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

Valid 12Z Thu Nov 12 2026 - 12Z Sun Nov 15 2026


...Northeast...
Days 1-2...

The first widespread accumulating snowfall of the season is
expected across primarily the higher elevations of Upstate NY, PA,
and New England late today through Saturday morning. The cause of
this snow will be a double-barrelled low pressure system which will
skirt south of New England and then lift northeast through
Saturday.

The first low will move off the Mid-Atlantic coast early Thursday
as a vorticity max swinging through the flow helps to amplify the
accompanying mid-level trough. Moisture advection will increase
during this time, and the accompanying theta-e ridge lifting off
the Atlantic may lift into a modest TROWAL which will pivot across
New England and back into NY/PA. This will result in an expanding
precipitation shield at the start of the period /12Z Thursday/ with
rain spreading as far north as central New England and coastal
Maine. Initially, all of this precipitation is expected to be rain,
with a rain/snow mix in the highest elevations of the Adirondacks
thanks to snow levels that are 1500-2000 ft during precipitation
onset.

Thereafter, as the first low pulls away, some cold advection
will begin in earnest in its wake, and at the same time the
secondary low will develop off the NJ coast 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 the cold 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 some banding potential NW
of the surface low. This occurring during the time of CAA will
dynamically cool the column to lower snow levels, and it is
possible even the valleys may receive some light accumulating
snowfall Friday morning. There continues to be a lot of uncertainty
as to how this band will evolve, but the WPC snowband tracker
indicates a threat for 1"/hr snowfall as this band pivots from near
the Catskills/Poconos, across eastern Upstate NY and into the
Litchfield Hills, Berkshires, Worcester Hills, and even into
southern VT/NH. Since p-type will be highly intensity-dependent,
accumulations in these areas will be modest, but the probability
for at least 1" of snow is as high as 10% in the valleys (Hudson,
Connecticut) with much greater probabilities at higher elevations.
The most significant snowfall, which has a 50-80% chance of
exceeding 4" will be across the Catskills, Adirondacks, and Greens,
where more of the precipitation can fall as snow regardless of the
intensity.


...Central Appalachians...
Days 2-3...

A surface low moving off the Mid-Atlantic Coast downstream of a
strengthening/negatively tilted upper trough will result as
intensifying cold advection (CAA) from the Great Lakes into the
Central Appalachians. This will not only drive upslope flow into
the terrain of the Central Appalachians, but also direct Great
Lakes moisture southward to enhance precipitation potential. The
guidance is surprisingly dry across the terrain Friday and
Saturday, but the setup will support at least periods of
precipitation, with rain mixing with and changing to snow as CAA
persists and snow levels drop, 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 in these areas if
40-60%, with difficult driving likely due to the heavy and wet
accumulating snow.


...Great Lakes...
Days 1-3...

Amplifying mid-level trough over the eastern third of the country
will swing slowly eastward through Saturday as it gets reinforced
by multiple 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 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 12 inches of snow during the period is 30-50%, with snowfall
rates occasionally peaking above 1"/hr. Elsewhere, LES has a 30-60%
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,
these slightly higher snow totals could occur.


...Sierra Nevada and Northern CA mountains...
Days 2-3...

A strong upper wave swinging into northern CA will direct a
weakening atmospheric river into the Sierra Nevada and southern
Oregon on Friday, with high probabilities (>90% chance) for IVT
exceeding 500 kg/m/s. This will produce a quick shot of heavy
upslope snow Friday afternoon into early Saturday morning with
snowfall rates for which the WPC prototype snowband tool suggests
could reach 2-3"/hr at times. Snow levels will initially be around
8000ft at the start of precipitation before modestly falling to
between 6000 and 7000ft during the day on Saturday. The character
of the snow will likely be fairly wet and heavy, with SLRs around
10-12:1 at best. Probabilities for at least 8" of snow are >50%
above 6500-7000ft, and for at least 12" of snow are >50% above
7000-7500ft. Higher end scenarios for those higher elevation bands
even point to 30-50% probabilities for accumulations of at least
24". The heavy snow will likely make for difficult travel across
some mountain passes, leading to 40-90% probabilities for moderate
impacts per the probabilistic WSSI, mainly above 7000-8000ft, with
some low-end potential for even major impacts across the higher
passes.


...Northern & Central Rockies...
Days 2-3...

A negatively-tilted but increasingly sheared upper trough is
forecast to move across the Great Basin and Intermountain West
Friday night into Saturday. Meanwhile, ahead of the wave, a plume
of deep and anomalous moisture from a remnant atmospheric river
will overspread the region. Increasing large scale forcing for
ascent highlighted by positive differential vorticity advection and
favorable positioning in the left exit region of a 100+ kt upper
jet streak will act on this moisture and lead to widespread
precipitation production and mountain snow across the higher
elevations of the northern and central Rockies. Snow levels will
initially reside above 8000-9000ft, but will fall to between
6000-7000ft across the northern Rockies as colder air filters in
during the day on Saturday and continues into Sunday.
Probabilities for snowfall accumulations of at least 4" are at
least 60% for the south-central mountains in ID, as high as 80%
across the Wasatch and Uintas of UT, and nearly 100% (and >50% for
8") across the San Juan mountains in western CO.


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

Weiss/Miller