Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 110038

Area Forecast Discussion
National Weather Service Green Bay WI
638 PM CST Sat Dec 10 2016

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Sunday
Issued at 258 PM CST Sat Dec 10 2016

The main concerns during the short term are onset of snowfall and
snow totals.

At 20Z radar showed, and surface observations confirmed, a band
of snow from southern Minnesota into central Illinois. The snow
developed in warm advection ahead of an approaching surface system
and mid level short wave. Models all showed this area making its
way to the east and reaching the southwest part of the forecast
area at around 00Z Sunday. This is expected to spread across the
area overnight and the snow should intensify on Sunday as a low
pressure system and mid level trough get closer to Wisconsin.

The combination of snow ratios averaging around 20:1, combined
with the forecast QPF yields the highest snow totals in parts of
central and east central Wisconsin and near Lake Michigan. Event
total snowfall should be around 7 inches in southern Waushara,
southern Winnebago, southern Calumet, and Manitowoc. These totals
will be spread out over a long enough period of time to justify
going with an advisory rather than a warning. Some locations in
the north may end up with totals too low to justify an advisory,
but totals there often end up higher than forecast so have opted
to include them in an advisory as well.

.LONG TERM...Sunday Night Through Saturday
Issued at 258 PM CST Sat Dec 10 2016

Forecast challenges to address early in the extended on Sunday
night as upper-level forcing associated with primary shortwave
trough tracks east across the forecast area. Rather complex
surface pattern Sunday evening as broad elongated surface trough
transitions to a modestly deepening surface low pressure center
forecast to be centered over northern Lake Michigan by about 06Z
Monday. During this transition, high resolution models have also
continued to suggest over the last few model cycles, a rather
sharp surface trough axis and associated convergence zone
developing from roughly Door County northwest across Marinette
County by late afternoon Sunday and persisting perhaps until
about 03Z Monday before shifting off to the northeast. Higher
resolution guidance has also been persistent in maintaining locally
higher QPF along this convergence zone as well. In addition, will
likely see several hours of southeasterly onshore boundary layer
flow off Lake Michigan into the trough axis. Despite short fetch,
given Lake Michigan water temperatures quite warm for this time
of year (40s F) and 850 temperatures around -8C, lake enhancement
will likely be a player. Current thinking is the highest snowfall
rates will be likely be seen over the far northeast portion of
the forecast area early Sunday evening before the primary upper level
shortwave swings east of the area by 06Z Monday. Perhaps another
inch of snow between 00Z and 03Z Monday would be seem reasonable.
Further upstream, an additional complication to the Sunday evening
forecast revolve around recent model trends of the primary upper-
level shortwave energy being a bit stronger and a bit further
north. This has introduced a somewhat more robust dry slot signal
that may begin to punch east across central and eastcentral
Wisconsin shortly after 00Z Monday. There may be a small window of
opportunity to cutoff ice crystal growth aloft before the
precipitation ends leading to a brief period of freezing drizzle.
Given uncertainty as to whether mid-level drying can occur before
the precipitation ends, will not include in the grids for Sunday
evening at this time.

Then the fun really begins as a strong arctic cold front is
forecast to sweep across Wisconsin Monday night ushering in the
coldest air of the season. Better consensus among the models with
respect to bringing a narrow band of snow shower activity with
the arctic front between about 03Z and 06Z Tuesday. Given the
speed of the front and paltry moisture expect little snow
accumulation except across the far north. In its wake, the onset
of the first arctic surge this winter will commence with
temperatures falling well below normal. Temperatures Monday night
will start out near normal, then fall throughout the day on Tuesday
with temperatures by sunset near zero central and 5 to 10 above
zero eastern WI. Wind chills by late Tuesday are progged to be as
cold as the mid teens below zero.

Little change with the forecast for the latter part of the work
week as bitterly cold arctic air mass settles over the Great
Lakes region. Lowest wind chills still appear to be Wednesday
morning and especially Thursday morning where values will be
approaching advisory criteria. Max temperatures Wednesday/Thursday
expected to only reach the single digits above zero. This pattern
would also tend to favor a more westerly wind component across
the northern Wisconsin snow belt region, but any subtle shift to
the west-northwest would allow some lake effect snow showers into
Vilas county. Based on latest medium range guidance confidence low
in any significant LES for Vilas County during this period.

Arctic airmass should begin to retreat Friday as next shortwave
approaches the High Plains with WAA devloping late Friday.
Although some timing differences, medium range guidance suggest
another shot at perhaps some substantial snow for the Western
Great Lakes for next weekend.

.AVIATION...for 00Z TAF Issuance
Issued at 638 PM CST Sat Dec 10 2016

Ceilings and visibilities will fall from southwest to
northeast tonight as snow arrives ahead of a winter storm
approaching from the southern Plains. IFR conditions are expected
over all of the area by daybreak, and will continue through the
afternoon Sunday. Snow accumulations of several inches are
expected. Locally heavier snow could fall near Lake Michigan.

Winter Weather Advisory until midnight CST Sunday night FOR

Winter Weather Advisory until midnight CST Sunday night FOR

Winter Weather Advisory from 6 AM Sunday to midnight CST Sunday
night FOR WIZ005-010>013-018>021-073-074.

Winter Weather Advisory from midnight tonight to 6 AM CST Monday


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