FOUS11 KWBC 111948
QPFHSD
Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
248 PM EST Tue Nov 11 2025
Valid 00Z Wed Nov 12 2025 - 00Z Sat Nov 15 2025
...Great Lakes and Northeast...
Days 1-3...
Persistent cyclonic flow across the northeast will result in
periods of light to moderate snowfall through Thursday, with
locally heavier lake effect snow also possible. Within this regime,
several shortwave impulses and accompanying vorticity maxima will
rotate southeast, providing renewed impetus for ascent and
resulting snowfall. The first of these will be exiting Downeast
Maine tonight, with rapid drying of the column in its wake leading
to a quick wane of snowfall across Northern New England. However,
this dry period will be somewhat short lived as yet another
shortwave quickly dives through the mid-level NW flow, emerging
from Ontario and shifting into the Mid-Atlantic states by the end
of D1 and into early D2 /00Z Thursday/. This will provide renewed
ascent for snow across much of the area from the U.P. of Michigan
through Northern New England.
During D1, the heaviest snow is likely downwind of Lakes Erie and
Ontario in response to increasing lake effect snow (LES). The setup
is not idea for LES despite warm lake temperatures as winds
gradually veer and mid-level temps modify, reducing the deltaT from
the lake surface to 850mb. Still, a focused band off Lake Erie
early D1, gradually transitioning as flow veers more to the NW by
the evening, will be followed by a focused band off Lake Ontario
later D1 and into early D2. A deepening DGZ is likely into which
theta-e lapse rates collapse suggesting at least a potential for
convective snow rates, but primary ascent below the DGZ and
warming temps aloft my limit the snowfall potential. Still, WPC
probabilities D1 are moderate to high (50-70%) for more than 6
inches of snow across the Tug Hill Plateau, with 2-4" expected
90% chance) near Buffalo, NY and into the far western
Adirondacks.
Then during D2 the elongated vorticity lobe streaks across with
transient height falls and PVA before SW flow later D2 results in
WAA and a reduction in LES, especially west of Lakes Erie and
Ontario. While flow is not ideal along any specific lake D2, an
upstream connection from Huron and the Georgian Bay will help
increase moisture SE of Lake Ontario, leading to a narrow corridor
of heavy snow from the Finger Lakes towards the Catskills. Again,
thermally the column is modest for LES and it is possible the
heavier snow is actually well removed from the lake itself due to
warm lake temps, and WPC probabilities are moderate (30-50%) for
more than 4 inches in parts of the Adirondacks and Greens, as well
as the higher terrain of the Presidential Range. Although snow
accumulations elsewhere across parts of PA, NY, and Northern New
England should be light, briefly heavy rates from convective snow
showers are possible across the region.
By D3, continued NW flow will support some additional modest LES
downstream of Lake Ontario, with upslope flow into VT, NH, and ME
leading to modest snowfall accumulations in the higher terrain of
those states as well.
...California and Nevada...
Days 2-3...
An impressive atmospheric river (AR) will spread onshore CA
beginning Wednesday night and persisting through Thursday. This AR
will be characterized by impressive IVT reflected by high
probabilities (>80% chance) of exceeding 750 kg/m/s, reflecting a
strong AR moving into CA, and in fact the +1 sigma within the
ensembles are above 1000 kg/m/s, which is above the climatological
record for mid-November according to NAEFS ensemble tables. This
will result in significant and heavy precipitation spreading into
CA Thursday and Friday.
The driver of this robust AR is a strong closed low moving across
the northern Pacific which will approach the CA/OR coast late
Thursday and then slowly open before moving onshore northern CA
Friday. This will result in strong ascent through the overlap of
height falls and narrow but impressive LFQ diffluence as a jet
streak pivots onshore as well. Despite the slow motion of this
upper low, the best moisture within the AR will be somewhat
transient, so the duration of heavy precipitation in any one
location across CA will be somewhat limited (around 24 hours), and
with pronounced WAA within the core of the AR, most of this
precipitation should fall as rain. However, the exception is likely
to be across the Sierra, initially above 8000 feet, then falling
to around 5000-5500 ft behind a cold front which will push eastward
into the Great Basin by Friday morning.
This lowering of snow levels will allow for heavy accumulations
across the Sierra, leading to significant impacts at the area
passes. WPC probabilities on D2 are high (>70%) for 6+ inches of
snow in the Sierra, but generally above 7000-8000 ft. As snow
levels lower D3, WPC probabilities indicate an additional high
chance (>90% in the higher terrain) of 6+ inches, leading to
event-total snowfall in the Sierra of 2-3 feet. This will produce
dangerous travel at many of the Sierra passes, with a heavy and wet
snow contributing to an increased chance for at least moderate
impacts from the WSSI-P due to snow load and snow amount.
Additional heavy snow is expected on D2 near Mt. Shasta, and on D3
extending into the Great Basin and some of the higher terrain of
Nevada.
...Pacific and Interior Northwest...
Day 3...
A sharp vorticity maxima will shed from the closed low approaching
CA (referenced above) and move onshore Washington State Friday.
This feature will be transient and of modest amplitude, but will
encounter elevated PWs as the S/SW AR downstream of the primary low
will spread higher moisture northward. This vorticity maxima may
lead to subtle surface low development as shown by some of the
ensembles, which would additionally enhance ascent, at least
briefly, followed by more confluent zonal mid-level flow in its
wake. While there are considerable differences among the various
models by Friday with respect to available moisture and timing of
these features, it appears likely that some heavy snow will occur
across the Cascades and potentially spread into portions of the
Northern Rockies, especially above 4000 ft in the Cascades and 5000
ft farther east. This will result in snowfall accumulations that
have a 30-50% chance of exceeding 6 inches, primarily in the higher
elevations of the Washington Cascades.
The probability of significant icing is less than 10 percent.
Weiss
$$
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