902
FXUS63 KGRB 251055
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
555 AM CDT Sat Jul 25 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Isolated strong to severe thunderstorms are possible late this
  afternoon and evening, primarily over far northeast Wisconsin,
  capable of producing hail and damaging winds.

- Additional rounds of severe storms remain possible late tonight
  into Sunday, and again on Monday.

- Heat and humidity will build across the region this weekend into
  early next week, with high temperatures in the upper 80s to low
  90s and heat indices reaching the middle to upper 90s on Sunday
  and 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 239 AM CDT Sat Jul 25 2026

The latest RAP analysis and satellite imagery show shortwave
energy pushing east across south-central Canada into northeast
North Dakota and northern Minnesota, driving a cold front into
northwest Minnesota. Thunderstorm activity ahead of this front has
been weakening across northern Minnesota as it outruns stronger
instability, though light showers continue to push into far north-
central Wisconsin early this morning. Further south and west, a
dominant subtropical ridge centered over the Intermountain West
continues to build northeast toward the northern Plains and Upper
Midwest, anchored by extreme heat over the Plains. Widespread low
stratus is lifting northeast across the state in a low-level warm
advection regime. Over the next 36 to 48 hours, the interaction
between embedded northern-stream shortwaves propagating along the
northern periphery of this building subtropical ridge and the
advancing surface front will serve as the primary driver for
convective potential and dangerous heat across the region.

Regarding thunderstorm potential, the decaying surface front will
track east today, losing definition near Lake Superior. Morning
cloud cover (and persistent light showers) over north-central
Wisconsin should mix out by afternoon, allowing for surface
heating and destabilization. Mid-afternoon CAPE values reaching
1000-2000 J/kg alongside lake breeze convergence will favor
isolated thunderstorm development from 4 PM to 9 PM, primarily
over far northeast Wisconsin. While weak overall convergence and
capping will keep storm coverage sparse, any cell that develops
could become conditionally severe, with large hail and damaging
winds as the primary threats. A second, more organized round of
convection is possible late night into Sunday morning as a 25-30
knot low-level jet over Minnesota supports a cluster of storms
riding elevated instability around 2000 J/kg into north- central
Wisconsin. Effective shear of 50-60 knots could organize these
early Sunday storms capable of damaging winds and hail before
dissipating. Depending on how morning convection modifies the
airmass, afternoon re-development on Sunday remains conditional
due to rising 700mb temperatures near 12C creating a stout cap.
Northeast Wisconsin remains the most favorable area for
thunderstorm development on Sunday afternoon, which is an area on
the edge of the mid-level cap. A third period of severe potential
arrives Monday as a stronger shortwave trough and cold front
sweep through the region, interacting with 2000-3000 J/kg CAPE and
deep- layer shear exceeding 40 knots. Much of the area is in a
slight risk of severe thunderstorms on Monday.

Regarding the heat threat, the expanding subtropical ridge will
usher a hot, muggy airmass into the region on Sunday and Monday.
Afternoon high temperatures in the upper 80s to lower 90s combined
with dew points climbing into the lower 70s will yield heat index
values peaking in the middle to upper 90s on both days,
particularly across central and east-central Wisconsin. While
convective debris and thunderstorm coverage introduce some
uncertainty regarding exact peak temperatures, conditions will
remain dangerously hot and humid for those outdoors. Headlines for
excessive heat may be required as confidence in storm coverage
improves. A cold front will cross the region late Monday into
Monday night, shifting winds to the north on Tuesday and
delivering a reprieve from the oppressive humidity. Highs Tuesday
through Thursday will moderate into the upper 70s to middle 80s
with much lower dew points, before another disturbance brings
renewed thunderstorm chances late Thursday into Friday.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 555 AM CDT Sat Jul 25 2026

Overcast MVFR and patchy IFR ceilings across western Wisconsin
will spread east across north-central and northeast WI early this
morning, affecting KAUW, KCWA, and KRHI before lifting to VFR by
mid- afternoon. KATW, KGRB, and KMTW will remain predominantly
VFR, but a brief period of broken MVFR ceilings are possible,
which will need to be monitored. Isolated thunderstorms are
possible late this afternoon and evening across northeast
Wisconsin, but confidence in direct terminal impacts remains too
low to include in the forecast. Once the MVFR ceilings lift, VFR
conditions will prevail through early tonight before light winds
and lingering moisture create favorable conditions for ground fog,
with LIFR/IFR visibilities most likely at KRHI after 08Z. High-
based thunderstorm chances increase late tonight into Sunday
morning, primarily for north- central terminals. Winds will remain
light, generally under 10 knots, shifting to south- southeast on
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...

Broken to overcast MVFR/IFR ceilings are making slow progress to
the east across western WI. The latest timing has these low clouds
(3-4 kft) making it to OSH by 12-13Z. A brief period of MVFR
ceilings looks possible from 14-17Z before ceilings rise and
return to VFR (4-5 kft in the afternoon). Partial clearing is
possible this afternoon.

Confidence is very low that any showers and storms will impact OSH
today or tonight despite an increasingly unstable airmass moving
into the region.

VFR flight conditions are forecast tonight. Conditions will be
supportive of patchy fog generally north of OSH.

&&

.MARINE...
Issued at 239 AM CDT Sat Jul 25 2026

Southern Green Bay buoy continues to gust from 20 to 25
kts so will hold onto the Small Craft Advisory until its 4 AM
expiration. South winds will remain 15 to 20 kts for much of the
day prompting persistent waves from 3 to 4 ft. With these waves,
there is a moderate risk for rip currents on Lake Michigan
beaches.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......MPC
MARINE.........MPC

NWS GRB Office Area Forecast Discussion