazardous driving conditions
and potential closures. Minor Impacts are most commonly seen as 
far west as eastern Montana and to the east across the Red River of the North.

...Northern & Central Appalachians... Day 3...

The aforementioned bowling ball of a 500mb closed low over the
Midwest on Wednesday will aid in the development of a new wave of
low pressure tracking over the northern Great Lakes Wednesday
evening. A strong cold front associated with the storm will push
through the Central Appalachians Wednesday night that will not only
lead to much of thermal profiles in the region being sub-freezing,
but a plume of 850-700mb moisture will reach the Central
Appalachians by Thursday morning. With 300-700mb mean flow
generally out of the west, this is a recipe for the first upslope
snow event of the season for the Central Appalachians. After the
first lobe of dynamical potential vorticity (PVU) swings through 
the Mid-Atlantic on Thursday (helping to kick-start the upslope 
event), the second PVU lobe will swing through Thursday night. This
will reinforce the ongoing upslope flow into the Central
Appalachians, and with this feature the initial low pressure 
system that pushed the strong cold front through Wednesday night, 
will introduce another surge in 850-700mb moisture over the region 
through Friday morning. WPC PWPF shows high chances (>70%) for 
snowfall >4" along the Allegheny Mountains and Laurel Highlands. In
fact, for elevations >3,000ft in the Allegheny Mountains, WPC PWPF
shows some moderate-to-high probabilities (50-70%) for >8" through
early Friday morning with additional snowfall still to come later 
in the day on Friday. The latest WSSI-P shows a large swath of
moderate-to-high chances (50-70%) for Minor Impacts from the 
Smokey's of North Carolina on north along the central Appalachians
and into the Laurel Highlands through Friday morning.

Farther north, similar to the original low pressure development
evolution over the Great Lakes on Wednesday, another area of low
pressure will form on the nose of the 2PVU rotating through the
Mid-Atlantic Thursday afternoon and deepen over Long Island
Thursday evening. This becomes a highly anomalous closed mid-upper
level low that is placed at the divergent left-exit region of a
strong 250mb jet streak over the Southeast. Geopotential heights 
over the Mid-Atlantic (500mb, 700mb, 850mb) are all pegged to be 
below the 1st climatological percentile Thursday night. As the 
storm deepens, strong frontogenetic forcing on the northwest flank 
of the 850mb low will result in a narrow deformation axis that is 
currently forecast to set up somewhere from northern PA on east 
through Upstate NY, northern NJ, and into southern New England by 
Friday morning. This evolution has been present on the EC-AIFS for 
over 24 hours. While boundary layer thermals may be questionable 
away from the deformation axis, this setup favors snowfall at 
elevations above 1,000ft in the Poconos, Catskills, Adirondacks, 
and Berkshires. It is worth noting that there remains some 
different solutions on snowfall totals and which areas see the 
heaviest amounts. Regardless, this would be the first accumulating 
snowfall of the season for these higher elevated areas. Should 
guidance trend toward a more robust deformation zone, some lower
elevated/valley locations in the Northeast could receive measurable
snowfall on Friday. At the moment, the WSSI-P shows moderate 
chances (40-60%) for Minor Impacts (generally hazardous driving 
conditions) in the Catskills, northern Poconos, and the Adirondacks
through Friday morning. 

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

Mullinax

...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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