FOUS11 KWBC 230755
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EST Sat Nov 23 2024

Valid 12Z Sat Nov 23 2024 - 12Z Tue Nov 26 2024

...Pacific Coast through the Intermountain West and Rockies... Days 1-3...

Strong upper low off the Pacific Northwest finally begins to
unravel while ejecting shortwaves inland across the West, with
persistent onshore flow and lowering snow levels leading to
widespread heavy mountain snowfall.

The first shortwave ejecting through the base of an anomalous
trough off the Pacific Coast will swing eastward into the Northwest
Saturday and then begin to amplify into a potent but fast-moving
impulse across the Northern Rockies and into the Northern Plains
Sunday. This deepening system will drive downstream ascent through
height falls, PVA, and divergence. At the same time, a zonally-
oriented jet streak will pivot south of the region, placing its
favorable LFQ atop the far northern High Plains and southern Canada,
interacting with increasing baroclinicity as a front wavers near
the international border. The overlap of synoptic lift with the
enhanced mesoscale ascent through fgen will result in weak
cyclogenesis, leading to an expanding area of precipitation falling
as snow. The column will be plenty cold for wintry precipitation
thanks to high pressure from Canada extending southward. As the WAA
along the warm front lifts north, an exceptionally deep DGZ will
support fluffy aggregate dendrites which should accumulate
efficiently, but the most favorable environment and most impactful
snowfall will likely be just north of the U.S.-Canada border. The
greatest impacts over the U.S. from this system appear to occur
over the northern Rockies on D1, where upslope flow will enhance
ascent. WPC probabilities for at least 8 inches of snow are high
(<70%) across northwest MT and Glacier NP, as well as the Salmon
River and Clearwater Mts of Idaho.

The Pacific mountain ranges spanning from the Cascades to the
Sierra Nevada will see a few separate shortwaves move inland
before the parent upper low finally pushes eastward on D3. The
ongoing AR over central CA will come to an end by the start of D1
and allow for snow levels to fall back to around 4000-5000 ft
across the Sierra and lower (2500-3000 feet) across the Cascades.
However, IVT values will also become very weak along the West Coast
until a resurgence across the Sierra on D3 in response to the
parent upper lows eastward movement. A few locally enhanced axes
of moisture are also possible across the Pac NW D2-D3 due to the
lingering low off the coast. This results in high probabilities
(>70%) for at least a foot of total snowfall over the next three
days across the Sierra Nevada, Shasta Siskiyous of northern
California, Oregon Cascades, and Olympic Mts. Areas above 5000 feet
can expect the potential for up to two feet of snow.

Leftover moisture spilling across the Intermountain West and
central Rockies could combine with favorable west-east upslope
enhancement over the Wasatch Mts of Utah and Colorado ranges on D2.
WPC probabilities for at least 8 inches of snow are low (10-30%).

...Northern New England... Days 1-2...

Stacked/wrapped upper and sfc low over the Northeast will complete its
counterclockwise loop this morning before a separate sfc low
develops and lifts north through Atlantic Canada along with the
larger upper low this weekend. Upslope snow into the central
Appalachians will persist through this evening via NW flow as
moisture has circulated from the Atlantic up and around
southeastern Canada back across the lower Great Lakes. A few
additional inches of snow are possible in elevations above 3000 ft
in West Virginia, with marginal surface temperatures limiting the
potential for heavier amounts.

Into New England, trailing vorticity lobe will aid this new coastal
sfc low east of Cape Cod that will track northward into Nova
Scotia by tonight, favoring increased northerly/cooler flow over
interior portions of Maine southwestward into VT. The column is
fairly mild to start over New England via SE flow ahead of the
initial front, but that will change on D1 with rain changing to
snow as the low approaches and deepens. QPF will be focused closer
to the coast, but light amounts of snow are likely for the interior
and especially the higher terrain above 2500ft or so (White
Mountains and northern ME) where WPC probabilities of at least 4
inches are >40%.

Additionally, lake effect snow showers are possible downwind of
Lake Ontario D1-D2 due to this persistent west-northwest flow
around the large low pressure system to the east. Warm boundary
layer temperatures will limit snowfall accumulations as upper
ridging quickly builds from the west. In, fact most showers may
simply fall as rain outside the higher elevations of Upstate New
York or within some of the locally heavier bands where dynamic
cooling can lower surface temperatures to near freezing.

...Upper Great Lakes... Day 3...

The fast-moving shortwave exiting the northern Plains on Sunday is
forecast to reach the Upper Great Lakes on Monday while also
taking on a negative tilt in response to a piece of upper energy
quickly rounding the southern end of the trough over the Midwest.
Surface low is expected to then deepen over the Great Lakes by the
end of D3 with upper diffluence and lake enhancement off Lake
Superior supporting periods of light to moderate snow over the
U.P. of Michigan. WPC probabilities for at least 4 inches of snow
are low (10-30%) for most of this region on D3, with medium
chances (40-60%) over the Porcupine Mts.

Snell
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