006
FXUS63 KGRB 011223
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
723 AM CDT Wed Jul 1 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- A Heat Advisory is in effect from 11 AM to 8 PM today for east-
  central and far northeast Wisconsin, including the lakeshore
  counties. Additional heat headlines may be needed Thursday.

- Isolated to scattered storms this morning. A few may be strong
  or severe. A greater chance for severe weather this afternoon
  into tonight with potential for damaging straight-line wind
  gusts, large hail, and isolated tornadoes. Isolated flash flood
  instances are also possible.

- Additional chances for thunderstorms through the holiday
  weekend.

&&

.DISCUSSION...
Issued at 258 AM CDT Wed Jul 1 2026

Precipitation/Thunder Chances...Ahead of a nearly stationary
boundary focused over northwest WI, very isolated showers and storms
were observed on satellite and radar imagery early this morning over
the southeast corner of the forecast area. Upstream, a mid-level
shortwave over SD/NE was producing scattered convection, some
severe, that extended into western IA. As the shortwave lifts
towards the region throughout the morning, it will interact with the
boundary and bring the area`s first round of convection. CAMs have
been bouncing around with where and when this convection is going to
develop, but ultimately, there is some agreement for it to
reach/develop in central WI around 7-8 AM in a more isolated
fashion. Uncertainty then increases with the east/northeastward
progression of these storms throughout the remainder of the morning.
There will be a risk of strong to severe storms given steep mid-
level lapse rates, sufficient instability of 1000-2000 J/kg, and
deep layer shear of 30-40 kts, with damaging winds and large hail
being the main threats. There should be a small lull in the precip
around midday before the next round of convection moves into the
area during the afternoon and late evening from the west associated
with the mid-level shortwave and increasing low-level jet (30-40
kts). Given the hot, moist airmass still in place, daytime
instability will jump to 2000-3000 J/kg with deep-layer shear still
around 35-45 kts. Similar to the previous forecast, CAMs are still
favoring bowing structures and potential supercells with this second
round, making for a greater severe threat during this time (compared
to the morning) with the potential for damaging straight-line wind
gusts up to 70+, large to very large hail, and a few embedded
tornadoes.

In addition to the multiple rounds of storms today, there will also
be a flooding threat given PWATs around 2 inches during times of
convection and the potential for the storms to move over the same
locations. While there is still an overall threat for flash
flooding, the threat in the afternoon/evening may be slightly lower
if the storms favor the bowing structure, which typically leads to
faster storm motions. But, if several rounds of bowing structures
develop through late tonight, like some CAMs are indicating, the
threat will be higher.

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. However, the
timing for the shortwave on Friday is uncertain (along with the
subsequent shortwaves), leading to uncertainty in severe potential.
But, given the warm, unstable airmass still over the Midwest, cannot
rule out the potential for strong or severe storms for Friday or
throughout the holiday weekend.

Heat/Temperatures...With afternoon heat index values peaking around
100-105 degrees today in east-central and far northeast WI, decided
to issue a Heat Advisory for these areas from 11 AM to 8 PM. It is
very possible some locations may not reach the 100 degree heat index
criteria in the advisory given the potential for the thunderstorms
throughout the day, but felt confident enough there will be window
for heat indices to rise. 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 12Z TAF Issuance
Issued at 722 AM CDT Wed Jul 1 2026

Periods of thunderstorms are expected throughout this TAF period.
The first period being this morning with ongoing thunderstorms,
some severe, moving northeast and expecting to impact the east-
central WI TAF sites between 12-13z Wed. Meanwhile, showers and
non-severe thunderstorms will lift over the central WI TAF sites
and potentially RHI. A lull in the showers and storms is
anticipated for the east-central WI TAF sites late this morning,
and for the central WI TAF sites throughout most of the afternoon.
The shower and thunderstorm activity will then ramp up again late
this afternoon and evening with strong to severe thunderstorms
anticipated capable of producing damaging wind gusts up to 70 mph,
large hail, and isolated tornadoes. Amendments will be needed as
this severe weather day progresses given the volatile pattern.

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 from 11 AM this morning to 8 PM CDT this evening
for WIZ020>022-031-037>040-045-048>050-073-074.
&&

$$

DISCUSSION.....Kruk
AVIATION.......Kruk

NWS GRB Office Area Forecast Discussion