445
FXUS63 KGRB 270002
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
702 PM CDT Sun Jul 26 2026

.KEY MESSAGES...

- Heat indices into the middle to upper 90s across central
  Wisconsin this afternoon and early evening, where a Heat
  Advisory is in effect for Wood, Portage, Waupaca, and Waushara
  counties.

- Strong to severe thunderstorms, and heavy rainfall and urban
  and small stream flooding, are possible tonight. There are
  lesser chances for severe storms on Monday. Damaging winds
  and large hail will be the primary hazards.

- A cold front passing through the region 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 225 PM CDT Sun Jul 26 2026

A weak wave of low pressure over sw Ontario will continue ewd
across nrn Lake Superior this evening along with an area of weak
PVA via vorticity maximums. A little farther to the south, persistent
850-700 mb warm, moist advection will continue across nrn MN to
over Lake Superior this afternoon then gradually shift swd into
Upper MI and nrn WI this evening via wly 850 mb flow of 25-30 kt.

A tongue of projected RAP MLCAPE is expected to rise to 3000-3500
J/KG from wrn WI to just and west of Duluth by late this afternoon
with a high CIN region over ne MN due to cloud cover, then greater
instability with less CIN in sw Ontario. The CAMs due initiate
convection over ne MN this afternoon within this high CIN region,
but it is possible initiation could occur farther to the south
where a weak boundary, from the passing sfc trough to the north,
will separate light wly winds from sswly winds to its south. It`s
also possible the showers and scattered storms over nw MN will
grow upscale when encountering the higher CAPE environment to its
se later this afternoon and evening.

Overall there is much uncertainty on the timing and location this
afternoon and early evening, but will count on the aforementioned
850-700 mb warm, moist advection that will gradually shift swd
across nrn and central WI through the evening and overnight to
bring some clarity to the forecast. MUCAPE should range from
1500-3000 J/KG from east to west across the forecast area tnt,
while the mid level cap likely stays over wrn WI. Some CAMs show
persistent north to south showers and storms from late evening
into Mon AM which agrees well with the persistent warm, moist
advection via wly flow aloft. High PWATs of 1.5-1.8 inches will be
available and will need to message heavy rain and urban and small
stream flooding. The recent dry conditions and the overall uncertainty
precludes the issuance of a Flood Watch at this time.

An Enhanced Risk is outlooked for WI but is counting on a mature
and severe MCS. Deep layer nwly shear of 50 kt is in place along
with the aforementioned values of CAPE and mdt to large DCAPE.
However, if more discrete cells or clusters of cells occur
instead of a large dominant MCS, large hail and damaging wet
microbursts will still remain fairly probable just with lesser
coverage. The tornado threat looks low given relatively weak low
level wind shear.

For Monday low pressure will track from east central MN across
central or srn WI, while a shortwave trough tracks from sw
Ontario into the nrn Great Lakes. Scattered showers and storms may
still be ongoing Mon AM for the overnight hours, but should
mostly dissipate by late morning. New shower and storm development
is expected but will focus over the ern half of the forecast
area. Believe temps will still rebound back into the 80s but there
is much uncertainty given the clouds and precipitation. This will
also affect the potential for severe storms which does look a bit
lower at this time.

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 40-50 percent chances for showers and
storms Thu nt-Sat. High temps in the lower to middle 80s are
expected for much of this period.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 626 PM CDT Sun Jul 26 2026

Storm chances will continue to increase throughout the evening
and overnight hours. Isolated storm development cannot be
entirely ruled out through 05Z, but confidence was not high
enough in convective initiation occurring near any particular
terminal to warrant mention in the TAFs. Widespread showers and
storms look more likely after 06Z as a cluster of storms move
into NW Wisconsin from Minnesota, reducing CIG/VIS to IFR and
occasionally LIFR. The strongest storms could also produce
strong surface wind gusts. Light precipitation may continue
briefly behind the complex before conditions improve early/mid
morning.


.KOSH...

Shower and thunderstorm chances are expected to continue
increasing over the course of the evening and overnight hours.
Guidance remains rather inconsistent, but a slowing trend has
been noted. As such, the start of -SHRA has been delayed until
08Z with the best TSRA window currently existing between 10Z
and 13Z. In particularly intense storms, strong surface gusts
(40+ kts) will be possible, as well as LIFR VIS and IFR CIG.
Light showers may linger behind the storms, but VIS/CIGs should
improve to VFR levels by mid-morning.


&&

.MARINE...
Issued at 225 PM CDT Sun Jul 26 2026

Light to modest sly winds this afternoon and evening will shift
wly late tnt into Mon AM, then back to southeasterly for Mon
afternoon. Nly winds will then develop Mon nt-Tue with waves
increasing to 2 to 4 feet. Patchy fog could develop at times
into tnt.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
WI...Heat Advisory until 9 PM CDT this evening for WIZ035>037-045.
MARINE...None.

&&

$$

DISCUSSION...MG
AVIATION...Macko
MARINE...MG

NWS GRB Office Area Forecast Discussion