399
FXUS63 KGRB 250740
AFDGRB

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

Forecast discussion for routine morning forecast 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 06Z TAF Issuance
Issued at 1056 PM CDT Fri Jul 24 2026

Skies ranged from mostly clear across central and east-central
Wisconsin, with a BKN-OVC deck across the north ahead of a cold
front and upstream convection. This cold front will approach from
the north overnight, which could bring some lower clouds across
the north along with some disorganized shower activity, affecting
primarily RHI. CAMs indicate less of a chance for this activity to
affect RHI; therefore, will only put a PROB30 group in this set of
TAFs. Further southwards these showers are expected to diminish
rapidly; therefore, no mention of showers was made at other sites
for now.

Outlook...Isolated to scattered thunderstorms and lower flight
categories are possible at times over Saturday evening and again
Sunday as a warm and unstable airmass shifts into the region.

.KOSH...

Mostly clear skies with only some high clouds are expected
overnight. An approaching cold front overnight into early
Saturday could bring some disorganized showers into northern
Wisconsin. Currently expect these showers to dissipate as they
move southwards, with around 10-15% chance of some sprinkles
reaching the site; therefore, did not add a mention of showers at
this time.

Fair weather cumulus returns Saturday afternoon. Looking ahead of
the current TAF period, Saturday evening will also bear watching
as it may include some pop-up showers and scattered storms. Best
signal remains west and northeast of the area, but will need to
monitor the forecast going forward for convective potential over
Oshkosh.

Winds will remain below 10 kts overnight. Southwest winds
increase Saturday morning at 5-10 kts with some brief gusts up to
15 kts through early Saturday evening.

&&

.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.......Kurimski
MARINE.........MPC

NWS GRB Office Area Forecast Discussion