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

Valid 00Z Mon Jan 28 2019 - 00Z Thu Jan 31 2019


...Northern Plains to Upper MS Valley and Great Lakes...
...Day 1 through Day 3...

A fast moving area of low pressure will drop through the Northern
Plains into the Upper Midwest tonight into Monday night. The
favorable warm/moist advection within the DGZ will produce an axis
of snow with very efficient SLRs. The strongest signal for the
heaviest snow is across portions of southeast MN across
southern/central WI then into the lower UP of Michigan where
probabilities of 8" are greater than 50 percent. Easterly flow
north of the low will likely lead to enhanced snowfall
accumulations across eastern WI as well as off Lake Huron.

Strong post-frontal cold advection across the upper Lakes will
lead to lake effect snows off Lake Superior and Lake Michigan.
With the steep low-level lapse rates and extremely cold air aloft,
forcing for ascent will be more than sufficient for several
additional inches of snow in the favored snow belts near the
lakes.


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...
...Day 1...

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 Monday, bringing
light snow into the mountains of north central CO.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...

As a cold front marches across the region late Monday night into
Tuesday morning, strong cold air advection will begin to undercut
the lower levels across portions of LA/MS/AL/GA/TN/NC. Enhanced
upper level divergence aided by a mid-level vorticity maximum
pivoting through will provide an opportunity for higher QPF in the
post-frontal environment. There appears to be a narrow spatial and
temporal 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, far northern GA into central/eastern TN, western NC
and then into more of the central Appalachians. In the higher
terrain areas of eastern TN, far northern GA and western NC
accumulations up to 4" are possible.

Overall, model agreement improved in the last model cycle and the
WPC QPF used for the snow amounts was based primarily on a blend
of the GFS/ECMWF with some inclusion of the NAM in Day 2.


...Northeast...
...Days 2-3...

The strong low pressure system trekking across the lower Great
Lakes will lift north into southern Ontario by Tuesday. A
secondary area of low pressure is then expected to develop off the
coast of Maine. The favorable forcing for ascent combined with
sufficient moisture in the column supports accumulating snow
across portions of upstate NY into New England and interior Maine.
Accumulations of 8-12" will be possible from central VT/NH into
the terrain areas of ME.


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

Taylor