Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EST Tue Nov 13 2018

Valid 12Z Tue Nov 13 2018 - 12Z Fri Nov 16 2018

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

Surface low pressure will exit New England this afternoon,
followed by a shortwave which will race eastward away from the
Great Lakes. Behind these two features, winds will become
unidirectionally NW from the surface up through 700mb, producing
an environment favorable for bands of lake effect snow. Cold
advection will drive increasing instability across the lakes, and
the NW flow will move across a favorably long fetch of water
across Lake Superior, Huron, and Ontario. Moderate instability and
potent omega across the lakes will drive the potential for heavy
snow rates, and an upstream moisture connection from Superior and
Huron supports a good chance for the greatest accumulations to
occur SE of Lake Ontario. Although WPC probabilities are high for
4 inches of snow in the U.P. of Michigan as well, the best chance
for 8 inches is in a narrow band SE of Lake Ontario across upstate
New York. Some of this snow may linger into the first half of
Wednesday southeast of Lake Ontario before winds shift to a less
favorable direction and shuts off the lake effect snow.


...Mid Mississippi Valley into the Ohio Valley...
Days 2-3...

Anomalously deep cutoff upper low will migrate slowly from the
Arklatex early on D2, into the Mid-Atlantic on D3. As this feature
lifts northeast, mid-level diffluence will increase downstream and
warm advection will transport moisture east of the low. This
moisture will then be wrapped back north of the low within the
warm conveyor belt producing a well defined trowal. At the same
time, enhanced frontogenesis between the warm air flooding in to
the east, and the cold pool beneath the upper low, will drive
strong lift into the trowal, which will then be enhanced by
significant mid-level deformation and steep lapse rates within the
upper cold pool. Forecast profiles suggest a saturated DGZ just
above the trowal, leading to efficient dendrite growth and likely
heavy snowfall rates. Guidance still differs into where the best
forcing will combine in the vicinity of this upper low, but heavy
snow appears likely, and WPC probabilities show a moderate risk
for at least 4 inches of snow, with 6 inches or more possible in
isolated locations.


...Northern New England and New York...
Day 1...

A northern stream trough moving through the Great Lakes will help
to drive surface cyclogenesis off the Mid-Atlantic coast which
will then lift through eastern Maine tonight. Warm and moist
advection ahead of this trough will produce precipitation across
the area, with snow likely away from the coast. A brief period of
strong mid-level frontogenesis will lift northeast into the
terrain producing heavy snow, but warm air ridging in from the
south will gradually change the snow over to rain, especially in
the lower elevations. Accumulations of 4 inches are expected only
in the Adirondacks and mountains of VT/NH/ME. The highest
accumulations are likely in the mountains of Maine, where low
probabilities exist for 8 inches of snowfall.


...Pacific Northwest to the Northern Rockies...
Days 1-3...

Two shortwaves will eject eastward within Pacific jet energy
bringing periods of snow to the northern Cascades and Rockies. The
first will move onshore late on D1 into D2, spawning an Alberta
Clipper surface low which will move towards the northern Plains.
Enhanced relative humidity and light snows will accompany this
feature. More significant snow is likely into D3 as a more potent
shortwave and associated jet maximum drops southeast atop the
Pacific ridge. Moisture will spill over the Canadian coastal
ranges and as the enhanced jet level diffluence moves overhead,
heavy snow will develop across most of the higher terrain of
Idaho, Montana, and into NW Wyoming. WPC probabilities are high
for 4 inches of snow in the northern Rockies near Glacier National
Park where forcing, upslope enhancement behind a surface cold
front, and column moisture combine most efficiently. The other
ranges from the Big Horns northwest into the Washington Cascades
will also receive snowfall, but with lower accumulations likely
due to the progressive nature of the mid-level trough.


...Appalachians into Southern New England...
Days 2-3...

Increasing confidence in a significant freezing rain event across
the Appalachians, with snow likely northward into the interior
Mid-Atlantic and Southern New England.

Deep upper low will drift northeast from the Mississippi Valley
into the Mid-Atlantic before opening and ejecting eastward late on
Day 3. At the surface, cold high pressure will center over New
England before retreating slowly to the northeast Thursday.
Isentropic lift will increase steadily atop the surface high as
mid-level flow remains from the S/SW, leading to precipitation
overspreading the area from the south. Temperatures will initially
be cold enough for snow at the onset, but a warm nose will rapidly
lift northward turning the precipitation to mixed. Despite the
surface high retreating, it will wedge down east of the mountains,
enhanced both by mid-level confluence and precipitation falling at
the surface. Forecast profiles depict an extended period of
freezing rain as surface temps remain below freezing despite
warming 850-700mb temperatures. Heavy QPF is likely, and much of
this may fall as freezing rain, especially in the terrain from the
southern Blue Ridge, up all the way into the Poconos of PA.
Guidance has increased its freezing rain accretion forecasts, and
despite heavy rainfall likely not accreting efficiently, there is
a good signal for a large area of more than 0.25 inches of
accretion, with 0.5 inches possible. WPC probabilities are now
above 50% for significant icing.

On the north side of this system on D3, the combination of
isentropic lift and synoptic ascent within the right front
quadrant of a departing upper jet will produce precipitation into
Southern New England. The column is cold enough for snow from PA
through NY and into southern/central New England where warm
advection snow may become heavy late on D3. WPC probabilities are
currently low for 4 inches of snow, but enough discrepancy still
exists in the surface low track off the Mid-Atlantic that snow may
end up heavier and further south than current probabilities
suggest.


Weiss