682
FXUS63 KGRB 252318
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
618 PM CDT Sat Jul 25 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Just slight chances for thunderstorms over far ne WI late this
  afternoon and evening. A strong storm is possible.

- 20 to 30 percent chances for thunderstorms Sunday afternoon
  over far eastern WI, but higher confidence for thunderstorms
  is for late Sunday night and Monday. Severe thunderstorms
  will be possible during this time.

- Heat and humidity will build across the region for Sunday and
  Monday. High temperatures in the upper 80s to low 90s and heat
  indices reaching the middle to upper 90s are forecast over
  central Wisconsin on Sunday, then slightly cooler for Monday.

- A cold front passing through the region on Monday will bring a
  shift to northerly winds and relief from the high humidity for
  Tuesday through mid-week, with lower thunderstorm chances
  until late in the work week.

&&

.DISCUSSION...
Issued at 216 PM CDT Sat Jul 25 2026

Broken to overcast stratocumulus clouds will continue for much
of the afternoon as moist air returns on swly winds. This will
keep high temps in the middle 70s to lower 80s depending on
location. Otherwise will be watching a very weak sfc trough and
wind shift that will affect Upper MI and far nern WI late this
afternoon and evening. It will be weakly supported aloft by a
vorticity maximum moving across the region. MLCAPE of 1000 J/KG
and effective shear of 40 kt is expected with some sunshine
possible late this afternoon. However, will only forecast 20
percent chances for showers and storms given the weak convergence.

With relative humidity remaining high today and weak sfc
ridging expected late tnt, areas of fog and low stratus will be
possible early Sunday morning especially over nern WI.

For Sunday a weak wave of low pressure will move ewd across nrn
Ontario and be supported aloft by a shortwave trough tracking
across the same region and toward srn Ontario late in the day.
Thus the better warm advection and PVA will be to the north with
weaker warm, moist advection shifting ewd across ne and east
central WI. Isolated to scattered storms (20-30 percent) will be
possible on the leading edge of the mid level cap, mostly over
far ern WI. The warm advection and much more sunshine than today
will boost high temps into the upper 80s to lower 90s, with
dewpoints into the lower 70s over central WI. Heat indices
reaching 100F are looking marginal at this time for central WI,
so no heat headlines, but will continue to message hot and
humid conditions.

For late Sun nt into Mon evening a shortwave trough will track
from the Canadian Prairies to the nrn Great Lakes with a 1000 mb
sfc low reaching far east central WI by 00Z Tue. Its swly low
level jet should initiate convection Sun nt over nrn MN toward
nw WI, with some storms moving into north central WI before 12Z
Mon. The showers and storms will then track sewd with new
development expected throughout the day on Mon via low to mid
level warm, moist advection and frontogenesis. The cold front
will follow the low and mostly pass through the area by early
evening. MUCAPE values are challenging to determine given clouds
and showers and storms to begin the day, but still have fairly
high confidence in minimal amounts of MUCAPE around 1500 J/KG.
Strong deep layer shear of 50 kt will be present with veering
winds in the low to mid levels from swly to nwly. Thus all
hazards including tornadoes are possible for Mon. The early
morning storms will also have a SVR wind and hail potential.

Afterward, this low pressure area and upper wave will occlude
and remain over portions of the Mid Atlantic States and New
England for Wed and the remainder of the week. This maintains
nwly flow aloft for the wrn Great Lakes with a sfc ridge
residing over the area for Tue nt-Thu. Shortwave troughs in the
flow aloft will then bring 20-40 percent chances for showers and
storms Thu-Sat. High temps in the lower to middle 80s are
expected for much of this period.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 617 PM CDT Sat Jul 25 2026

VFR/MVFR stratus continues to impact the region early this
evening. Despite previous model runs scattering out these clouds,
current model trends indicate this moisture will get trapped below
an inversion and stick around overnight. In fact, CIGs should
lower overnight as the inversion lowers to MVFR and even IFR at
some locations.

There is a chance for patchy fog, mainly in the Wisconsin River
Valley and across northern WI early Sunday morning (9-12Z). If
fog does develop it should dissipate shortly after sunrise
Sunday.

Outlook: Intermittent periods of thunderstorms are expected on
Sunday and Monday, but confidence is low regarding the timing and
area to be impacted.

.KOSH...

Stratus over OSH looks to stick around overnight as an inversion
develops after sunset. CIGS will start out VFR, then lower to MVFR
overnight as the inversion lowers. These clouds are expected to
dissipate Sunday morning around 16-18Z with VFR conditions the
rest of the day. Steady south to southwesterly winds around 4-
8 kts is expected through Sunday. The risk for any showers or
thunderstorms impacting OSH Sunday afternoon is too low to include
in this set of TAFs.

There is a slight chance for patchy fog to develop in the Fox
Valley early Sunday morning, however, confidence any fog
impacting OSH is too low to include at this TAF issuance.

&&

.MARINE...
Issued at 216 PM CDT Sat Jul 25 2026

Modest sly flow this afternoon will become light wly winds tnt
followed by modest sly winds once again from late Sunday morning
into the evening. A moderate swim risk is forecast this
afternoon and evening for the Lake MI beaches via 2 to 4 foot
waves.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....MG
AVIATION.......Kurimski
MARINE.........MG

NWS GRB Office Area Forecast Discussion