Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EST Sun Nov 10 2019

Valid 00Z Mon Nov 11 2019 - 00Z Thu Nov 14 2019

...Northern Plains and Rockies...
Day 1...
A positively tilted upper level trough axis pushes south from
Montana this evening. This is behind a cold front already reaching
the southern Plains and ahead of a sprawling high shifting south
from Alberta. Ongoing moderate to locally heavy snow will push
south across eastern WY, the Black Hills of SD, and western NE
tonight where probabilities for four inches are high. However,
upper level forcing shifts east as the cold air spills south,
leading to lighter snows over CO Monday morning.

...Great Lakes...
Days 1-3...
An upper level trough swinging across the Great Lakes Monday
through Tuesday will maintain a very favorable setup for lake
effect snow through Tuesday. N/NW flow across the western Lakes
from sprawling high pressure shifting down the Great Plains into
Monday night will keep rounds of Lake Effect bands in the
favorable locations downwind of Superior, Michigan, and Huron. The
heaviest lake effect snow is likely Monday and Monday night when
sfc-700mb lapse rates steepen due to strong CAA, and the inversion
heights climb towards 10 kft. Robust convergence and increasingly
unidirectional flow will support intensifying and slow moving
bands during this period, and convective snow rates are possible,
especially across the favored regions of MI. WPC probabilities for
18 inches in 48 hours are highest for D1-D2 for portions of the
eastern UP, northwest LP, and thumb of MI. 

Low pressure developing over the Ohio Valley Monday will focus a
swath of moderate snow over eastern IA, northern IL, and southern
MI late tonight through midday Monday. In the wake of this low,
lake effect over southern lake MI and Erie comes into play. 48hr
probabilities for 6" in D2 are low over southern MI and higher in
the preferred lees of SW MI/northern IN and far northeast OH, NW
PA into western NY.

...Northeast...
Day 3...
Low pressure develops along a cold front and moves northeast from
the Ohio Valley to across interior New England Monday through
Tuesday. Guidance continues to affirm a farther inland track than
prior runs, so the heaviest snowfall will likely extend in a swath
from Buffalo through northern New York and northern New England.
The heaviest snowfall should occur in a narrow band through the
Adirondacks and into the Green Mtns of VT where WPC probabilities
through D2 are likely for one foot. This is where the best overlap
of jet level diffluence, 700-600mb fgen, and WAA occurs. Periods
of moderate snow are likely as the low moves quickly to the
northeast, but modest SLRs and the progressiveness of the system
will limit total accumulations to generally under a foot outside
of the highest Northeast mountains. The track is farther inland
than before with the southern limit of 20% probabilities for 4"
now limited to the Berkshires, and areas west of the I-95/Route 9
corridor in ME.

A transition zone with rain/freezing rain/snow/sleet mix is
expected over central interior New England to interior Maine. WAA
ahead of the surface low drives a warm nose to >0C, but sfc temps
remain just below freezing as high pressure tries to wedge down
through the area. The low is not expected to intensify quickly,
limiting the isallobaric drainage/cooling potential, but there
still exists an increasing threat for moderate accretions of
freezing rain, and WPC probabilities have increased again to
20-30% for 0.1" from the southern Adirondacks, central VT/NH, to
much of interior Maine.


Jackson