FOUS11 KWBC 191926
QPFHSD

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

Valid 00Z Wed Nov 20 2024 - 00Z Sat Nov 23 2024

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

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

An extremely amplified pattern over the Pacific characterized by an
intense +4 sigma ridge near the Aleutians will help drive a
downstream closed low which is progged by NAEFS to drop to as low
as -6 sigma with respect to 850mb heights off the Washington coast
to start the period. This intense feature will be accompanied by a
strong surface low, and although this surface low will generally
stall and then retrograde back to the north into Friday, the
accompanying ascent and moisture will surge onshore through the
period.

South of this closed low, pinched mid-level flow combined with a
persistent 130kt Pacific jet streak will cause strong warm and
moist advection to push onshore. This will create a potent
atmospheric river /AR/ which both the ECENS and GEFS probabilities
suggest will exceed 750 kg/m/s IVT. As the parent low retrogrades,
the associated frontal structure will stall in a NE to SW
orientation, providing a favorable setup for a long duration of
impressive IVT pushing onshore, focused into northern CA, but with
downstream PW anomalies reaching as high as +2 sigma all the way
into the Great Basin and interior Northwest. This suggests, and is
agreed upon by nearly all global models and ensembles, that
persistent and widespread moderate to heavy precipitation will
occur through the next 48 hours, with dynamic ascent through
upslope flow enhancing the intensity of precipitation rates.

Snow levels initially will be low, 1500-3000 ft from north to south
tonight, suggesting that the heavy precipitation will begin as snow
in many areas outside of the lowlands, but including the passes.
Snowfall rates will likely peak around 3"/hr in the Cascades and
northern CA ranges tonight, which when combined with gusty winds
will produce treacherous and at times impossible travel. During D2,
the focus will shift to be more aligned SW to NE within the
greatest IVT plume from the Shasta/Trinity region of CA through the
Northern Rockies, but at the same time snow levels will steadily
climb, reaching above most pass levels by the end of D2, around
8000 ft in CA and 5000 ft in MT. WPC probabilities D1 are high for
more than 12 inches in the Shasta/Trinity region of northern CA,
along the spine of the Cascades, and into the Olympics. Pass level
impacts are also expected D1. By D2 the highest probabilities shift
east as snow levels rise, reaching above 70% for 12+ inches in the
Salmon River/Sawtooth area. In the highest terrain, especially
around Mt. Shasta, snowfall of 4-6 feet s likely.

During D3 snow levels continue to rise within the IVT plume, but
also more broadly as a secondary surface low lifts near the WA
coast to move onshore British Columbia by the end of the forecast
period. This will enhance moisture and ascent once again, while at
the same time a cold front drops southward from Canada into the
Northern Rockies. Together, this could result in an enhanced swath
of heavy snow across the Okanogan Highlands eastward to the
Northern Rockies where both upslope flow and enhanced fgen will
combine. WPC probabilities D3 are highest along the WA Cascades
eastward through the Northern Rockies where they reach 50-70% for=20
an additional 6+ inches.


...Northern Plains...=20
Day 1...

An impressive closed 500mb low with 500-700mb heights falling below
the 2.5 percentile according to the NAEFS ensemble tables will be=20
positioned across ND to start the forecast period. This low will=20
drift only slowly southeast as the trough continues to amplified in
response to vorticity lobes swinging cyclonically through the base
of the trough, resulting in additional deepening of the trough and
a pull southeast into the Upper Midwest of the primary trough=20
axis. Beneath this, an occluded surface low will track gradually=20
eastward across southern Manitoba, dropping into MN by the end of=20
D1 while filling. Although this won't result in intense synoptic=20
lift as features will be vertically stacked, impressive mesoscale=20
ascent will persist to produce heavy snow across the Dakotas and=20
into Minnesota.

The evolution of this low will result in persistent theta-e=20
advection, driving a pronounced TROWAL within the 600-500mb layer=20
which will rotate cyclonically southward across the Dakotas and=20
into Minnesota Tuesday night into Wednesday. This TROWAL will=20
overlap efficiently with an axis of mid-level deformation and=20
correlated 850-600mb fgen which will drive a narrow corridor of=20
enhanced ascent into a deepening DGZ (probs for 100mb of depth from
the SREF exceed 50% Wednesday morning). In general, ascent will be
waning through D1, but this axis of strong mesoscale lift combined
with strong winds and a cold column should result in moderate to=20
at times heavy snow rates exceeding 1"/hr, which combined with the=20
gusty winds will produce limited visibility and hazardous travel=20
conditions. Total snowfall may me somewhat moderate, with locally=20
heavier amounts likely. WPC probabilities for more than 4 inches of
snow are above 70% from the Turtle Mountains along the ND/Canada=20
border and then stretching as far south as the Sisseton Hills of=20
northeast South Dakota.


...Ohio Valley, Appalachians, & Northeast...=20
Days 2-3...

A complex upper pattern will produce dual interacting surface low
pressures across the region through Friday. Confidence in placement
and evolution is lower than typical, but at least some areas are
likely to receive heavy snowfall beginning Thursday.

The strong closed low, characterized by widespread 500-700mb height
anomalies of -2 to -3 sigma according to NAEFS, will encompass much
of the region from the Great Lakes, Northeast, and Mid-Atlantic,
with spokes of vorticity periodically rotating through the trough.
The strongest of these vorticity maxima will shift across the Mid-
Atlantic to south of New England Thursday aftn, which will drive
secondary cyclogenesis (the primary low beneath the core of the
closed low over Michigan). These dual low pressures will then
pinwheel around each other through Friday producing periods of
intense ascent within a moistening column to drive rounds of mixed
precipitation.

The challenge through the period is where will the heaviest snow
accumulate, and, especially outside of higher terrain, how much
snow can accumulate. The high confidence portion of this forecast
involves the Central Appalachians where post frontal NW flow will
produce impressive upslope ascent into a cold column, while at the
same time the primary low from MI rotates southward to enhance
ascent locally across that same area. This will result in a 2-day
period of moderate to heavy snow, and WPC probabilities on D2 reach
30-50% for 4+ inches, with greater probabilities reaching above=20
90% for additional snowfall exceeding 4 inches on D3. Total=20
accumulation may exceed 12 inches in the highest terrain of WV and=20
the MD Panhandle, with locally as much as 4-6" as far south as the=20
NC/TN border.

In other places, the broadly forced ascent results in lower
confidence forecast but with high risk potential for rounds of=20
heavy snowfall rates. In the vicinity of both the primary low=20
dropping from Michigan across the Ohio Valley and into the Central=20
Appalachians, as well as across New England, New York, Pennsylvania
near the secondary low as it retrogrades, periods of intense=20
deformation are likely, and in fact several of the global models=20
indicate rotating plumes of negative theta-e lapse rates which=20
could support CSI in bands of heavy precipitation. Timing and=20
placement of any of these more intense bands is quite uncertain,=20
but with marginal thermal structures in place, it will likely=20
require these heavier rates to drive dynamic cooling sufficient for
significant snowfall accumulations. The most likely regions will=20
be in higher elevations above around 1500 ft including the=20
Catskills and Poconos, where WPC probabilities have increased to
50-70% for 6+ inches, and across portions of the Ohio Valley where
they are 30-50% for 4+ inches.


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

Weiss



...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!6x9i5vOfghMBoXUymLE8NedzgOf8ZMj3ZFI5VZnH4SdQ5=
RpMmQEoOm2gPWanMC788RS09vg-2qy13FS35HcHYf_YhKo$=20

$$

=3D =3D =3D
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