389
FXUS63 KGRB 120411
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1111 PM CDT Sat Jul 11 2026

Updated aviation portion for 06Z TAF issuance

.UPDATE...
Issued at 852 PM CDT Sat Jul 11 2026

Warm evening across northeast WI with temps still in the 70s to
near 80. Main focus for the update is on northern MN and adjacent
Ontario/Manitoba to see if storms start to develop ahead of weak
shortwave on instability gradient. Modest LLJ 30-35kt is forecast
to poke into this area by late evening, so that may be what some
guidance keys on to develop storms. Based on 850-300mb thicknesses,
if they form they could impact northern WI 7am-10am on Sunday.
Gusty winds over 40 mph and brief heavy downpours would be the
primary impacts.

Then for later Sunday, saw enough evidence in guidance/CAMs to
place 15-20% pops into far northeast WI (Marinette County across
the bay into Door County). Prefer the models that show convective
initiation (CI) over central Upper Michigan near Lake Superior
lake breeze. Mean storm motion is NNW-SSE, but enough instability
(MLCAPEs 1500-2500 J/kg) and effective shear of 30-40 kts that
there is non-zero chance that strong to severe storm could form
following right-moving motion vector of NNE-SSW, which would bring
storms more into far northeast WI, instead of mainly staying
northeast of our area. Highly conditional threat for severe as many
other models are quiet.

00z models are just starting to roll in. We`ll see if they offer any
notable trends the convection chances on Sunday.

&&

.KEY MESSAGES...

- A few showers and storms are possible across the north Sunday
  afternoon and evening. If the storms develop, a few may become
  strong or severe capable of producing damaging wind gusts and
  hail.

- Building heat and humidity early next week will bring a period
  of heat-related impacts, with heat index values forecast to rise
  into the mid 90s to up to 105F on Monday and Tuesday.

&&

.DISCUSSION...
Issued at 314 PM CDT Sat Jul 11 2026

Precipitation Chances...A cu field developed over the area north and
west of the Fox Valley this afternoon, with a couple of showers
popping up at times. But with a lack in shear, the showers have been
struggling to sustain themselves enough to produce lightning. Given
these showers are diurnally driven, they will end soon after sunset
this evening. Dry conditions are then anticipated overnight through
much of Sunday. A few models are indicating showers and storms
developing along a Lake Superior lake breeze Sunday
afternoon/evening and driving south/southwest over northern WI. Not
exactly sold on this solution with little guidance supporting it.
However, if these storms develop, cannot rule out the potential for
a strong or severe storm with 2000-3000 J/kg of SBCAPE and 0-6km
shear around 35 kts present. If one of these stronger storms
develop, damaging wind gusts and hail will be the main hazards.

Dry conditions are then expected for Monday through at least
Wednesday with an upper-level ridge prevailing over the area.
Wednesday into Thursday, the pattern transitions to a northwest
flow, leaving the forecast area in the line of fire for any
shortwaves riding the ridge. This brings the next chances for
showers and storms to the area from Thursday through the end of next
week.

Temperatures/Upcoming Heat...Above normal temperatures are expected
through this forecast period, peaking on Monday and Tuesday when
925mb temps climb to 27C to 32C over the area. However, this high
heat for Monday and Tuesday is slightly different than a typical
heatwave pattern. Drier air coming from the Dakotas/MN circulating
on the northern part of the upper to mid-level ridge will keep heat
indices near or slightly above the air temperature across north-
central WI due to dew points in the low to mid 60s. Meanwhile, the
better corn-aided evapotranspiration will be focused across central
and east-central WI where dew points in the upper 60s to low 70s are
forecast; however, the hotter air aloft will still be focused across
the north. Regardless, Monday and Tuesday will see the greatest risk
for heat-related illnesses with air temperatures in the 90s, with
some spots approaching 100. This will translate to heat indices in
the mid 90s to up to 105, but determining exactly where the highest
heat indices will occur is still uncertain; therefore, decided not
to issue any heat advisories or warnings at this time, but they will
be needed. As the ridge breaks down to the northwest flow midweek,
temperatures will not be as hot as Monday and Tuesday, but will
still remain above normal in the 80s to low 90s.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1110 PM CDT Sat Jul 11 2026

Mostly clear overnight, but still keeping eye on northern MN to
see if cluster of storms form through 06z. If they do, signal
present in guidance that they could drop into far northern WI
(including RHI) 12z-15z Sunday. Since models keep steadily backing
off on developing these storms, continue to just include scattered
altocu/stratus in central and north-central TAF sites for now.
New wrinkle is the potenital for additional storms dropping in
behind these storms mid to late morning, but confidence remains
low given how models have been verifying thus far upstream.

Eventually do expect cumulus field to blossom over northern WI on
Sunday afternoon. Isolated showers or storms may develop (

NWS GRB Office Area Forecast Discussion