453
FXUS63 KGRB 212255
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
555 PM CDT Fri Aug 21 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Isolated to scattered storms (40-75% chance) are expected to
  develop over eastern WI through 9 PM today. Strong to severe
  storms are possible (level 1 of 5 risk).

- Showers and storms (40-60% chance) are expected over north-
  central to eastern WI on Saturday, mainly in the afternoon.

- Dry weather expected Sunday and Monday, with active weather
  returning for the middle of next week.

&&

.DISCUSSION...
Issued at 145 PM CDT Fri Aug 21 2026

Severe Weather Threat Through This Evening...An area of stratus
is slowly being replaced by more convective cloud types as a cold
front approaches from the west. Already radar returns are popping
up in the area where some partial clearing has occurred across
north-central and western Wisconsin, just ahead of the cold front.
This activity is expected to increase in coverage and intensity
this afternoon as the partial clearing works its way east,
allowing instability to build across eastern and northeast
Wisconsin. Forecast instabilities are expected to rise to 1-2 J/g,
with the highest values across east- central Wisconsin. Bulk
shear looks a bit better than yesterday, clocking in at 20-30
knots, with the best values across east- central Wisconsin
coincident with the best instability. That being said, the window
for clearing and building instability appears to be fairly short
looking at satellite imagery with the best instability and shear
south of our area; therefore, the pulling back of the slight risk
out of the area per SPC seems appropriate. Current thinking is any
storms that develop would be pulsy, with a threat for strong to
severe storms this afternoon into this evening in the 3-9PM time
frame. The main threat would be damaging winds and large hail.
Locally heavy rain will also be possible with any storm. The
threat from flooding given the recent dry spell seems to be low;
however, if an area gets repeated rounds of activity the threat
could increase. The threat greatly diminishes this evening with
the loss of daytime heating.

This Weekend...Northwest, cyclonic flow will prevail across the
Great Lakes. A shortwave trough moving through this flow will
create chances (20-60%) for showers and a few storms across north-
central and eastern Wisconsin on Saturday, mainly during the
afternoon and early evening. Weak deep-layer shear will prevent
significant storm organization, keeping the severe weather risk
low; however, inverted-V soundings and steep low- level lapse
rates would promote some gustier winds and small hail in any
stronger shower/storm.

A Dry Period...High pressure will then build in Saturday night,
providing dry weather across the region Sunday into Monday.

Next Week...another shortwave trough is forecast to move across
the region late Tuesday into Wednesday, bringing the next chance
for showers and thunderstorms. Instability appears modest with
this mid-week feature, keeping severe probabilities low.

Temperatures into next week will stay fairly close to late-
summer norms, with daily highs mostly ranging from the 70s to
lower 80s and overnight lows in the upper 40s to lower 60s, with
a couple nights with lows in the low-mid 40s across northern WI
this weekend.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 554 PM CDT Fri Aug 21 2026

A cold front will be moving across eastern Wisconsin through late
evening before exiting over Lake Michigan. Scattered showers and
thunderstorms will be impacting the eastern WI terminals through
02-03Z before exiting. Brief reductions in visibility to IFR along
with wind gusts to 30 kts will be possible in the strongest
storms.

Winds will shift to the northwest as the front passes. Scattered
VFR clouds will likely persist into the overnight hours before
clearing occurs for a short time.

Clouds increase on Saturday morning as an upper disturbance dives
into the region from the northwest. While sct-bkn MVFR ceilings
could move into far north-central WI after 10Z, expect low end VFR clouds
to build with daytime heating in the 15-16Z timeframe. Expect
scattered showers and isolated storms to develop over northeast WI
in the 19-23Z timeframe which could impact GRB/MTW so added a
prob30 group consequently. ATW is on the western edge of the
shower potential, but will continue to monitor trends.

Northwest winds will be a bit breezy on Saturday with gusts to 20
kts at all terminals.

&&

.MARINE...
Issued at 145 PM CDT Fri Aug 21 2026

Persistent S/SSW winds will build waves to 2-4 feet on
Lake Michigan late this afternoon. A few 5 foot waves are
possible in the open waters. A few gusts to 25 kts possible this
afternoon as well. Conditions look to be just shy of Small Craft
Criteria, so will hold off on an advisory. As winds turn to the NW
this weekend, a few gusts to 25 kts will be possible, especially
near Death`s Door on Saturday.

&&

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

&&

$$

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