Severe Thunderstorm Warning 25, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
746 PM CDT Mon Apr 13 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Thunderstorms will develop late this afternoon and continue
  into the overnight hours with hail in excess of 2 inches,
  damaging winds and a tornado possible with the strongest
  storms. The highest risk is across central Wisconsin.

- Rainfall amounts of 1 to 3 inches across parts of central into
  east-central Wisconsin may lead to additional flood concerns
  through tonight.

- An active stretch of weather continues through late week with
  continued periods of showers and storms. The risk area for
  severe storms shifts south on Tuesday, but extends north into
  parts of central and east-central Wisconsin.

- Minor to moderate river flooding is ongoing or forecast to
  occur on several area rivers. Expect many rivers to rise this
  week due to additional rain and runoff from Upper Michigan,
  with the greatest impacts expected on the Menominee River,
  where some locations may see major flooding during the mid to
  late part of the week.

&&

.DISCUSSION...
Issued at 245 PM CDT Mon Apr 13 2026

Tonight: Severe Storms/Heavy Rain

Early afternoon surface analysis indicated a surface low over
central Nebraska with a warm front snaking eastward across
northern Iowa into central Wisconsin. MLCAPE has increased to
around 1000 J/kg across central Wisconsin with some inhibition
noted.

Late this afternoon, expect more focused storm
initiation/intensification generally in the vicinity of the
warm front and nose of the low- level jet. Surface-based
instability is maximized over central Wisconsin (1000-2000 J/kg)
by late this afternoon with effective shear around 40-50 kts.
Combined with steep lapse rates already sampled by the 18Z GRB
RAOB, the environment will be supportive of large to very large
hail. The tornado risk likely will be highest along the warm
front from west-central Wisconsin into perhaps central
Wisconsin as the low-level jet increases early this evening and
overlaps the warm front/instability axis. Damaging wind risk
will depend on the degree of upscale growth/organization of
storms. Overall, the severe weather risk lessens with northward
extent.

Precipitable water values remain near the top of climatology for
mid April. HREF probability matched mean 24-hr rainfall is in
the 1 to 3 inch range across parts of central into east-central
Wisconsin through tonight, on the nose of the low-level jet.
However, rainfall amounts will depend on the organization and
degree of training. This rainfall could exacerbate already
saturated soils.

Areas of fog may also develop overnight behind the storms.

Tuesday: Additional storms

The front slides south on Tuesday with the instability axis also
displaced south. The placement of the boundary will impact the
northward extent of severe weather during the
afternoon/evening. RAP hodographs show lesser low-level shear
although an elongated profile favorable for large hail and
winds. Currently the highest risk of severe storms is across
southern Wisconsin, but could extend into parts of east-central
Wisconsin.

Wednesday-Monday: Active into the weekend

Southwest flow aloft persists into late week keeping the area in
a warm, wet pattern. Another shortwave trough will bring
additional shower (40-80%) chances Wednesday into Thursday. A
more dynamic trough is slated to impact the region late Friday
into the weekend bringing additional widespread rainfall and
storms. The northward extent of greater instability is somewhat
uncertain with this system, impacting severe weather potential,
but this will be something to watch in the coming days. Drier
and cooler weather is expected behind this system for early next
week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 746 PM CDT Mon Apr 13 2026

Widespread showers and storms will continue to impact AUW/CWA/ATW
/GRB/MTW this evening. The strongest storms could contain heavy
rain, hail and gusty winds. Some fog will along linger near Lake
Michigan. The activity will end west to east from 03-07z. Low
clouds and fog will linger overnight, with clouds lifting and
scattering out later Tuesday morning and afternoon. Another round
of storms is possible at ATW/GRB/MTW Tuesday afternoon.

&&

.HYDROLOGY...
Issued at 245 PM CDT Mon Apr 13 2026

Flooding is expected to continue on several area rivers during
the week, and will be exacerbated by periodic heavy rainfall.
Runoff from Upper Michigan is still expected to bring moderate
to major flooding along the Menominee River, especially during
the mid to late part of the week. If you live near a river or
stream, keep a close eye on water levels this week and monitor
the latest Flood Warnings from the National Weather Service in
Green Bay.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 7 AM CDT Tuesday for WIZ013-018>022-030-031-
035>040-045-048>050-073-074.
&&

DISCUSSION.....JM
AVIATION.......Bersch
HYDROLOGY......Kieckbusch/JM

Hazardous Weather Outlook
National Weather Service Green Bay WI
251 PM CDT Mon Apr 13 2026

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-142000-
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-
251 PM CDT Mon Apr 13 2026

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

.DAY ONE...Tonight

Elevated water levels or flooding continue along some rivers.

Severe storms, including heavy rainfall, large to very large hail,
damaging winds and tornado or two are possible late this afternoon
into tonight. The highest risk of severe storms is across central
into east- central Wisconsin.

Additional flooding may occur for areas that receive heavy
rainfall tonight.

.DAYS TWO THROUGH SEVEN...Tuesday through Monday

There is a chance of severe thunderstorms south of a line from
Wisconsin Rapids to Sturgeon bay during the late afternoon and
evening on Tuesday. Large hail and strong winds will be the
primary threats.

Additional thunderstorms are possible Wednesday afternoon into
Wednesday night, and again Friday night into Saturday.

Elevated water levels and at least minor flooding will continue
along many rivers through the week. Runoff from the Upper
Peninsula will likely result in moderate to possibly major flooding
on the Menominee River during the mid to late part of the week.

.SPOTTER INFORMATION STATEMENT...

Spotters should report severe weather and flooding to the
National Weather Service in Green Bay.

LMZ521-522-541>543-142000-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
251 PM CDT Mon Apr 13 2026

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

.DAY ONE...Tonight

Thunderstorms, some possibly strong to severe, are expected
tonight.

Areas of fog are possible.

.DAYS TWO THROUGH SEVEN...Tuesday through Monday

Thunderstorms will be possible at times Tuesday through Wednesday
night, and again Friday night into Saturday. Severe storms will
be possible Tuesday evening.

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