Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Thu Nov 12 2026

Valid 00Z Fri Nov 13 2026 - 00Z Mon Nov 16 2026


...Northeast...
Day 1...

The first widespread accumulating snowfall of the season will ramp
up across primarily the higher elevations of PA, Upstate NY, and
New England tonight through Friday evening as a secondary low
pressure system strengthens off the NJ coast. This strengthening
will be in response to increasing baroclinicity offshore and a
secondary vort max causing the primary trough axis to tilt
negatively. This will have a two pronged effect of intensifying
cold air advection through ageostrophic drainage into the deepening
secondary low while also enhancing ascent as some modest 850-700mb
fgen overlaps an intensifying deformation axis to produce banded
precipitation NW of the surface low. This dynamical response will
help cool the column to lower snow levels, changing any rain or
rain/snow mix to all snow in higher elevations, mainly above
1500ft. However, given the impressive forcing for ascent and
potential for heavy banded precipitation, it's possible snow levels
are dragged closer to ground level in some of the lower elevations
of the interior Northeast as well.

The latest trends in hi-res guidance (particularly the
HRRR/RGEM/RRFS) have been for a colder and more southerly storm
track, placing the overlap of favorable dynamics and thermals over
northeast PA and south-central Upstate NY. Timing of this overlap
also happens to be during the nighttime hours tonight into the
morning daylight hours on Friday, removing or reducing most melting
effects from solar insolation. Taken altogether, there is
increasing concern for a thump of heavy wet snow (especially above
1500ft) with rates possibly exceeding 1"/hr at times. The
probability for snowfall exceeding 4" is up to 40-70% in the
higher elevations of the northern tier of PA, and up to 50-80% from
the southern tier of NY extending into the Catskills. There are
even some spots of 10-20% probabilities for snowfall >8" in
northern PA and 20-50% in the Catskills, highlighting a worse case
scenario. While the most significant accumulations have largely
shifted south of the Adirondacks and Greens, there are still some
low to moderate probabilities for >4" in southern VT.


...Central Appalachians...
Days 1-2...

Cold air advection within northwesterly flow will upslope into the
central Appalachians, with modest moisture sourced from the Great
Lakes supporting at least periods of precipitation through
Saturday. Precipitation will initially fall as rain before mixing
with and changing to snow this evening into tonight as CAA persists
and snow levels drop, all aided by steepening lapse rates and
dynamic cooling. Periods of accumulating snow are likely,
especially above 2000 ft in WV and 3000 ft along the NC/TN border.
The probability for at least 4 inches of snow is 20-50% in WV and
50-80% in the high spots of TN/NC, particularly in the Smoky
mountains with difficult driving conditions likely due to the heavy
and wet accumulating snow.


...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 more
than 8 inches of snow during the period is 30-50%, with snowfall
rates occasionally peaking above 1"/hr. Elsewhere, LES has a 30-50%
chance of producing at least 4" of snow where a single band will
occasionally pivot south of Lake Michigan into far northeast
IL/northwest IN, as well as the NW L.P. of MI in the vicinity of
Traverse City, and across the remainder of the U.P. as well.
Thermally, the setup is more marginal in these areas, but should a
band linger long enough with sufficient lift to cool the column,
locally higher snow totals could occur.


...Sierra Nevada and Northern CA mountains...
Days 1-2...

A strong upper wave will cross northern CA Friday/Friday night.
Ample moisture ahead of the associated cold front will bring a
somewhat short burst of upslope enhanced snow to terrain over
northern CA and the length of the Sierra Nevada. Snow levels ahead
of the cold front should remain around 7000-7500ft through the
event Friday with levels dropping to around 5500ft behind the cold
front as snow tapers off friday night. Heavy, wet snow can be
expected at and above the snow level with SPC REFS snow rates
generally 2-3"/hr for a 3-4 hour window down the length of the
Sierra Nevada from 22Z Friday to 06Z Saturday. Day 1.5 (12Z Fri-
12Z Sat) PWPF snow probs for >8" are 50-80% for the higher Sierra
as well as the highest CA Cascades and Klamath Mtns. Expect some
travel disruptions on I-80 at Donner Pass Friday afternoon.

Precip diminishes Friday night with a lull between waves then
until Sunday. The next wave reaches far northern CA Sunday with
pre-frontal snow levels back up to around 6500ft Sunday morning in
the Klamath Mtns. This snow should reach the Sierra Nevada by
Monday.


...Central Rockies...
Day 2...

A negatively-tilted, but increasingly sheared trough crosses the
Great Basin and Intermountain West Friday night and Colorado on
Saturday. Decent Pacific-sourced moisture streams ahead of the
trough with favorable ascent from the left exit region of a 100+kt
NWly jet crossing southern NV/UT. Snow levels start around 9000ft
across NV/UT/CO and drop to around 6000ft behind the cold front a
precip tapers off. Day 2 Probabilities for snowfall >6" are 40 to
60% for the higher central NV ranges and 50-90% for southern UT
ranges up to the southern Wasatch and Uinta and the San Juans. Some
30-50% probs area also there for the Sangre de Cristos in both CO
and NM, Grand Mesa in CO. Snow quickly tapers off Saturday night
across CO with a lull in precip over the Intermountain West then
into Monday.


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


Miller/Jackson/Weiss