Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Mon Nov 12 2018

Valid 00Z Tue Nov 13 2018 - 00Z Fri Nov 16 2018

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

The initial thrust of lake effect snow occurs within the cold
advection crossing Lake Superior with resultant snow showers in
lee shore convergence areas located in the UP of MI.  Long cross
lake trajectories and more sustained low level rising motion
focuses slightly higher amounts in the eastern UP of MI.

On Day 2 (Tue night-early Wed), the focus remains on the eastern
UP of MI as well defined boundary layer moisture convergence
continues with long cross lake trajectories favoring higher snow
amounts in lee shore areas towards the east end of Lake Superior. 
Western areas dry as a low level ridge approaches and drying
develops aloft.

The heavier lake effect areas then develop off Georgian Bay and
downstream off Lake Ontario in western New York, with focused
moisture convergence in a band near the southeast corner of Lake
Ontario that persists a few counties inland with strong low level
winds favoring inland propagation. The higher resolution NAM Conus
nest was given the most weighting, with support from the high
resolution models as well. The potential for the band to remain
focused in the low directional shear environment supports locally
heavy snow near the southeast corner of Lake Ontario.

With the low level ridge moving east into the lower Lakes, and
residual bands of snow should taper Wed night, when little
additional accumulations are forecast.


...Western PA/New York/Northern New New England Day 1...

A mid level trough crossing the Great Lakes merges with a
circulation moving up from the southeast.  The result is warm and
moist advection will occur ahead of this trough, with snow from
northwest PA
across western and northern NY and northern New England.  The warm
advection quickly changes precip over to rain in eastern PA across
southeast NY, southern New England, and  downeast Maine. 
Probabilities for 4 inches of snow are still only high for the
terrain of the Adirondacks, as well as the mountains of Vermont,
New Hampshire, and Maine, where a slight risk for 8 inches of snow
exists.  Lower probabilities exist due to uncertainties where a
mixture of precip types are possible in parts of the upper Hudson
Valley, Mohawk Valley, and Champlain Valley.  The precip from the
synoptic system fades later Tue as the main low departs New
England.

...Southern to Central Appalachians Day 3...

Anomalously deep cutoff upper low will drift across the
Mississippi Valley on Wednesday before ejecting slowly to the
northeast while filling. A surface wave will develop along the
Southeast coast with deep moist advection lifting into the
Southern Appalachians Wednesday night. Cold high pressure to the
north will only slowly retreat, and as isentropic lift
intensifies, precipitation will overspread the region. Despite a
warm nose rapidly lifting to the north, surface temperatures
within the wedge of high pressure will remain below freezing, at
least during precipitation onset, and a period of freezing rain is
becoming more likely Wednesday night. There is good model
agreement in freezing rain, but placement and intensity vary.
There is enough consensus for low probabilities for 0.25 inches of
accretion across the terrain of NC, VA, and WV.

...Lower to Mid MS Valley Day 3...

The models and ensembles forecast the southern stream low to
develop Tue evening somewhere near the TX border with OK and AR,
drifting northeast into the lower MS Valley Wed night.  Moisture
wrapping around the north and west side of the circulation where
the models forecast a cold pool below freezing support a
transition to snow across portions of northeast AR into eastern
MO, southern IL, and adjacent far western KY and TN.  Precip type
uncertainties exist on the east side of the low.  Another area of
uncertainty is how far west of the closed low the precip extends. 
The 12z UKMET was a fast outlier and produced less QPF in
northeast AR and eastern MO. This solution was given little
weighting given better clustering among the NAM, ECMWF, GFS, and
Canadian global models.

...Pacific Northwest to Northern Rockies Days 2/3...

A shortwave trough pushes onshore from the eastern Pacific across
WA State on day 2, with a cold front crossing the WA Cascades. 
Initial snow levels will be high and with the frontal passage
start to decrease the last several hours of day 2.  The longer
duration of where temperatures are cold enough for snow will be in
the higher elevations of the WA Cascades. On day 3, a succeeding
wave crosses from British Columbia and the the northern Rockies
near Glacier National Park Wednesday night. Light snows are
expected but the progressive wave leads to a short period of
ascent and snow, so probabilities are low for heavy snow.

Petersen