Irma, Wisconsin Weather Forecast Discussion

908
FXUS63 KGRB 171912
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
212 PM CDT Wed Sep 17 2025
Forecast discussion for routine afternoon forecast issuance
.KEY MESSAGES...
- Isolated strong storms could produce gusty winds in excess of 40
  mph and small hail over northern Wisconsin late this afternoon
  and early this evening.
- Additional scattered showers and storms are possible at times
  Thursday through Monday. The risk of severe weather is low.
- Temperatures about 10 to 15 degrees above normal today, before
  returning to near normal from Friday into the weekend.
- Conditions hazardous to small craft will be possible on Lake
  Michigan south of Two Rivers on Friday night and Saturday
  morning. There will also be an increased risk of rip currents on
  Lake Michigan beaches along this section of shoreline.
&&
.DISCUSSION...
Issued at 212 PM CDT Wed Sep 17 2025
Short Term...Tonight and Thursday
Latest satellite analysis continues to show a weak cold front
boundary stretching Wisconsin into the Upper Peninsula, roughly
from Phillips in Price County to just south of the city of
Marquette. Surface parameters in the region meanwhile show
building surface based CAPE values to around 1500-2000 J/kg of
CAPE, indicating a favorable unstable conditions. As the front
interacts with this instability, scattered rain and thunderstorm
activity is anticipated in the afternoon, largely focused towards
northern Wisconsin. Indeed, as of this early afternoon a couple of
storms have already developed over the Upper Peninsula of
Michigan. As we get into the evening, a few showers or storms may
ride outflow southwards and expand active weather coverage
southwards, but are still largely expected to stay well north of
Highway 29, including the Fox Valley and Green Bay areas. Shear
remains virtually nonexistent, so any storms will likely be
fairly pulsy in nature and slow moving, making any impacts fairly
localized. Any thunderstorms will have the potential for heavy
rain, small hail, and brief but strong wind gusts up to 40 mph.
As the rain away from the cold front will be largely diurnally
driven, quieter conditions are expected to return rapidly
overnight, although a few showers could continue in the far north
along an area of better moisture convergence. The front will also
bring some drier air in, which should mitigate much of the fog
potential for northern Wisconsin. Areas further south could still
see some patchy fog, namely for central and east-central, but
dense fog is not expected.
Thursday will see warm conditions continue, but cooler air will
bring high temperatures down by around 5 degrees compared to
today, with highs ranging from the middle 70s to the lower 80s.
Scattered shower activity and a rumble of thunder will still be
possible, mainly along the stalled cold front in northern
Wisconsin and an area of weak convergence across central Wisconsin
brought in by a larger low pressure system to our west. Without
more widespread support however, coverage of active weather will
be somewhat limited.
Long Term...Thursday Night Through Wednesday
The synoptic blocking over CONUS will break down over the first
few days of the extended, allowing the upper trough to progress
across the region through the weekend. Ahead of this, isolated
shower activity is likely to continue across portions of central
and northern Wisconsin Friday and Saturday before the more
widespread forcing arrives. A round of more widespread rain and
storms is then likely for Saturday night into Sunday. Return flow
accompanying the trough will bring in another surge of warm air,
holding high temperatures in the 70s into early next week.
Active weather will accompany the warmer air, with storms and rain
likely again Sunday evening and early next week when a larger low
pressure system is expected to develop over the Central Plains.
&&
.AVIATION...for 18Z TAF Issuance
Issued at 1247 PM CDT Wed Sep 17 2025
Scattered shower and storm activity is still anticipated for the
afternoon period, focused largely across northern Wisconsin.
Coverage remains relatively small, so continued to carry mention
only in the RHI TAF, while areas further south are not expected to
be impacted at this time. Any storms could produce brief gusty
winds, brief MVFR ceilings, and small hail.
Behind the rain, another round of fog is possible across the area.
Fog in the north will likely be relegated to those that receive
rain as winds turning to the north will bring in drier air that
may inhibit fog in these areas. This dry air may not make it to
portions of central and east-central in time to fully inhibit fog,
so kept a small mention in the current TAF. Future updates may
still remove these if the moisture appears to be more absent
tonight.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION.....Uhlmann/ - Default -
AVIATION.......Uhlmann

NWS GRB Office Area Forecast Discussion