Area Forecast Discussion
Issued by NWS Green Bay, WI

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263
FXUS63 KGRB 100437
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1037 PM CST Fri Dec 9 2016

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Saturday
Issued at 356 PM CST Fri Dec 9 2016

Overall quiet weather tonight into Saturday as a ridge of high
pressure drifts over the region. A weekend storm system has been
trending with a later arrival time for Saturday but still left a
small chc pop for a small part of central Wisconsin late Saturday
afternoon. So Primary forecast challenge centers on cloud trends
and temperature forecast due to the clouds tonight.

Visible satellite imagery showed a persistent area of lower level
clouds over the northwest third of the state and gradually spreading
eastward. Further west, a weak system in the near zonal flow over
the Northern Plains will be spreading a period of low end vfr
clouds into at least the south half of the state overnight.
meanwhile over eastern Wisconsin, convective looking clouds with
weak returns were developing with 1.5 km lapse rates near 9.5. As
a result, these cloud trends support more clouds over northern
wisconsin and the diurnal nature of the clouds over the east may
dissipate by 6 pm. So low temperatures will be challenging to pin
down tonight due to the variable clouds. Did not deviate much
from the going forecast. Cloud trends will continue to impact
Saturday max temperatures, depending on rate of arrival of thicker
system clouds for the weekend.

.LONG TERM...Saturday Night Through Friday
Issued at 356 PM CST Fri Dec 9 2016

Focus in the long term revolves around the system moving through
the area Saturday night into Monday.

Models are coming into some sort of agreement on the track and
timing of this system. The next 24 hours will be interesting as
this feature finally pushing into the NW Coast and can be sampled
better. Hopefully models dont change again once this system comes
ashore.

Current thinking would have a prolonged period of snow starting
Saturday night. Strong WAA ahead of the main low will set up over
the southern part of the state and help develop a nice swath of
snow Saturday night. After that, the better synoptic forcing will
move in on Sunday as a complex jet couplet and short wave shift
through the Great Lakes region.

TIMING...

Models moving towards a later start time for the onset of snow,
with snow likely not making its way into central Wisconsin until
after 00Z Saturday. As WAA get cranking in southeast Wisconsin
Saturday night, snow should overspread the rest of NE through the
night. Some question on how much snow will fall through the
overnight period as models continue to disagree on the strength of
the 500mb shortwave that will be swinging through the mean zonal
flow aloft Saturday night. In general though, at least light snow
should falling across most of the CWA by Sunday morning. Snow will
continue through Sunday and should pick up in
intensity...especially in central and east central Wisconsin which
will be closer to the better low level dynamics as the low tracks
through northern Illinois into lower Michigan...and then taper off
from west to east Sunday night. There is continued uncertainty
about when the snow will actually end in eastern Wisconsin as some
of the models linger light QPF through 12Z Monday. Higher Res
models show a sfc inverted trough hanging back over the northern
lakeshore through far NE WI overnight Sunday into Monday
morning...and some lingering QPF through the time period.

Impacts...

The main impact for this system will be the snow. A lot of impacts
from this event will be determined by small scale
features/banding which should be resolved better as we get closer
to the event.

Winds should not be much of a concern, though minor
blowing and drifting is possible Sunday morning with higher snow
to liquid rations. Snow should start close to 20:1 before falling
back to 15:1 or so Sunday later morning as the low levels warm
slightly...especially near Lake Michigan.

Travel will be an obvious concern. Low visibilities in heavier
burst of snow are expected. Heavy snow is certainly possible,
especially in central and eastern Wisconsin where strong lifting
through a deep snow growth region will occur Sunday morning.

Lake enhancement is also a concern Sunday afternoon. Lake
Michigan waters are still in the 40s and 850mb temps are cold.
Delta Ts will be over 15C and 850 wind should turn from SW to SE/E
Sunday afternoon. Favorable conditions exists for a time period of
heavy snow Sunday afternoon somewhere in eastern Wisconsin. There
is obvious disagreement among models this far out about the
extent of this mesoscale feature, but it will bear watching.

Given the potential for over 6 inches of snow exists and the high
impact it may cause to travel later this weekend, decided to issue
a winter storm watch for south of highway 29. Also included Door
County in the WSW due to the potential for lake effect enhancement
to push snow amounts to the 6 inch range. Further north, a couple
inches of snow is looking likely but the confidence of over 6
inches of snow accumulating is low at this time.

The rest of the forecast...

Models disagree on the timing of a few weak pieces of energy for
next week, though at this time none of the systems look of high
impact. The bigger issue next week will be the cold temperatures
that filter into the area behind an arctic front. Exactly how cold
it will get is still uncertain, however model guidance is trending
colder with temperatures for the second half of next week. Below
zero low temperatures are likely across at least central and
northern Wisconsin Tuesday night through Thursday night as arctic
high pressure settles in over the region. Well below zero wind
chills can be expected across Northeast Wisconsin at times through
that period.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1036 PM CST Fri Dec 9 2016

There could be some areas of MVFR ceilings and snow
flurries in central and northcentral Wisconsin overnight, but
otherwise VFR conditions are expected through Saturday evening.

A winter storm moving from the southern Plains towards Lake
Michigan will bring snow and IFR conditions to much of the area
late Saturday night and Sunday. Snow accumulations of six inches
or more are possible roughly south of a Wisconsin Rapids to
Washington Island line.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Saturday evening through Sunday evening
FOR WIZ037>040-048>050.

Winter Storm Watch from Saturday evening through Sunday evening
FOR WIZ035-036-045.

Winter Storm Watch from late Saturday night through late Sunday
night FOR WIZ022.
&&

$$

SHORT TERM.....TDH
LONG TERM......Allen
AVIATION.......RDM



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