River Flood Warning 20, NWS Green Bay, WI

Area Forecast Discussion
National Weather Service Green Bay WI
609 AM CDT Mon Apr 13 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Potential for severe thunderstorms with very large hail,
  damaging winds and a few tornadoes has increased for late this
  afternoon and evening.

- Additional rainfall amounts of 1 to 3 inches will occur south
  of Hwy 64 late this afternoon into tonight. Given this area
  already has saturated soils and rising water levels on rivers
  and streams, a Flood Watch has been issued.

- There is a chance of severe thunderstorms with large hail and
  damaging winds south of a line from Wisconsin Rapids to Sturgeon
  Bay during the late afternoon and evening on Tuesday.

- 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.

- Much above normal temperatures expected through Friday, with
  highs in the 60s and 70s.

&&

.DISCUSSION...
Issued at 314 AM CDT Mon Apr 13 2026

Rain associated with short-wave trough has shifted east of the
region, while a weakening band of showers associated with a cold
front moved through northern WI. Stratocumulus clouds covered
most of the forecast area and post-frontal low stratus was
dropping into northwest and north central WI, but some clearing
was poised to move into central WI. Temperatures were very mild,
in the middle 50s to lower 60s.

Forecast concerns include severe thunderstorms with all hazards,
heavy rainfall and possible flooding later this afternoon into
tonight. Additional severe thunderstorms could clip the southeast
part of the forecast area Tuesday afternoon and evening. River
flooding will also be rampimg up regionwide, with the greatest
impacts expected on the Menominee River.

Severe Thunderstorm Potential Later this Afternoon into Tonight:
the cold front currently moving through the northwest part of the
forecast area is expected to set up west to east across central WI
this afternoon. Partial clearing near this boundary should allow
temperatures to rise into the middle to upper 70s as dew point
approach 60 degrees. Surface based CAPE is expected to increase to
around 2000 j/kg near the boundary, with substantial elevated
instability expected farther north due to steep mid-level lapse
rates. CAMs show scattered supercells developing in central WI
during the late afternoon, then expanding into a large cluster as
a strenthening low-level jet ascends the frontal zone. The initial
supercell development near the frontal boundary in the late
afternoon/early evening could produce very large hail (2+ inch
diameter) and possibly a couple tornadoes, given STP values of 2-3
and 0-1km SRH of 200-300 m2/s2. The greatest potential for
tornadic development looks to be southwest of a line from Wausau
to Oshkosh, including the Marshfield, Wisconsin Rapids, Stevens
Point, Waupaca and Wautoma areas. This impact area may need to be
adjusted if the front sets up farther north or south. North of the
boundary, the main threat should be large hail, given a more
elevated environment, with lower severe probabilities over far
northern WI.

Heavy Rainfall and Flooding Potential late this Afternoon into
Tonight: Have issued a Flood Watch for roughly the southern two-
thirds of the forecast area from late this afternoon through
tonight. This area is expected to receive 1-3 inches of rain
tonight, based on the favored Canadian, Canadian Ensemble mean
and HRRR models. The HRRR is quite ominous, as it shows potential
for significant training of the storms over the southern part of
the forecast area. Much of this region has saturated soils from
the 1-2 inches of rain that fell Saturday night into Sunday, and
several rivers and streams are already running high or flooding.
Additional heavy rainfall could cause significant flooding problems.

Severe Potential Tuesday Afternoon and Evening: The front will get
shoved south by tonight's convection, and will likely set up just
to our southeast Tuesday afternoon and evening. CAMs show strong
to possibly severe convection developing over the southeast third
of the forecast area (roughly south of a ISW-SUE line) between
21z/Tue-03z/Wed. Large hail would be the primary threat, though
damaging winds are also possible.

Additional significant rainfall is expected as a low pressure
system lifts through the area late Wednesday into Wednesday night,
and with a strong cold frontal passage Friday night into
Saturday.

Milder temperatures are expected through the period, with highs
reaching into the 60s and 70s from today through Friday.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 601 AM CDT Mon Apr 13 2026

Low stratus will cover much of northern WI this morning, though
the edges will likely erode due to daytime heating. The low
clouds should shift south along the lakeshore and persist as
surface winds become SE. Strong to severe thunderstorms will
develop in central WI late this afternoon, then increase in
coverage and expand north and east into the evening hours. Most of
this activity should taper off during the early overnight hours.
Widespread low clouds and fog will develop regionwide tonight, due
to shower and thunderstorm activity and E-SE flow north of a warm
front and off Lake Michigan.

&&

.HYDROLOGY...
Issued at 314 AM CDT Mon Apr 13 2026

Minor flooding is expected to continue on several area rivers
during the week, and will be exacerbated by periodic heavy
rainfall (starting with tonight's Flood Watch event, which is
detailed above). Runoff from Upper Michigan is still expected to
bring moderate to possibly major flooding on 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 from 4 PM CDT this afternoon through Tuesday morning
for WIZ013-018>022-030-031-035>040-045-048>050-073-074.
&&

DISCUSSION.....Kieckbusch
AVIATION.......Kieckbusch
HYDROLOGY......Kieckbusch

Hazardous Weather Outlook
National Weather Service Green Bay WI
337 AM CDT Mon Apr 13 2026

WIZ005-010>013-018>022-030-031-035>040-045-048>050-073-074-140845-
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-
337 AM CDT Mon Apr 13 2026

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

.DAY ONE...Today and Tonight

Elevated water levels continue along many rivers, with minor
flooding occurring on the Wolf, Oconto, Menominee, Pine and
Embarrass Rivers, and a small portion of the Wisconsin River.

There is a Slight risk of severe thunderstorms (level 2 out of 5)
over most of the forecast area, with an Enhanced Risk (level 3
out of 5) over central Wisconsin late this afternoon and evening.
Large hail and damaging winds are possible regionwide, with a
threat of very large hail (2 inches or greater) and a couple
tornadoes in central Wisconsin late this afternoon and evening.
The greatest threat of tornadoes should occur southwest of a line
from Wausau to Oshkosh, including the Marshfield, Wisconsin
Rapids, Stevens Point, Waupaca and Wautoma areas.

A Flood Watch has also been issued for areas along and south of
Highway 64, where an additional 1-3 inches of rain is expected
to fall on saturated soils. Many rivers and streams are already
running high or flooding in this area, so additional heavy
rainfall may cause significant problems. Urban flooding and water
on roads will result in hazardous travel conditions.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

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 should be the primary threat.

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.

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

.SPOTTER INFORMATION STATEMENT...

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

LMZ521-522-541>543-140845-
The Bay of Green Bay south of a line from Cedar River MI to Rock
Island Passage-Lake Michigan from Washington Island to Sheboygan-
337 AM CDT Mon Apr 13 2026

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

.DAY ONE...Today and Tonight

Conditions hazardous to small craft are expected on Lake Michigan
early this morning. Dense fog is also anticipated this morning,
especially on Lake Michigan north of Sturgeon Bay.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

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

Kieckbusch