Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Sun Jan 27 2019

Valid 12Z Sun Jan 27 2019 - 12Z Wed Jan 30 2019


...Northern Plains to Upper MS Valley and Great
Lakes/PA/NY/western New England...
...Day 1 through Day 3...

A clipper low pressure system will dive southeastward across the
northern plains/Midwest late Sun into Mon along an existing
frontal zone.  The mid level warm/moist advection produces lift in
the frontal zone, in conjunction with upper divergence/lower
convergence maxima, supporting a swath of snow across much of the
northern plains, Upper Midwest, and upper Great Lakes.  The
strongest signal in the guidance for heavy snow is from southern
WI east into lower Michigan, with snowfall totals in this area
likely to be 8 to 12 inches in a mesoscale band just north of the
surface low track. Lake enhancement off lake Huron north of the
low leads to higher totals in northeast lower MI Mon.

On Mon night-Tue, the surface low will move east-northeast out of
MI into Ontario/Quebec, with the synoptic snow coming to an end. 
On Tuesday, the trailing cold front moves east from Ohio across PA
and NY.  A wave of low pressure develops along the front in
central Pa/NY late Tue to Tue evening, which allows a post-frontal
period of snow and potential for several inches of snow.
Downstream from the developing wave, low-mid level warm/moisture
advection produces a period of snow in interior New York into
western New England.
The low level warming produces a precip type transition zone in
southeast New York and southern New England, with snow likely to
change to rain.

With post-frontal cold advection across the upper Lakes Tue, lake
effect snow develops downstream from Lake Superior and lake MI,
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 with extremely cold air aloft as 850 mb temps drop below
-30c favor ascent in the moist air mass near the lakes. 

...Ranges of central to southern MT/WY and CO...
...Days 1-2...

Upper divergence maxima cross MT near a jet streak and into WY and
also the Black Hills of adjacent SD, targeting the Judith
range/Bighorn Mountains/Black Hills on day 1 with several inches
of snow.  The jet is focused near the Bighorns centered around 00z
and then drops south to the CO border by 12z Mon, bringing light
snow into the mountains of north central CO. On day 2 (Mon),
lighter amounts are expected focused on the front range of CO into
the Sangre de Cristo mountains of southeast CO/northeast NM as the
upper jet builds south across these areas.  Amounts and coverage
taper Monday afternoon/evening as drier air gets advected into the
region, causing snow to end.


...Lower MS Valley/TN Valley/Southeast/Southern to Central
Appalachians...
...Days 2/3...

A cold front will steadily progress across the region late Mon
into Tue with largely ana frontal precipitation suggested by model
guidance. Precipitation is expected to begin as rain for many
areas but should quickly change to sleet and then snow as the cold
air mass rushes in behind the front. A swath of accumulating snow
is expected from portions of the lower MS Valley northeast to the
TN Valley, interior Southeast, and southern to central
Appalachians.

Few models tonight depict a weak wave of low pressure along the
front, so there is a slightly lower probability of enhanced
snowfall amounts in a mesoscale band in the post-frontal area of
snow in the southeast to TN.  A general 1-3 inches is shown in the
respective SREF, GEFS and ECMWF ensemble means, as well as the
operational ECMWF, whose progressive 700 mb wave leads to lower
snow totals.  A little more confidence exists for higher amounts
in the southern Appalachians, with amounts tapering once you get
east of the mountains. band would set up. 

...Northeast...
...Day 1...

An energetic shortwave causing snow across western NY and PA is
forecast to continue moving northeast across the lower Great Lakes
and Northeast today.  The system will produce relatively light
snowfall amounts for most areas.  Lake enhancement and orographic
lift combine with low level warm advection to increase snowfall
totals across the Tug Hill plateau of New York downwind of lake
Ontario. Tonight, the moist layer and ascent moves downstream out
of New Yo into northern New England, where 3-5 inches of snow is
expected in portions of the mountains of northern VT/northern
NH/northern ME. 

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

Petersen