290
FXUS63 KGRB 291144
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
644 AM CDT Mon Jun 29 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- An Extreme Heat Warning is in effect from this afternoon through
  Tuesday for central Wisconsin and the Fox Valley. A Heat
  Advisory is in effect for the rest of northern and northeastern
  Wisconsin for the same time period.

- High heat and humidity will result in continued heat-related
  impacts for the middle to late part of the week.

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

- 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.

&&

.DISCUSSION...
Issued at 1257 AM CDT Mon Jun 29 2026

Severe Weather Potential This Morning...

Convection is having issues getting going across the region as a
warm front lifts north across the region. A recent KGRB RAOB
sounding indicates this activity is fighting several capping
inversions and dry air aloft, which will probably not be overcome
overnight. Therefore, isolated to scattered thunderstorms are
anticipated this morning instead of a huge MCS rolling through the
region. This dry air and elevated warm layers should act to limit
the severe weather potential this morning.

Storms tonight and Tuesday Night...

Ample moisture and heat will provide the instability for storm
development through the upcoming week. However, lingering warm
air aloft will limit surface based storms at times. Thus, the
placement of the upper ridge and associated `heat dome` over
eastern CONUS will remain the critical components to watch for
convective potential for our area. The first shortwave will cross
the upper Midwest tonight into Tuesday, but will largely initiate
in Minnesota. This system will then spread eastwards through the
evening, but capping over our area should help weaken storms as
they arrive. This will limit the best severe potential to our
west, with the Marginal Risk currently limited to portions of
central to north-central Wisconsin.

This scenario then repeats Tuesday evening as the next shortwave
arrives, but this time the ridge will be slightly more east,
possibly bringing the storms more into our area. That said, the
details of convection each day are muddy, largely depending on
when/where storms developed the previous day. That said, the upper
ridge is expected to continuing shifting eastwards through the
work week, bringing us into the line of fire of any shortwaves
along the upper ridge. Any storms will have the potential to be
strong to severe given the instability and moisture around. The
most likely hazard will be very heavy rainfall, especially if
storms repeat each day and soils become heavily saturated.

Dangerous Heat...

Active weather aside, the heat accompanying the upper ridge will
be a hazard itself. Dewpoints well into the 70s accompanied by
high temperatures into the 90s will bring heat indices well into
the upper 90s to mid-100s today and Tuesday. With the strong heat
signal over central Wisconsin and probabilistic guidance still
suggesting around 60-80% chance of exceeding 104 heat indices,
will keep the current headlines of an Extreme Heat Warning for
all of central to east-central Wisconsin. The most likely areas
to get the extreme heat will be central Wisconsin into the Fox
Valley, but also included the lakeshore areas for the inland
portions of the counties. There is also a signal on Tuesday that
southern Oconto and southern Marinette counties may reach Extreme
Heat Warning criteria Tuesday afternoon; however, will hold off on
any upgrades for these areas since overnight convection or clouds
could impact temperatures on Tuesday.

Those living along the shoreline of Lake Michigan and north of
Sturgeon Bay should see significantly lower impacts than those
further inland. North of the Extreme Heat Warning, heat indices
will remain just below advisory criteria during the afternoon,
lingering in the 95-99 range. However, overnight temperatures are
unlikely to drop below 75 in many areas, which typically means
little relief from the heat for those without shelter and means of
cooling. Given that this makes for a long stretch of heat, and
also that the forecast keeps overnight temperatures up even as
high temperatures drop slightly by Wednesday, decided to issue a
Heat Advisory for the rest of the region.

As we get into the mid to late week, heat indices should see a
minor drop, but temperatures are currently forecast to remain in
the upper 80s to lower 90s each day. The main complicating factor
for headlines during this period will be convective weather, as
any storms could provide some relief and lower the daytime high
in some areas. On the other hand, the forecast keeps heat indices
for most of the area above 70 through Friday, which would
compound heat fatigue effects for those without shelter as
mentioned above. Depending on how these convective trends and
clouds play out, an extension of heat headlines is not out of the
question.

4th of July Weekend...

Taking a peek into the 4th of July weekend, current trends keep
the dangerous heat across the region on Friday, then push the
warmest air to the south Saturday and Sunday as a semi-
stationary front remains across Wisconsin. This will come with a
downside, as there will be lingering chances for showers and
thunderstorms at times as the front becomes the focus for
convection as several shortwaves track along the zonal flow across
the western Great Lakes region. At this time the best window for
convection appears to be Friday afternoon and Friday night as well
as Saturday afternoon into Saturday night; however, given how far
out this forecast is this could easily move to a different periods
based on what happens earlier in the week. Although the risk for
severe weather is difficult to assess this far out, the warm,
moist airmass will provide instability, which means strong to
severe storms are certainly a possibility for the weekend.

Currently temperatures look to stay in the upper 80s to lower 90s
on Friday, then cool slightly into the 80s Saturday and Sunday.
Heat index values on Friday will still be well into the 90s, with
values possibly approaching 100, cooling into the lower 90s by
Saturday and Sunday. Despite the "cooler" temperatures, we will
be coming off a prolonged period of hot weather; therefore an
extension of warm temperatures through the weekend may have a
compounding effect.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 644 AM CDT Mon Jun 29 2026

Overnight rain and storms have left behind messy skies and areas
of low stratus over much of central and north-central Wisconsin.
Therefore expect a period of IFR ceilings for the beginning of the
TAF period for many areas, except the lakeshore which has offered
some clearing. As we get into what should be a fairly hot day,
expect additional clearing across the area with VFR conditions for
most of the region. Active weather for the day seems relatively
unlikely for most of the region. The only exception could be along
the northern Wisconsin border, but confidence was not high enough
to include any mention in the RHI TAF over this issuance.

Winds turn southwards today with gusts up to 15 to 20 knots during
the afternoon.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon today to 7 PM CDT Tuesday for WIZ005-
010>013-018>021-073-074.

Beach Hazards Statement through this evening for WIZ022-040-050.

Extreme Heat Warning from noon today to 7 PM CDT Tuesday for
WIZ022-030-031-035>040-045-048>050.
&&

$$

DISCUSSION.....Kurimski
AVIATION.......Uhlmann

NWS GRB Office Area Forecast Discussion