FOUS11 KWBC 120806
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 AM EST Thu Dec 12 2024

Valid 12Z Thu Dec 12 2024 - 12Z Sun Dec 15 2024

...Great Lakes.. Days 1-2...

The core of an anomalous upper low (-2 sigma at 500mb according to
NAEFS with a 510dm center) will begin to fill and drift eastward
across southern Canada on D1, with the primary mid-level trough
axis pivoting into New England by 00Z Saturday. This will be
replaced by brief shortwave ridging downstream of a potent trough
digging across the Central Plains. Until that happens, however, the
environment will remain favorable for another round of widespread
and heavy lake-effect snow (LES) in the W/NW snow belts.

At the start of the forecast period, 850mb temps will be generally
-15C to -20C, and continue to plunge to -18C to -23C by 00Z Friday
 before slow warming begins. This CAA supplying the cold air will
 move across Great Lakes water temperatures that are still warm,
 generally +5 to +9C, supporting deltaT that is well above
 threshold for heavy LES, and will drive lake-induced equilibrium
 levels 10,000 - 15,000 ft, which will be well above the -10C
 isotherm suggesting potential lightning in the heavier bands. This
 will support snowfall rates of 1-2"/hr in many of the bands, with
 locally 3-4"/hr possible downstream of Lakes Erie and Ontario.
 Combined with the cold temperatures and gusty winds, significant
 travel impacts are expected within LES.

The heaviest LES is likely D1 before a slow ease of CAA (and onset
of subtle WAA) wanes LES from west to east during D2, but not
before multiple feet of snow occur in the more persistent and
intense banding. WPC probabilities D1 are high (>70%) for more than
8 inches in the eastern U.P., near Traverse Bay, along the
Chautauqua Ridge, and into the Tug Hill Plateau, with high
probabilities continuing on D2 only in the Tug Hill.

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

Two waves of precipitation will spread onshore the West Coast and
then inland through the forecast period bringing widespread snow
to the higher elevations.

The first wave of moisture will be ongoing to start the period as a
slowly filling closed low drops into northern CA with 500mb height
anomalies falling below -1 sigma according to NAEFS. Height falls
combined with the LFQ of an accompanying upper level jet streak
will drive ascent onshore, leading to locally heavy snow, primarily
confined to the Sierra Nevada. The strongest forcing is transient
and pushes south of the area halfway through D1, but at least a
short duration of 1"/hr snow rates is likely across the Sierra,
generally above 5000 ft. During D1, WPC probabilities for more than
6 inches of snow are above 70% in the Sierra, but probabilities for
more than 12 inches are only 10-30%.

A more significant system will then approach the coast from the
Pacific Friday into Saturday, leading to a more substantial
precipitation event after only a brief break Thursday night into
Friday. This system will again be driven by a pronounced closed low
offshore, opening into a potent trough as it shifts across CA/OR
late Saturday, reaching the Great Basin by the end of the forecast
period. Once again, downstream divergence, warm/moist advection,
and robust jet energy will combine to drive ascent. However, this
second system will be more broad, reflected by 500mb height
anomalies below -1 sigma across much of the Pacific Coast, combined
with modestly coupled jet streaks to push a strong surface low into
British Columbia and a second wave across CA. This overall more
impressive system is reflected as well by GEFS and ECENS
probabilities for 500 kg/m/s IVT lifting onshore exceeding 60%.

Snow levels across the West with this second system will generally
remain around 4000-5000 ft, with a narrow corridor exceeding 6000
ft in the strongest IVT/WAA. However strong ascent into an
environment with a deep layer of lapse rates stronger than moist-
adiabatic could lower snow levels beyond forecasts, and the NBM
25th percentile may be more representative, which reaches as low as
3000-4000 ft during the heaviest precipitation, and falls even
lower as precip begins to shift eastward. This suggests the
potential for more widespread pass-level impacts, reflected by
WSSI-P above 70% for moderate impacts due primarily to snow load
and snow rate in the Sierra and Shasta/Trinity region. Lesser, but
still notable impacts, are possible as far north as the WA Cascades.

WPC probabilities for more than 8 inches of snow on D2 are high
(>70%) but confined to the Shasta/Trinity region of northern CA and
the highest peak of the WA/OR Cascades. By D3, high WPC
probabilities for more than 8 inches spread across the length of
the Sierra and into the Sawtooth/Salmon River Ranges, while also
continuing near Mt. Shasta.

...Central Plains into the Upper Midwest... Days 2-3...

A closed 500mb low emerging from the Central Rockies will track
eastward while deepening and pulling south in response to a potent
vorticity lobe rotating around its base. The trend in the global
guidance has been for this feature to be a little weaker and
farther south, and while this will likely result in more
interaction with a stationary front/modest low-level baroclinic
zone, the resultant downstream moisture advection may be somewhat
muted. Still, as this advects east into Missouri by Saturday
evening, it will combine with strengthening fgen in response to
surging WAA to drive moisture northward reflected by a narrow
channel of +1 sigma PWs on the NAEFS tables, while also helping to
create a surface low moving across the Central Plains and the Upper
Midwest by the end of the forecast period.

Downstream of this low, a sprawling high pressure centered over the
Mid-Atlantic states will gradually retreat to the east. This will
allow confluent flow to merge into the Corn Belt and then Upper
Midwest, with overrunning WAA leading to a period of mixed
precipitation including sleet and freezing rain, with snow farther
north. Even where precipitation is snow, it may begin as a period
of ZR/IP due to antecedent low RH within the DGZ (no ice growth)
noted in regional soundings.

There remains considerable spread in the latitudinal gain of warm
air, but with the high retreating steadily and WAA increasing,
solutions with a farther north depiction of accumulating snow and
ice seem more reasonable, and this could result in hazardous
accretions of freezing rain, and modest snowfall as well. Current
WPC probabilities for more than 4 inches of snow are less than 10%
with this system, but the probability for at least 0.1 inches of
ice are 10-30% D2 in central IA, increasing to 50-70% D3 when
locally as much as 0.2 inches is possible.

Weiss

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