Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 AM EST Mon Jan 28 2019

Valid 12Z Mon Jan 28 2019 - 12Z Thu Jan 31 2019



...Upper MS Valley and Great Lakes...
...Day 1 through Day 3...

A circulation sfc and aloft will move out of the Upper Midwest and
into MI through tonight. The snow over northern IL and much of WI
will move downstream into lower MI as favorable warm/moist
advection within the DGZ will produce an axis of snow, followed by
snow continuing within the mid level deformation zone until the
circulation departs Monday night. 

The higher snows are likely where it lasts longer in northeast
lower MI, where there is also a likely lake enhancement off Lake
Huron.  In this area the probabilities of 8" of snow are greater
than 50 percent.  Snow moving east from MN into the UP of MI is
aided by 700 mb convergence, with forcing for ascent helping
produce several additional inches of snow in the favored snow
belts near Lake Superior today-tonight.

With post-frontal cold advection across the upper Lakes Tue and
the cold air mass remaining in place Wed, lake effect snow is
forecast downstream from Lake Superior and lake Michigan, with
higher amounts where longer cross-lake trajectories move onshore
into the UP of MI and northwest MI near Traverse Bay and southwest
MI near the wider part of Lake Michigan.  High lapse rates
resulting from extremely cold air aloft as 850 mb temps drop below
-30c favor ascent in the moist air mass near the lakes.  The high
lapse rates also favor high snow to liquid ratios so high amounts
are expected in areas with persistent snow showers.

On Wed, lake effect snow picks up in coverage and intensity
downstream from Lakes Erie and Ontario as the cold air ventures
across these lakes.  Heavy snow bands are likely as long cross
lake Erie trajectories move onshore into southwest New York, and
corresponding long cross Lake Ontario trajectories move onshore
into the Tug Hill and western Adirondacks.

...Central/Eastern New York across interior New England...
...Days 2-3...

The strong low pressure system trekking across the lower Great
Lakes will lift north into southern Ontario Tuesday. A secondary
area of low pressure is then expected to develop along the
trailing front in southeast New York Tue evening and then move
northeast across New England to New Brunswick by Wed evening. The
favorable forcing for ascent combined with sufficient moisture in
the column supports snow just west of the low track across
portions of upstate NY into interior New England.  Accumulations
of 6-12" will be possible from upstate New York across portions of
VT/interior NH and ME. The precip type transition zone is expected
along the low level circulation track, with snow changing to mixed
precip types and/or rain south of the low track in coastal areas
of NY and southern New England, continuing along the NH/Maine
coasts.  Considerable QPF differences among the models/ensembles
members lead to snow amount differences in interior central to
eastern/northern New York, VT, NH, and ME.

...Lower MS Valley/TN Valley/Southeast/Southern to Central
Appalachians/interior Mid Atlantic...
...Days 1-2...

As a cold front marches across the region late Monday night into
Tuesday morning, strong cold air advection will occur across
portions of MS/AL/GA/TN/NC. Enhanced upper level divergence
supports lift that is aided by a mid-level vorticity maximum
passing through.  This combination will provide an opportunity for
precip continuing in the post-frontal environment. There appears
to be a narrow window for accumulating snow before the mid levels
quickly dry out in the wake of the system.  Probabilities of
accumulations of 2" will primarily be found from central MS into
northern AL, northwest GA into central/eastern TN, western NC and
then into the central Appalachians. In the higher terrain areas of
eastern TN, far northern GA and western NC accumulations up to 4"
are possible. 

Lack of low pressure development on the front and the progressive
motion of the 700 mb wave limits the potential for higher snow
amounts.  East of the mountains from the Carolinas to VA/MD/PA,
the precip begins as rain with post-frontal cooling allowing a
transition to snow.  A longer duration as cooler temps provides
gradually increasing snow amounts heading towards northeast PA,
where several inches are possible.

...Ranges of CO/northeast NM...
...Day 1...

The NAM shows a well defined 700 mb convergence axis moving north
to south today down the foothills and front range of the CO
mountains into the Sangre de Cristo mountains of southeast
CO/northeast NM.  As the wave departs this evening, the 700 mb
convergence dissipates. Amounts and coverage taper this evening as
drier air gets advected into the region, causing snow to end.

The probability for significant icing is less than 10 percent
nationwide days 1-3.

Petersen