078
FXUS63 KGRB 301158
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
658 AM CDT Tue Jun 30 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- The dangerous heat peaks today, but the high heat and humidity
  will continue through late this week, resulting in continued
  heat-related impacts. An Extreme Heat Warning is in effect
  through 7 PM today (Tuesday) for central, east-central, and far
  northeast Wisconsin. A Heat Advisory is in effect for north-
  central Wisconsin for the same time period.

- Periodic chances for thunderstorms expected at times this week.
  Some of these storms could become strong or severe. Main
  hazards would be damaging winds, large hail, and heavy rain.

- Gusty onshore southerly winds will result in high waves and
  dangerous currents on Lake Michigan beaches through late this
  evening.

&&

.DISCUSSION...
Issued at 300 AM CDT Tue Jun 30 2026

Precipitation/Thunder Chances...An upper-level ridge axis was
focused over the western Great Lakes early this morning, while a
ridge rider (shortwave) was moving over MN. As a result, severe
weather was ongoing in northeast MN. Meanwhile, over our forecast
area, satellite and radar imagery showed areas of low clouds with an
area of light rain and a few rumbles of thunder just to the west of
the forecast area. Given the forecast area is under a large cap, any
of the storms in northeast MN are not anticipated to impact the
forecast area. However, the area of rain and rumbles of thunder will
move into portions of central and north-central WI early this
morning, but are anticipated to dissipate as they move eastward.
Otherwise, much of the day is forecast to remain dry until the
afternoon when a low-level southwesterly jet increases and meets up
with a surface boundary. With a very moist atmosphere (dew points in
the low 70s), deep-layer shear of 30-40 kts, and more than enough
instability, there will be potential for severe thunderstorms to
develop. The latest CAM guidance would suggest convective initiation
beginning in the late afternoon over the forecast area and
continuing through the late evening. Large hail, damaging winds, and
heavy rainfall will be the main threats if these severe storms
develop. There is uncertainty in the coverage as some models suggest
portions of the area may be capped, limiting storm potential.

The boundary associated with today`s convection is progged to sag
south over the state tonight and gradually north into Wednesday. As
destabilization occurs during the afternoon and evening, this
boundary will be the trigger for additional thunderstorms to develop
and ride along the instability gradient. However, there is
uncertainty where this boundary will set up. Regardless, there will
still be ample heat and instability to work with to produce some
strong/severe storms during Wednesday afternoon into the late
evening.

Models indicate additional convection may develop on Thursday as the
upper-level ridge breaks down; however this will depend greatly on
the evolution of convection on Wednesday. A greater chance for
showers and thunderstorms appears to be on Friday with the passage
of a mid-level shortwave, but it is too early to determine the
severe potential. There are signs of an additional shortwave
sometime over the 4th of July holiday weekend, but there are timing
and placement differences.

Dangerous Heat...Temperatures early this morning were still in the
low 70s to low 80s across the region with dew points in the 70s. As
a result, heat index values were in the low 80s to low 90s. With the
abnormally warm airmass remaining in place today, continued to keep
the heat headlines as is with the same end time of 7 pm today.
However, given temperatures are progged to be warmer in far
northeast WI, decided to upgrade Menominee, Oconto, Marinette, and
Florence counties into the Extreme Heat Warning. High temperatures
today are expected to climb into the upper 80s to mid 90s across the
region with dew points in the low to mid 70s, resulting in afternoon
peak heat indices ranging from 95 to 110 degrees. Of course,
depending on when and if the convection begins, some locations may
not see these peak values.

High temperatures and dew points on Wednesday will not be as high as
today, keeping heat index values right around 100 mainly for
locations in east-central WI. An additional Heat Advisory may be
needed. With the upper-level ridge breaking down a bit midweek,
temperatures and dew points will again drop slightly, with heat
index values topping out in the 80s or low 90s for the remainder of
the forecast period, including during the 4th of July holiday
weekend. Therefore, aside from Wednesday, the need for additional
heat headlines looks to come to an end through at least the end of
the weekend.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 658 AM CDT Tue Jun 30 2026

A mix of MVFR and VFR cigs prevailed across the region this
morning with the MVFR cigs mainly focused across the central and
north-central WI areas. A few showers have fallen at the RHI TAF
site early this morning as well. Upstream, showers and a few
storms were observed in west-central WI. Confidence is high enough
to include -SHRA at AUW/CWA/RHI in a TEMPO group for this morning,
but not high enough to include the mention of thunder given trends
showing the shower activity dissipating as it moves
east/northeastward.

VFR conditions are anticipated for the remainder of the day and
overnight. However, there are some high-resolution models
indicating isolated to scattered thunderstorms developing in
central or north-central WI this afternoon and evening, but the
atmosphere will be fairly capped. Therefore, confidence is still
not high enough to include a mention in this TAF issuance.

Southwest winds will increase today, with gusts up to 20 kts.
Winds subside late this evening.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for WIZ005-010-011-018-
019.

Extreme Heat Warning until 7 PM CDT this evening for WIZ012-013-
020>022-030-031-035>040-045-048>050-073-074.

Beach Hazards Statement through late tonight for WIZ022-040-050.
&&

$$

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

NWS GRB Office Area Forecast Discussion