Severe Thunderstorm Warning 109, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
630 PM CDT Mon Jul 20 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Level 3 of 5 risk of severe storms today. Damaging winds, large
  hail, and a few tornadoes all possible. Highest risk is mid
  afternoon to early evening (3 to 8 PM).

- Drier and cooler for mid-week.

&&

.DISCUSSION...
Issued at 255 PM CDT Mon Jul 20 2026

MLCAPE of 2500-3000 J/KG is building into north central WI at
this time on swly flow ahead of a weak sfc trough. Meanwhile 0-8km
shear has increased to 50-60 kt across the area. 994 mb low
pressure over north central Ontario will deepen slightly 991 mb
as it moves near the srn tip of James Bay by 12Z Tue. The pre-
frontal sfc trough over nw WI will move across the nrn half of
WI late this afternoon and early evening. Sfc convergence along
this feature will be modest, but aided by the main upper wave
over sw Ontario moving over nrn Lake Superior for late
afternoon, and then deepen slightly as it moves across ern Lake
Superior this evening. Overall 12hr 500 mb ht. falls of 60-80
meters are expected across the nrn half of WI. Without strong
forcing, believe the CAMs are correct with discrete supercells
after mainly 21Z into the early evening. Cannot rule out some
consolidation of cells farther to the east toward Lake MI and
will forecast 50-70 percent chances for showers and storms.
Effective SRH of 200 m2/s2 is forecast with 0-1KM SRH of 100-150
m2/s2. LCLs and LFCs will be sufficiently favorable for
tornadoes along with the aforementioned low level wind shear.
DCAPE of 1200-1600 J/KG will be a limiting factor for tornadoes,
and a stronger factor for damaging winds. Overall though, all
hazards, possibly significant severe, are forecast.

The actual cold front will then pass late tnt on increasing nwly
winds. Breezy nwly winds and cold advection will prevail into
Tue nt. Broken stratocumulus, relatively cold air aloft and lake
effect rain showers from Lake Superior may lead to a few
showers over far north central WI late tnt-Tue. Relatively cool
temps are forecast Tue-Wed but with less clouds for Wed when
high pressure approaches from the west.

Nwly flow aloft will then prevail into the weekend. High
pressure over MN and IA on Wed will track across the Ohio Valley
and into New England for Thu-Fri. Temperatures will climb back
to normal for Thu-Fri then above normal for the weekend as swly
winds return. Chances for showers and storms return to the
forecast for Sat nt-Mon as a weak Canadian cold front will pass
or stall over the region.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 614 PM CDT Mon Jul 20 2026

KOSH - Scattered thunderstorms will move into eastern WI this
evening. The highest chances of storms will be from 01-03z.
The storms will be capable of producing gusty winds, hail,
and brief heavy rain. Since the storms are scattered, will wait
closer to arrival to better pinpoint wind gusts if they should
move across the airport. Synoptic winds will gust to 20 to 25 kts
through 03z, then wind gusts diminish to around 18 knots into
early Tuesday morning. Gusty northwest winds are expected on
Tuesday, generally around 25 knots from mid morning through much
of the afternoon.

For the rest of the area, scattered thunderstorms will end from
west to east later this evening. Some of the strongest storms
could produce strong winds, hail and brief heavy rain.

&&

.MARINE...
Issued at 255 PM CDT Mon Jul 20 2026

Breezy sly winds this evening will become breezy nw winds Tue-
Tnt as deep low pressure moves across srn Canada. A Small Craft
Advisory is in effect into tnt but will likely need to be
extended for Tue-Tue nt. A Beach Hazards Statement will continue
through tnt for the Lake MI beaches.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement through Tuesday morning for WIZ022-040-
050.
&&

DISCUSSION.....MG
AVIATION.......Eckberg
MARINE.........MG

Hazardous Weather Outlook
National Weather Service Green Bay WI
225 PM CDT Mon Jul 20 2026

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-211930-
Vilas-Oneida-Forest-Florence-Northern Marinette County-Lincoln-
Langlade-Menominee-Northern Oconto County-Door-Marathon-Shawano-
Wood-Portage-Waupaca-Outagamie-Brown-Kewaunee-Waushara-Winnebago-
Calumet-Manitowoc-Southern Marinette County-
Southern Oconto County-
225 PM CDT Mon Jul 20 2026

This Hazardous Weather Outlook is for north-central and northeast
Wisconsin.

.DAY ONE...Tonight

There is a level 3 of 5 risk of severe storms with damaging
winds, large hail, tornadoes, and locally heavy rain into this
evening.

.DAYS TWO THROUGH SEVEN...Tuesday through Monday

The probability for widespread hazardous weather is low.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to report peak wind gusts to the National
Weather Service in Green Bay.

LMZ521-522-541>543-211930-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
225 PM CDT Mon Jul 20 2026

This Hazardous Weather Outlook is for portions of northwest Lake
Michigan and the waters of Green Bay.

.DAY ONE...Tonight

Conditions hazardous to small craft are expected. A Beach Hazards
Statement is also in effect.

There is a chance of thunderstorms late this afternoon and
evening that would pose a hazard to mariners

.DAYS TWO THROUGH SEVEN...Tuesday through Monday

Conditions hazardous to small craft are possible Tuesday into
Tuesday night.

Preliminary Local Storm Report
National Weather Service Green Bay WI
736 PM CDT Mon Jul 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0658 PM     Funnel Cloud     1 WNW Little Chicago    45.05N 89.87W
07/20/2026                   Marathon           WI   Public

            Report on social media.

&&

HEK

Preliminary Local Storm Report
National Weather Service Green Bay WI
737 PM CDT Mon Jul 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0656 PM     Funnel Cloud     2 S Hamburg             45.06N 89.88W
07/20/2026                   Marathon           WI   Public

            Report on social media.

&&

HEK

Preliminary Local Storm Report
National Weather Service Green Bay WI
743 PM CDT Mon Jul 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0654 PM     Funnel Cloud     1 E Athens              45.03N 90.05W
07/20/2026                   Marathon           WI   Public

            Report on social media, location
            approximated by radar.

&&

HEK

Preliminary Local Storm Report
National Weather Service Green Bay WI
801 PM CDT Mon Jul 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0745 PM     Funnel Cloud     1 NE Aniwa              45.01N 89.20W
07/20/2026                   Shawano            WI   Public

            mPING, funnel cloud reported 2 mi NE of
            location.

&&

HEK

Preliminary Local Storm Report
National Weather Service Green Bay WI
812 PM CDT Mon Jul 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0639 PM     Funnel Cloud     2 SSE Little Chicago    45.01N 89.83W
07/20/2026                   Marathon           WI   Public

            Funnel cloud reported about 3 mi west of
            location. mPING.

&&

HEK

Preliminary Local Storm Report
National Weather Service Green Bay WI
815 PM CDT Mon Jul 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0735 PM     Marine Tstm Wind 1 NW Washington Island  45.39N 86.92W
07/20/2026  M39 MPH          Door               WI   AWOS

            A wind gust to 34 knots was reported at the
            Washington Island Airport.

&&