FOUS11 KWBC 130641
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
141 AM EST Fri Nov 13 2026

Valid 12Z Fri Nov 13 2026 - 12Z Mon Nov 16 2026


...Northeast...
Days 1 & 2...

Secondary coastal low will be deepening very close to the NJ coast
Friday morning before slowly pivoting away to the east through D1
in response to an amplifying trough over the Northeast being
reinforced by a vorticity maxima rotating off the Carolinas and
through the base of this trough. At the same time, a downstream jet
streak will intensify and arc towards the Canadian Maritimes,
leading to impressive deep layer ascent through D1.

Northeast of this surface low, precipitation will continue within a
weakening TROWAL, especially along the comma head/deformation axis
that is likely to pivot E/SE as the surface low gradually pulls
away. The thermal structure of the column will be marginal for
wintry precipitation, but impressive ascent through 850-600mb fgen
and sharpening deformation will likely create a band of heavier
precipitation dropping E/SE from near the Catskills/Finger Lakes
region of NY through the Poconos and eventually into eastern
PA/northern NJ. Some elevated instability within the TROWAL
combined with the fgen should produce heavy snow rates within this
band, and the WPC prototype snowband tool indicates rates that will
reach 1-2"/hr at times. With the thermal structure being marginal
for this event even as CAA intensifies behind the low, snowfall
accumulations will be heavily elevation and rate dependent, with
the best chance for significant accumulations occurring in the
higher terrain above 1000 ft.

The probability for more than 4" is generally 20-30%, highest
across the Poconos, but the banding that has occurred tonight has over-performed so locally higher totals are possible. This also
suggests that in the lower elevations of eastern PA, NJ, and even
into parts of CT, small slushy accumulations are possible even into
the valleys should any intense banding drive dynamic cooling
sufficient for lower-elevation accumulations to occur.

As this system pulls away, the mean elongated trough across the
east will persist, through which yet another vorticity lobe will
swing southward out of Canada on Saturday, driving a surface
trough/weak cold front southward with it as the core of the trough
shifts across Maine. This will bring a resurgence of snowfall to
the higher elevations of Upstate NY and northern New England, with
upslope flow offsetting some of the drying in the atmosphere behind
Friday's system. Steep lapse rates combined with upslope flow on NW
winds will promote periods of snow showers which may accumulate to
several inches in the higher terrain of the Adirondacks, Greens,
and Whites where there exists a 20-30% chance of an additional 4
inches of much fluffier snow than what occurs on D1.


...Central Appalachians...
Day 1...

Cold air advection (CAA) continues behind the departing Mid-
Atlantic low thanks to persistent troughing amplified along the
east coast. This will result in N/NW flow pushing into the Central
Appalachians today, with the resulting upslope flow tapping into
Great Lakes enhanced moisture to spawn periods of snow in the
higher elevations of WV, as well as along the NC/TN border. With
marginal overall thermals in place, at times during weaker
ascent/lighter precipitation, this may mix with rain below
2000-3000 ft, but should be all snow above these elevations. With
steepening lapse rates dropping into the region this aftn, some
heavier rates within convective snow showers could drop snow levels
lower, but accumulation below 2000 ft should be minimal. However,
in the higher terrain, there is 10-20% chance for more than 4" in
WV, with a 50-80% chance in the higher elevations of the
Smokeys/Blue Ridge of TN/NC.


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

An amplifying mid-level trough over the eastern third of the
country will swing slowly eastward through Saturday as it gets
reinforced by multiple embedded shortwaves digging out of Canada.
In response to this evolution, 850mb temperatures will plummet to
as low as -5 to -10C, coldest across Lake Superior, causing
impressive lake-induced instability due to lake surface temps that
are +8C to +12C (Superior) and +10C to +15C (Michigan). This will
cause sfc-850mb deltaT values as high as +20C, supporting elevated
ELs and periods of heavy lake effect snow.

The lakes being still so warm suggests that the immediate
shorelines will likely experience primarily rain, or mixed rain and
snow, leading to limited, if any, accumulations. However, across
the U.P. of MI, multi-bands of snow will occur at times through the
forecast period, with heavy accumulations likely in the higher
elevations of the Huron Mountains. Here, the probability of
additional snowfall exceeding 4" is 20-30%, with snowfall rates
occasionally peaking above 1"/hr. Elsewhere, LES has a 30-50%
chance of producing a least an additional 4" of snow where a
single band will occasionally pivot south of Lake Michigan into far
northeast IL/northwest IN.

Then during D2, another vorticity lobe will swing through the mean
trough and drive a weak cold front/surface trough across the lakes
and down to the southeast. This will result in a renewed surface
of CAA, with winds shifting to be W/NW than N/NW, providing a
better chance for some lake effect precipitation across northeast
OH, northwest PA, and Upstate NY. Thermally the profile is
marginal, and near the lake shores precipitation may be a rain/snow
mix, but otherwise there is a 20-40% chance of at least 2" of snow
in these areas, highest near the Finger Lakes of NY.


...Western United States...
Days 1-3...

A series of atmospheric rivers (ARs) will translate onshore during
the next few days two mid-level troughs advect across the West.

The first of these will occur today and Saturday as dual closed
lows approach the Pacific coast. One of these will come onshore
across southern CA this morning, while the primary upper low lags
and moves into British Columbia late tonight. Between these two
features, impressive IVT will push onshore, reflected by high
probabilities (>90%) for 500 kg/m/s pushing into northern CA and
southern OR beneath a zonally oriented jet streak approaching the
coast and then lifting into the Great Basin by Saturday morning.
This AR will be accompanied by raising snow levels within the WAA,
so snow levels will rise to 7000-8000 ft before a cold front moves
onshore this evening. Ahead of this front, the impressive moisture
plume will be wrung out as precipitation both through the deep
layer ascent (height falls, PVA, jet level diffluence) as well as
intense upslope enhancement, especially into the Sierra and
northern CA ranges. It is this area of CA that will experience the
heaviest snow, which has a 50-90% chance of exceeding 12" today,
with locally up to 2 feet possible.

This entire trough and cold front shift into the Intermountain West
during D2, with snow levels fluctuating between 5500-7500 ft. While
the trough will remain relatively progressive, especially as the
upper low itself opens over Canada, there will be sufficient above
these snow levels to produce areas of heavy accumulations late D1
into D2. The greatest snowfall is likely across the Wasatch and
Uintas of UT, as well as the San Juans, CO Rockies, and Sangre de
Cristos. In these ranges, 2-day snowfall (ending 12Z Sunday) has a
greater than 80% chance of exceeding 6", with more than 12"
possible (20-40% chance).

Then, during D3, after a brief period of shortwave ridging, yet
another trough will approach the Pacific with a renewed IVT surge
peaking around 800 kg/m/s within the next AR lifting into WA/OR.
This next AR has a more focused /narrow/ moisture channel, but
appears more intense and thus will have greater moisture but also a
return of elevated snow levels climbing to near 8000 ft. As the
accompanying trough swings onshore late D3 and pivots the upper jet
streak southward, periods of heavy snow are possible along the
length of the Cascades and into northern CA/the Sierra once again.
An additional 6+" of snow is possible across these areas resulting
in difficult travel over the passes.


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


Weiss


$$
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