Forecast Discussion for GRB NWS Office
FXUS63 KGRB 080402

Area Forecast Discussion
National Weather Service Green Bay WI
1102 PM CDT Tue Apr 7 2020

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Wednesday
Issued at 419 PM CDT Tue Apr 7 2020

Thunderstorm trends for the rest of the afternoon, fog trends,
and precipitation chances with a frontal system on Wednesday, are
the main forecast concerns.

A surface low was moving through central WI this afternoon. A warm
front briefly lifted into the southern Fox Valley earlier, but has
since slipped just south of the forecast area. Isolated strong to
severe elevated thunderstorms with hail have developed well north
of the surface warm front, in an environment characterized by
strong deep layer shear (around 60 kts), weak elevated CAPE but
impressive mid-level lapse rates (8-8.5 C/KM), and low wet-bulb
zero heights around 7500 feet. A isolated severe hail threat will
continue through about 6 pm in the Fox Valley, bayshore and
lakeshore areas.

Marine fog was developing rapidly this afternoon, as low-level
moisture advected over the cold waters of Lake Michigan and Green
Bay. Have issued a Marine Dense Fog Advisory for the entire
nearshore area through late this evening. Light offshore winds
should help push the thicker fog over the open waters by that

The showers will exit before sunset, with clearing expected from
west to east this evening. This clearing, combined with light
winds and lingering low-level moisture, will lead to fog
development overnight, especially in north central and central WI.
Lows will be in the 30s to lower 40s.

A strong upper level disturbance and associated cold front will
bring showers back to the region on Wednesday, especially over
the southern half in the afternoon. Gusty west to northwest winds
will develop in the afternoon, in the wake of the cold frontal
passage. Highs will be in the 50s and lower 60s.

.LONG TERM...Wednesday Night Through Tuesday
Issued at 419 PM CDT Tue Apr 7 2020

Wednesday night and Thursday...cold front will be exiting the
area late Wednesday afternoon, taking the widespread precip with
it. Lingering showers will be possible, mainly across north
central WI through the night, as the shortwave tracks across the
area. Best chances for clearing will be the farther south you go
as the better moisture remains closer to Lake Superior. CAA will
kick in behind the front, dropping 850mb into the -6 to -10C
range. This cold air aloft, combined with cyclonic flow and a
shortwave dropping across the area, and some daytime heating,
expect some rain or snow showers to develop. Most widespread
activity looks to be over northern WI, where a little moisture
boost from Lake Superior will be present. Steep low level lapse
rates could lead to some graupel in the stronger showers. Winds
will be rather gusty through the day as well, with wind profiles
showing 30-35kt winds mixing down, highest during peak mixing
during the day. It will be noticeably cooler on Thursday as well,
with highs in the 30s across the north to the lower 40s in the
Fox Valley. Any lingering shower activity will end by early to mid
evening as loss of daytime heating and the shortwave departs.
Winds will also diminish through the evening hours.

Friday...high pressure settles over the area, bringing dry
conditions. Expected some sunshine and lighter winds. Temps will
moderate back into the 40s for most locations away from the
immediate shores of Lake Michigan.

Rest of the extended...chances for precip return this weekend as
return flow behind the departing high pressure system tries to
makes it way into the Great Lakes and another weak front pushes
into the area. Models still on different pages on how things will
play out as the upper pattern shifts to westerly then
southwesterly. Models showing a low pressure system tracking into
the Great Lakes Sunday into Monday, bringing a better chance for
widespread precip. This system could bring significant weather to
the region, including wind, winter and possibly some flooding
(including lakeshore) impacts. Phasing will be an issue with
these system, so models will likely struggle with the details.
Stay tuned!.

.AVIATION...for 06Z TAF Issuance
Issued at 1100 PM CDT Tue Apr 7 2020

Clearing skies can be expected across the area for much of this
TAF period. The exception will be where vertical visibility drops
below minimums in the densest fog. The clearing skies has led to
fog development across the area, in some cases it has dropped
below airport minimums.The fog is expected to mix out by mid
morning Wednesday. Some light showers and possibly MVFR conditions
may develop Wednesday afternoon as another system brushes by to
the south. The best chance would at ATW and MTW, but can`t be
completely ruled out at CWA/AUW.




SHORT TERM.....Kieckbusch
LONG TERM......Bersch

NWS GRB Office Area Forecast Discussion