Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
349 AM EST Mon Nov 11 2019

Valid 12Z Mon Nov 11 2019 - 12Z Thu Nov 14 2019


...Great Lakes...
Days 1-3...
...Significant and widespread Lake Effect snow event likely in the
favored N/NW snow belts on Day 1...

Strong CAA behind a cold front and low pressure system moving into
New England will create extremely favorable conditions for Lake
Effect snow in the favorable N/NW snow belts. This includes the
eastern U.P. of MI, as well as portions of the L.P. including near
Saginaw Bay. Persistent N/NW unidirectional low-level flow and
extremely cold 850mb temps falling to -20C will create steep lapse
rates and deep inversions towards 10kft. SLR's may reach 25:1 in
the cooling airmass, and the intense lift may be sufficient for
convection as theta-e lapse rates fall to 0, evidence for at least
conditional instability. Some of the high res guidance is
indicating the potential for mesovorticies in the strongly
convergent environment which could further enhance snowfall. The
only limiting factor is subtle wind shifts which could drift the
bands W/E slightly, but where bands are most persistent, well over
12" of snow is likely, and probabilities for 18" are as high as
30% across parts of MI. 

Low pressure developing over the Ohio Valley today will move into
New England, and in the wake of this low, lake effect over Erie
comes into play. 48hr probabilities for 6" in D2 are moderate in
the preferred lees if northeast OH, NW PA and into western NY.

After a brief respite in MI on D2, renewed Lake Effect snowfall
may again develop on D3 ahead of a trough, but on SW flow this
time. Temperatures will still be cold enough to produce the
low-level deltaT required for instability and LES. At this time
WPC probabilities for 4 inches in the favorable S/SW snow belts
are modest, but this could increase with later issuances and will
need to be monitored for heavy snowfall once again.


...Northeast...
Days 1-2...
Low pressure moving along a cold front will push northeast from
the Ohio Valley and across Southern/Central New England into
Tuesday. The track of the surface low has again shifted a bit NW,
pushing the axis of heaviest snowfall further north, while
increasing the threat for freezing rain in portions of the
Northeast. The heaviest snow will remain confined to a narrow band
collocated with jet exit region ascent and 850-600mb fgen pivoting
northeast across the Adirondacks and into portions of northwest
Vermont, with lighter snows on either side of this band due to
WAA/ascent. Although the system is progressive, periods of heavy
snow are possible, and WPC probabilities are high for 8 inches in
the northern Adirondacks and the Greens of VT, and isolated
amounts to 12" are possible. WPC probabilities for 8" are also
high across portions of northern ME on day 2.

Further south and east into central New England and closer to the
coast, strong WAA which will enhance snowfall north, will also
produce a warm nose >0C to cause a mixture of snow/sleet/freezing
rain. Where sfc temps remain below freezing, a period of freezing
rain is likely, and WPC probabilities for 0.1" are above 40% for
parts of NH and ME.


...Texas to Tennessee...
Day 1-2...
Strong cold advection behind a surface front will produce a
situation with cold air chasing the moisture. As the column begins
to cool, soundings indicate a period of time where surface temps
fall below freezing while the warm nose/inversion persists. Then,
as the entire column cools below 0C, the DGZ begins to lose
saturation. This suggests the potential for an extended period of
light freezing rain/freezing drizzle, and WPC probabilities have
increased to 10-20% for 0.1" of accretion, with isolated higher
probabilities.

Weiss