Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
351 AM EST Sun Nov 11 2018

Valid 12Z Sun Nov 11 2018 - 12Z Wed Nov 14 2018

...Great Lakes...

Days 1-3...

Continued periods of lake effect snow are likely through mid-week.
The best chance for lake effect will be today and again on
Tuesday. A trough of low pressure will move across the lakes today
with some light warm advection snow, before column winds shift to
the W/NW producing the potential for more lake effect snow across
the U.P. and northwestern L.P. of Michigan. Some light lake effect
snow will persist Monday. More significant snowfall is probable
Tuesday as synoptic lift associated with an impressive jet streak
combines with increasing isentropic lift in the vicinity of a
coastal low pressure moving through New England. This will produce
a few inches of snow near the eastern Great Lakes, followed by
increasing lake effect snow once again as winds shift to the NW
once again behind a shortwave ejecting to the east Tuesday night.
The highest probabilities for 4 inches of snow during this period
are on day 1 over NW Michigan, and then again on day 3 east of
Lake Ontario and Lake Erie.


...Northern Rockies...

Day 1...

Shortwave energy diving into the Rockies from Canada will be
accompanied by a jet maximum as it digs into the Four Corners area
by tonight. A brief period of enhanced lift will occur coincident
with higher relative humidity to produce light snows across the
mountains of MT/WY. A few inches of snow are likely, with a small
chance for more than 4 inches in the Big Horn and Absaroka Ranges.


...Southern Rockies across south-central Plains...

Days 1-2...

An upper level trough will dig into the Four Corners region today
driving a positively tilted longwave trough across the middle of
the country. This will be reinforced by a second impulse on
Monday. Concurrently, a low-level cold front will drop southward
into NM/TX. Persistent SW flow between 700-500mb atop the
increasingly E/NE low-level flow will produce isentropic lift from
NM eastward into the Southern Plains. This isentropic lift will
moisten the column sufficiently for precipitation to develop
across the area, and will be enhanced both synoptically and on the
mesoscale to produce two areas of heavy snow.

The first will be in the terrain of Colorado into northern New
Mexico, along the Front Range and the Sangre De Cristo Range.
Here, jet level diffluence and vorticity advection will combine
with upslope enhancement along the E/NE ridges to produce heavy
snow. WPC probabilities are moderately high for 8 inches across
the Front Range, Sangre De Cristos, and northeastern San Juans,
with 12 inches possible. The remaining Rocky Mountain terrain has
a moderate chance for 4 inches of accumulation.

The other area of heavy snow will be along an axis from eastern NM
through the Panhandle of TX/OK and through southern KS. Here,
mid-level deformation and frontogenesis will drive ascent, coupled
with the right entrance region to a departing but strengthening
upper jet streak. Guidance remains in very good agreement that a
stripe of accumulations of more than 4 inches will occur, with a
small chance for 8 inches, focused near the Panhandle of Texas.
Despite relative short duration of high 1000-500mb saturation,
intense snowfall rates of 1"/hr are possible where upright
convection occurs beneath the best coupled forcing in a saturated
DGZ. The column is cold enough throughout for snow, and SLRs will
likely exceed climatological norms. There has been a shift
northward in the guidance this morning suggesting a better chance
for moderate snowfall accumulations into much of southern Kansas
beneath the departing jet streak. WPC probabilities have ramped
upward, now reaching up to 50 percent for 4 inches of snow.

A band of snow is likely to extend northeast across KS/MO/AR in
response to the best diffluent region of the upper jet streak.
Light snowfall is likely within this corridor, although
uncertainty into exactly how far north, and how much, snow will
accumulate. As the subtropical jet streak begins to phase with the
polar jet energy on day 2, it will minimize the omega driving this
snowfall, so a break will occur, likely near the IL/MO border, and
WPC probabilities for 2 inches fall below 50 percent in that
location.


...Interior Mid-Atlantic and Northeast...

Days 2-3...

Positively tilted longwave trough will gradually eject eastward
into the Ohio Valley Monday into Tuesday, spawning a
slow-developing surface low pressure along the Gulf Coast Monday
that ejects northeast, reaching New England Tuesday. Significant
moist advection will occur ahead of this trough, with a
strengthening baroclinic gradient producing the potential for snow
well north of the surface low track beginning Monday night in the
Ohio Valley. Rapid cyclogenesis northeast from the Mid-Atlantic
Tuesday would broaden the precipitation shield to the eastern
Great Lakes with areas roughly along the northern Appalachians and
west under threat for snow. The global model consensus for the
surface low placement has shifted eastward tonight, likely due to
a slightly slower mid-level trough crossing the Great Lakes. This
will allow for colder air to remain across much of the interior
northeast from Pennsylvania and points north into Maine.
Significant QPF should fall as snow in the higher terrain from the
Adirondacks, through the Greens, and into the Whites, where WPC
probabilities are high for 4 inches of snow, with 8 inch
probabilities now approaching 30 percent in Maine.


The probability for significant ice (0.25 inches or more) is less
than 10 percent for all three days.


Weiss