FOUS11 KWBC 130813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Fri Dec 13 2024

Valid 12Z Fri Dec 13 2024 - 12Z Mon Dec 16 2024


...Great Lakes.. Day 1...

Residual heavy lake-effect snow (LES) will persist through the
first half of Friday, especially east of Lake Ontario into the Tug
Hill Plateau, before increasing SW flow downstream of a shortwave
moving through the middle of the country causes WAA and shuts off
LES by the end of D1. Before that occurs, the environment remains
favorable for 2+"/hr snow rates, primarily downstream of Lake
Ontario, before intensity wanes and LES ends later in the day.
Despite a generally short duration of continuing heavy LES, WPC
probabilities for more than 6 inches are high (>70%) downstream of
Lake Ontario, with locally up to 12 inches possible.

...Western U.S...

A closed low over the eastern Pacific will amplify and shed
periodic vorticity lobes towards the Pacific Coast of the United
States and Canada. Each of these will combine with increased
moisture to cause widespread winter weather across the region.

California... Days 1-2...

One of these shortwaves will intensify into a pronounced trough,
taking on a negative tilt as it shifts into central CA Saturday
aftn /early D2/. This will act in tandem with modestly coupled jet
streaks to drive ascent, while impressive moist advection surges
IVT to 500-750 kg/m/s according to both GEFS and ECENS
probabilities. Snow levels will rise to around 5000 ft within the
core of this IVT, highest in the Trinity/Shasta/Siskiyou region,
but still support heavy snow in the higher terrain, with
accumulations aided by nearly ideal upslope flow. Although the
heaviest snowfall, which will likely feature rates above 1"/hr will
occur within the warmer snow levels/higher IVT, even as snow levels
fall behind the primary trough axis moderate to heavy snow will
persist through D2 before waning. Additionally, heavy snow through
synoptic and impressive upslope flow will extend along the length
of the Sierra, bringing substantial impacts to many of the Crest
Passes. WPC probabilities for more than 12 inches of snow is
extremely high (>90%) in the Shasta/Trinity region and the northern
Sierra D1, continuing above 70% D2 while extending down the length
of the Sierra. Local snowfall maxima of more than 4 feet are likely
in the highest terrain.

Pacific Northwest through Northern Rockies... Days 1-2...

Farther north, a stronger closed mid-level low will push into
British Columbia as it remains embedded in the downstream southerly
flow ahead of the larger scale trough axis. This will drive
persistent moist advection into the Pacific Northwest D1 into D2,
with this moisture then merging with the aforementioned shortwave
ejecting from CA to drive widespread heavy precipitation into the
Northern and Central Rockies. Snow levels will fluctuate through
the period, but generally remain around 4000-5000 ft. However,
strong ascent and ample moisture could result in locally lower snow
levels through cold-air dragging on intense snowfall rates,
especially in regions of intense upslope ascent or modest elevated
instability.

WPC probabilities for more than 6 inches of snow are high (>80%)
along the WA/OR Cascades, the Olympics, and into the Salmon
River/Sawtooth region on D1, with the highest probabilities
focusing across parts of ID and into the Tetons D2, while
remaining, albeit more modestly, in the Cascades. By D3 the highest
WPC probabilities for more than 6 inches of snowfall drop
considerably to 30-50%, focused almost exclusively in the Tetons.
Storm total snowfall of 20-30 inches likely in the higher terrain
of many of these ranges. With snow levels ranging between around
4000-5000 ft, some of the higher passes could experience
considerable impacts as well.

...Central Plains into the Upper Midwest... Days 1-2...

A trough ejecting from the Central Rockies Friday night will
amplify into a closed low over the Central Plains as it digs E/SE
through Saturday, and then lifts northeast towards the Great Lakes
on Sunday. Height falls downstream of this deepening trough will
combine with weak but coupled jet streaks aloft and a residual
low-level baroclinic gradient to drive surface cyclogenesis in the
lee of the Rockies Friday night. As this low moves progressively to
the east, it will draw increased moisture northward from the Gulf
of Mexico, channeling an axis of PW anomalies as high as +2 sigma
into MO/IA Saturday. This moisture has steadily shown an increased
trend with recent model runs, likely in response to a subtly deeper
upper low.

As this feature moves east and deepens, it will interact with a
retreating but sprawling surface high (max pressure around
1050mb!). This will leave an environment that is cold enough for
wintry precipitation, but as the WAA intensifies, the high will
lose its favorable position for cold advection leading to a column
that will become overwhelmed by the WAA and warm with time.
Additionally, it will take some time for the DGZ to saturate as the
antecedent air is quite dry, so this setup continues to look more
favorable for IP (and mostly ZR), with just some snow on the
northern side. However, the stronger low could also result in some
more intense deformation to the north of the surface low, leading
to at least some modest snowfall accumulations in MN/WI.

Despite that, the primary hazard appears to be freezing rain, and
WPC probabilities have increased for significant accretion, with
locally damaging icing now possible as reflected by max
probabilities reaching around 30% for > 0.25" of ice, highest
across central IA.

...Central Appalachians... Day 3...

The same high pressure retreating eastward from the Upper Midwest
(above) will push into New England and then favorable wedge down
the coast in a Cold Air Damming (CAD) setup east of the
Appalachians. As moisture from a low pressure moving into the Great
Lakes shifts eastward, it will encounter cold air, especially in
the higher elevations, supportive of wintry precipitation. However,
the robust WAA driving the expanding precipitation shield is
likely to overrun the cold air, leading to a corridor of
significant freezing rain, with only light snow accompanying. WPC
probabilities for more than 0.1" of freezing rain have increased to
as high as 50-70% across the higher terrain from eastern WV,
through the western Panhandle of MD, and into the Laurel Highlands of PA.

Weiss

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