Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 251942
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
242 PM CDT Tue Sep 25 2018

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Wednesday
Issued at 241 PM CDT Tue Sep 25 2018

The latest RAP analysis and satellite/radar imagery show a cold
front extending from around Marinette to Wautoma early this
afternoon. Partial clearing over northeast WI has allowed some
instability to develop up to the 600-1000 j/kg range. Meanwhile,
a shortwave trough over Minnesota, and associated surface low over
Iowa continue to push an area of showers and storms to the
northeast and across southwest Wisconsin. This area of
precipitation remains on track to pass over the area from mid-
afternoon through the evening. Given the instability over east-
central WI, concern remains for a few strong storms to develop,
mainly from about 21-00z time period. The main threat will likely
be strong, gusty winds from any storms.

Tonight...Cold front, and weak surface low, will be departing over
central Lake Michigan at the start of the period. Although the
potential for strong storms will likely have passed on to the
east by 00z, deformation showers and a few embedded storms are
expected to continue across the region through the evening before
exiting northeast WI shortly after midnight. Partial clearing is
possible overnight, mainly over central and east- central WI, but
additional cloud cover from a trailing shortwave will approach
late. With gusty northwest winds developing behind the front, lows
will fall back into the 40s at most locations.

Wednesday...Upper troughing will be moving across the region.
Progged soundings indicate the thermal profile will be supportive
of an ample cu field developing at locations that clear overnight.
With a breezy west wind and scattered to broken cloud cover, temps
will range from the mid 50s north to low 60s south.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 241 PM CDT Tue Sep 25 2018

Another cold front is forecast to arrive late Wednesday night
into Thursday, along with the next chance of showers. Models show
little instability with this system, so will hold off on any
mention of thunder at this time. Drier weather is anticipated
Thursday night, but a short-wave trof and secondary cold front
could bring scattered showers on Friday.

Canadian high pressure is expected to be the dominant weather
feature over the weekend, and the 00z ECMWF would suggest that
the high would keep mainly dry conditions in place through Sunday
morning. The GFS is dry Friday night into Saturday, but quickly
shifts the Canadian high east on Saturday night, allowing showers
to develop due to increasing WAA/isentropic lift.

An unseasonably cool air mass will arrive Friday night, with
850 mb temperatures dropping to -5 to -6 C over north central WI
by Saturday morning. Will need to keep an eye on Friday night and
Saturday night, as forecast lows are cold enough to be concerned
about frost potential. However, sky cover and wind will play a
significant role in whether or not frost forms, and it is too far
out to ascertain those details.

A significant isentropic lift/overrunning event is possible
across the forecast area from Sunday night into Monday night.
There may be enough elevated instability to support thunderstorm
development, especially over the south half of the forecast area.
This is shaping up as a potentially very wet period.

Temperatures should be near normal on Thursday, drop below normal
over the weekend, then recover to near normal early in the work
week.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1123 AM CDT Tue Sep 25 2018

Scattered to overcast mvfr/ifr cigs continue along a cold front,
currently positioned from near Marinette to Wautoma late this
morning. Area of showers and storms over Minnesota and Iowa will
move across the taf sites this afternoon and evening. The best
chance for thunderstorms will be at the eastern WI TAF sites late
this afternoon and early this evening. Winds will shift from the
south, to the west then northwest at the front passes.

Cigs will likely stay in the low end mvfr range or high end ifr
range tonight behind the front. Some clearing is possible over
central and northeast WI late, though confidence is relatively
low. Low clouds should evolve into a cu field by late morning on
Wednesday.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....MPC
LONG TERM......Kieckbusch
AVIATION.......MPC



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