559
FXUS63 KGRB 012335
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
635 PM CDT Wed Jul 1 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- A Heat Advisory is in effect until 8 PM today for east-central
  and far northeast Wisconsin, including the lakeshore counties.
  Additional heat headlines may be needed Thursday.

- Chance for severe weather late this afternoon into tonight with
  potential for damaging straight-line wind gusts, large hail, and
  isolated tornadoes. Isolated flash flooding is also possible.

- Additional chances for thunderstorms through the holiday
  weekend.

&&

.DISCUSSION...
Issued at 249 PM CDT Wed Jul 1 2026

Morning and early convection has largely shifted to the east as of
2PM. Expect a lull in thunderstorm activity across the forecast area
over the next 4-6 hours before the next round of thunderstorms
begins to develop and move into central and east-central WI. This
round of thunderstorms is expected to be focused along a nearly
stationary boundary that that extended from near Green Bay back
toward La Crosse and then toward central IA. A mid-level short-wave
is also evident on water-vapor imagery over western IA and southern
MN. As this short-waves moves east it will be overspreading a highly
unstable environment with dew points in the low to middle 70s and
temperatures quickly warming through the 80s. At 500mb the region is
located under the right rear quad of an 80kt jet streak resulting in
0-6 shear of 40-50kts. This creates a favorable parameter space for
strong to severe thunderstorms capable of producing damaging wind
gusts, large hail, and possible tornados. The tornado threat is
conditional on any storms remain discreet for a period this evening.
As storms grow upscale this may transition a damaging wind event
with 60-80mph wind gusts possible. The window for these higher end
severe hazards is 7PM to midnight. 18Z HRRR also shows convection
reinvigorate overnight as a LLJ noises into central and east-central
WI. This round of storms is very conditional on where this evenings
convection shifts the stationary boundary. If storms do develop
there would be a marginal threat for severe winds and hail.

In addition to the multiple rounds of storms this evening and
overnight, there will is a flooding threat given deep warm cloud
depths and PWATs around 2 inches during times of convection. The
flash flood threat is also conditional on training storms as storms
are expected to be moving quickly to the east, especially if they
develop into a bowing complex.

Models indicate additional convection may develop on Thursday,
however this will depend greatly on the evolution of today/tonight`s
convection and where outflow boundaries reside. Latest guidance
would suggest convection developing somewhere from central WI to
southern WI during Thursday afternoon, likely along the instability
gradient. If the strong to severe storms end up in our forecast
area, damaging winds would be the main threat.

The next chance for widespread showers and storms appears to be on
Friday with the passage of the first of multiple mid-level
shortwaves throughout the 4th of July holiday weekend. Timing and
intensity of storms related to this short-wave are highly uncertain.
However, one signal that is persistent for Friday in a round of heavy
rainfall across central and east-central WI as the region remains
under a very humid air mass. Additional rounds of thunderstorms are
possible on Saturday and Sunday, however, however, given the
volatile nature of this air mass it predictability with any
details at this time is low. Beyond Sunday global ensembles and ML
models do trend toward a drier period through the first half of
next week.

Heat/Temperatures...Heat Advisory for central and east-central WI
remains in effect until 8 PM this evening as clearing skies are
quickly leading to rapid warming with temperatures in the low to
middle 80s and heat indices in the middle to upper 90s. A few spots
may near 100 degree heat indices again on Thursday, but will again
depend on thunderstorm activity. For Friday through the holiday
weekend, temperatures and dew points look to slightly decrease, but
will continue to monitor the need for additional heat headlines.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 635 PM CDT Wed Jul 1 2026

Active weather will be the main aviation concern for the early TAF
period through the overnight. Strong to possibly severe
thunderstorms are expected to develop to our west and make their
way into the region through the early evening. Included a
prevailing TSRA group for the best timing of the arrival of
storms, but would expect some rain behind the initial line of
storms. Strongest storms will be capable of producing damaging
wind gusts up to 70 mph, large hail, and isolated tornadoes.
Ending time of active weather remains uncertain, as recent
guidance does suggest the potential for continued rain or a second
round early Thursday morning, so kept this mention in the current
TAFs. Finally, another round of thunderstorms will be possible
Thursday PM, with the strongest storms expected south of a CWA to
GRB line. Amendments will be needed as this severe weather day
progresses given the volatile pattern.

Ahead of these stronger storms inbound from the west at the
beginning of the period, a couple weaker storms are possible for
ATW and MTW as suggested by recent satellite and radar trends
around the TAF issuance. Added a brief TEMPO and PROB 30 for
these areas.

As the showers and storms come to an end sometime late tonight or
towards the end of the TAF period, there are signs of fog
development across north-central WI with MVFR/IFR cigs.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT this evening for WIZ020>022-031-
037>040-045-048>050-073-074.

Flood Watch through Thursday morning for WIZ020-030-031-035>040-
045-048>050-074.
&&

$$

DISCUSSION.....GK
AVIATION.......Uhlmann

NWS GRB Office Area Forecast Discussion