652
FXUS63 KGRB 151054
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
554 AM CDT Tue Sep 15 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Light to moderate rain will exit by mid to late morning,
  followed by breezy west winds gusting 20 to 30 mph this
  afternoon.

- Hazardous marine conditions with gale-force gusts on Green Bay
  and Lake Michigan will transition to Small Craft Advisories
  later this morning. A dangerous rip current risk along Lake
  Michigan beaches will continue through this afternoon.

- A stronger system brings widespread rain late Friday into
  Saturday, with a 40 to 50 percent chance of rainfall exceeding 1
  inch across much of north-central and northeast Wisconsin.

&&

.DISCUSSION...
Issued at 133 AM CDT Tue Sep 15 2026

A potent surface low pressure system centered over southern
Manitoba early this morning is tracking northeast toward Hudson
Bay, pulling a cold front eastward across Minnesota while a warm
front lifts northward into Iowa. Aloft, a strong 50 to 60 knot
low-level jet directed into northeast Wisconsin has generated
widespread showers and scattered embedded thunderstorms early
this morning. Latest radar imagery and surface observations
confirm a swath of 1 to 3 inches of rain, with localized reports
exceeding 3 inches, extending from Portage and Waushara counties
northeastward into Outagamie and Brown counties. Although local
county dispatch reported no widespread flooding overnight, radar
estimates and private weather stations indicated rainfall rates
exceeding 6-hour flash flood guidance, maintaining a localized
urban and small stream flooding risk early this morning. Will
maintain the Flood Advisory for east-central WI until expiration.
As the low- level jet veers eastward, the heaviest precipitation
is shifting south and east of the area.

Light to moderate rain today through Wednesday Night...
Light to moderate showers along with isolate storms will linger
through mid to late morning until the cold front sweeps across the
area. Additional rainfall amounts up to 0.50 inches will be
possible across east- central and northeast WI. Strong boundary
layer mixing behind the front will yield clearing skies and breezy
west winds this afternoon, with frequent gusts of 20 to 30 mph
expected before winds diminish rapidly near sunset.

Surface high pressure building across the northern Great Lakes
will bring dry and tranquil weather for tonight and Wednesday.
Radiational cooling beneath clear to partly cloudy skies and light
winds will promote the formation of patchy ground fog late
tonight into early Wednesday morning, particularly across far
northern Wisconsin. High temperatures Wednesday will be
seasonable, reaching the upper 60s to low 70s. As the frontal zone
slowly returns north Wednesday night, elevated warm advection
will bring a 30 to 50 percent chance of light showers to central
and east- central Wisconsin. However, moisture availability is
modest and elevated instability reaches only 100 to 200 J/kg,
keeping the probability of rainfall exceeding 0.50 inches under 10
to 20 percent in the southern Fox Valley, with no heavy rain
impacts anticipated.

Widespread rain late Friday into Saturday...
A more dynamic weather system is forecast to impact north-central
and northeast Wisconsin late Friday into Saturday as a shortwave
ejects from the western United States trough. Supported by a 40
to 50 knot low-level jet and precipitable water values rising to
around 1.70 inches (roughly 95 percent of seasonal normal),
widespread showers and embedded thunderstorms will spread across
the region. Model guidance indicates a 40 to 50 percent
probability for 24-hour rainfall amounts to exceed 1 inch across
much of the area from Friday night through Saturday morning.
Elevated instability remains limited with Most Unstable CAPE
generally under 500 J/kg, keeping the severe thunderstorm risk
low. Forecast predictability decreases markedly Sunday into Monday
depending on how quickly the main western trough moves eastward,
but lingering mid-level energy will maintain a 20 to 30 percent
chance of showers through Sunday.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 553 AM CDT Tue Sep 15 2026

Through 00Z Wed:
A cold front over NW WI will move east today. Ahead of the front,
low CIGS with patchy DZ and scattered -SHRA will bring MVFR to
KGRB/KATW/KMTW and LIFR to KRHI/KAUW/KCWA. Flight conditions will
improve to VFR from west to east after 15-16Z as skies clear.
West winds will become gusty to 20-25 kts this afternoon before
subsiding around sunset.

After 00Z Wed Through 12Z Wed:
High pressure brings clear skies and light winds, promoting
patchy fog late tonight. LIFR VSBYs are most favored at KRHI after
08Z (added TEMPO), with patchy MVFR/IFR fog at KAUW/KCWA. VFR
will prevail at KGRB/KATW/KMTW.

High confidence in VFR return this afternoon. Medium confidence
in late-night fog impacts.

&&

.MARINE...
Issued at 133 AM CDT Tue Sep 15 2026

Hazardous marine conditions remain present early this morning with
gale force gusts measured at Sheboygan Light to the northern Lake
Michigan buoy. These gale force gusts look mainly confined to Lake
Michigan. Southerly wind gusts up to 35 knots will subside
through early morning as the cold front approaches and the
pressure gradient relaxes somewhat. Gales will expire by 7 AM CDT,
giving way to Small Craft Advisories across all nearshore waters
through the afternoon as winds shift west at 15 to 25 knots.
Additionally, residual high wave action along Lake Michigan
beaches will create a high risk of dangerous rip currents through
this afternoon, supporting an active Beach Hazards Statement for
the shoreline of Door, Kewaunee, and Manitowoc counties.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 7 PM this evening for WIZ022.

Beach Hazards Statement until 4 PM CDT this afternoon for WIZ040-
050.
&&

$$

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

NWS GRB Office Area Forecast Discussion