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

Valid 12Z Sun Nov 10 2019 - 12Z Wed Nov 13 2019

...Northern Plains and Rockies...
Day 1...
Mid-level impulse diving southward from Canada will amplify a
longwave trough across the east, while pushing a cold front south
into the Central Plains. Anomalously cold air will follow this
front in response to cross-polar flow, which will support most
precipitation in the form of snowfall. A swath of snow is likely
from near Glacier NP southeast into Nebraska, where jet level
diffluence and mid-level frontogenesis along the baroclinic
boundary will drive ascent. While much of the snow is expected to
be light as the best upper forcing quickly pulls off to the
northeast, enhanced omega through upslope flow behind the front
will provide a better chance for heavy snow in the terrain of MT
and WY, as well as into the Black Hills of SD and Pine Ridge of
NE. Here, WPC probabilities are moderate for 6 inches, but low in
the Pine Ridge. Elsewhere, WPC probabilities are moderate for 2
inches across the western Dakotas and into central Nebraska.

...Great Lakes...
Days 1-3...
Persistent upper level trough will induce a favorable setup for
lake effect snowfall through early next week. N/NW flow across the
Lakes will induce rounds of Lake Effect bands in the favorable NW
flow locations downwind of Superior, Michigan, Huron, and Erie.
The heaviest snowfall 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 bands during this
period, and convective snow rates are possible, especially across
the favored regions of MI. WPC probabilities for 6 inches are
highest on D2 /Monday/ across the eastern U.P., NW L.P. of MI, as
well as SW MI and into N IN downwind of Lake Michigan.
Accumulations of 12" are possible on D2 across the eastern U.P.
3-day total snowfall probabilities are greater than 50% for 18" in
the eastern U.P., with low probabilities also existing in the
other favorable NW flow locations across the state of MI.

...Northeast...
Day 3...
A wave of low pressure will develop along a cold front and move
northeast across New England through Tuesday. Guidance has shifted
the track of this feature a bit further N/NW today, so the
heaviest snowfall will likely extend in a swath from southern
Michigan through interior 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 the best overlap
of jet level diffluence, 700-600mb fgen, and WAA overlaps. 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. On Monday, WPC probabilities
indicate a high chance for 6 inches from central NY into VT and
northern NH, with the heaviest snow axis extending into northern
Maine on D3. Isolated amounts to 10" are possible in the high
terrain of NY and VT. SE of the heaviest snow axis, a warmer
column, weaker forcing, and shorter duration for potential snow
will limit accumulations, and WPC probabilities for 2" are above
20% only as far SE as the Worcester Hills of MA to roughly the
I-95/Route 9 corridor in ME.

Further to the SE, a transition zone is likely with rain/freezing
rain/snow mix all possible. The further NW solution has led to an
uptick in freezing rain potential where WAA aloft 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 to 10-20% for 0.1" in NH and ME.


Weiss