551
FXUS63 KGRB 171155
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
655 AM CDT Sun May 17 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Strong to severe storms and locally heavy rainfall possible from
  this afternoon through Monday night. There is low confidence on
  the timing and coverage of severe storms.

- A return to drier and cooler weather is expected for the mid to
  late part of the next work week. Frost or freeze headlines may
  be needed over northern WI Tuesday night and Wednesday night.

&&

.DISCUSSION...
Issued at 247 AM CDT Sun May 17 2026

At the surface, a back door cold front had pushed through the Fox
Valley, generating NE winds gusting to 25 to 35 mph, and bringing
cooler air into the region. Showers and a few thunderstorms were
already pushing toward our southwest counties early this morning,
a bit ahead of schedule. These showers were forming in an area of
moisture convergence on the leading edge of a returning moist and
unstable air mass.

An 850 mb warm front and short-wave trough will lift north through
the forecast area today, resulting in an increase in showers and
embedded thunderstorms. Steeper mid-level lapse rates over
8 C/km pushing in from the south could aid in a few stronger
storms late in the afternoon, especially in central WI, where the
nose of 40 kt low-level jet will be focused. With persistent NE-E
winds, a cooler air mass in place and increasing clouds/showers,
have gone considerably cooler than NBM guidance for highs, which
should mainly be in the upper 50s to middle 60s.

The steeper lapse rates associated with an Elevated Mixed Layer
will overspread the forecast area this evening. There will be a
good chance of showers and storms over northern WI early, until
the 850 mb warm front shifts north of the region. Capping should
limit convection for a bit, but a weak surfave wave and short-wave
will impact the northwest part of the forecast area overnight,
leading to a significant uptick in showers and thunderstorms.
Would not be surprised to see a few severe storms in north central
WI as this occurs.

The degree of surface based instability on Monday is in question,
as significant cloud cover could limit daytime heating. In
addition, models suggest significant capping through much of the
day. As a result, have limited pops to the chance category for
most of the day. One thing to watch will be a short-wave that
brushes through the southeast part of the forecast area during the
middle of the day, as this could potentially kick off a round of
strong convection. The current forecast calls for highs in the
upper 70s to middle 80s, though it is possible these may be a bit
on the high side.

There should be an increase in convection Monday evening as
capping erodes and another surface low and associated cold front
approaches. However, the actual frontal passage looks to occur
overnight into Tuesday morning, when overall instability should
be waning. Even so, the degree of deep layer shear (40-60 kts)
and synoptic forcing support a risk of severe storms overnight
into early Tuesday morning. Dry weather looks to return in the
wake of the cold front Tuesday afternoon.

One last warm day is expected Tuesday with highs in the lower to
middle 70s in eastern WI, and 60s farther northwest. High
pressure brings dry weather and cooler temperatures for the mid to
late part of the week. The only main impact could be the need for
Frost/Freeze headlines over northern WI Tuesday night and
Wednesday night.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 650 AM CDT Sun May 17 2026

Scattered showers and isolated thunderstorms have already
developed over C/EC WI early this morning, and will continue
to increase from south to north through the day as a short-wave
and 850 mb warm front move through. Flight conditions will
deteriorate to MVFR/IFR as the steadier showers move in.
Have attempted to outline the better periods for thunderstorms
with tempo groups in the TAFs, but overall confidence in timing is
low. Gusty NE-E winds are expected through the day, but will
gradually turn SE-S as a warm front moves through tonight. Once
the warm front moves through, flight conditions will improve to
MVFR/VFR.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....Kieckbusch
AVIATION.......Kieckbusch

NWS GRB Office Area Forecast Discussion