363
FXUS63 KGRB 121109
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
609 AM CDT Sat Sep 12 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Scattered storms early this morning, mainly in north-central
  WI. Severe weather threat is very low, but brief gusty winds are
  possible.

- A round or two of heavy rain and storms is possible Monday into
 Tuesday. The severe weather threat is low.

- Gusty south-southwest winds will generate high waves along Lake
  Michigan through this morning, causing hazardous conditions to
  small craft and dangerous swimming conditions at Lake Michigan
  beaches. Another period of breezy winds and high waves is
  expected Monday into Tuesday.

&&

.DISCUSSION...
Issued at 1229 AM CDT Sat Sep 12 2026

Shower/Storm Early this Morning...A broken line of storms has
developed across northwest WI early this morning ahead of an
eastward advancing cold front. RAP analysis and recent radar
trends suggest these storms are elevated and rooted about a stable
boundary layer. This will limit the severe threat through the
early morning hours, however, with a 45-50kt LLJ can`t entirely
rule out a few isolated stronger wind gusts to 30-40mph with any
collapsing cells. As storms progress further east across WI they
will get further away from the stronger forcing and become more
isolated. As the cold front works to clear the area this morning
may see a brief uptick in storm activity near the lakeshore around
mid-morning as a narrow corridor of instability builds ahead of
the front. Once the front is offshore dry conditions are expected
for the rest of today as high pressure builds back into the
region. The mostly dry conditions should continue through Sunday
with only a low-end (10-20%) chance for scattered showers south of
HWY 10 Sunday as weak mid-level f-gen develops.

Heavy Rain and Storm Potential Next Week...Ensemble and
deterministic models continue to be in fairly good agreement that a
strong low pressure system and shortwave will track across the
northern Plains early next week, which will usher in a seasonally
moist air mass (PWATs up to 2", near the 99th percentile of
climatology). Increasing WAA associated with a LLJ up on Monday will
likely lead to a widespread area of showers and storms  to our
southwest and move into the area Monday afternoon into Tuesday
morning, with the exact timing still to be ironed out. Severe
weather threat looks low due to limited instability and a strong
capping inversion, but elevated instability and the mean flow also
briefly becomes more parallel with the boundary layer flow which
will promote training convection. This could lead to some heavy rain
and a soaking rain for many spots. Question is just how long it will
sit over our area and how many rounds of rain any one location will
see. NBM probs have lowered a little, down to 30-45% chance for
greater than 2" of rain Monday into early Tuesday, with the highest
chances across central WI. WPC has also shifted a Slight Chance for
excessive rain a little to the south. Will still need to monitor for
heavy rain with the high PWATs and possible training convection.

Rest of the Forecast...Questions remain on how far south a boundary
will set up next week within the zonal flow over the Great Lakes and
if we will hit with additional showers and storms or if high
pressure wins out and we stay mainly dry. NBM brings low end PoPs
(20-40%), especially across the south, for much of the week, which
is basically just portraying the differences in location of the
boundary. Hopefully the picture gets a little clearer as we get
closer. If we were to get more bouts of heavy rain later in the
week, this could increase the overall flood threat and will need to
be monitored. Some cooler air will filter into the region behind the
system on Tuesday, with some chilly air in store if the high wins
out.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 608 AM CDT Sat Sep 12 2026

Line of broken showers, currently along a line from KIMT to KISW,
will continue to drift east over the next few hours. Brief showers
are possible through about 14Z at the eastern WI TAF sites.
Instability is very weak so don`t expect any thunder with these
showers over the next few hours. Behind the line of showers and area
of OVC MVFR status has develop over northwest WI. Expect these MVFR
cigs to drift east over the central and northern WI TAF sites this
morning before slowly rising and scattering out after 17-18Z. Once
this status deck clears out expect mostly clear skies for the
remainder of the TAF period.

Winds will shift to the west-northwest this morning behind a cold
front is currently moving across western WI, with gusts to 20 kts.
The west-northwest winds will subside into Saturday evening.

&&

.MARINE...
Issued at 1229 AM CDT Sat Sep 12 2026

No changes to the current marine/beach headlines as southerly winds
continue to support 3-5ft waves along the Lake Michigan shoreline. A
high swim risk also continues into this morning with a decreasing
swim risk this afternoon as winds become offshore.

Another period of breezy southeast to south winds will lead to
increasing waves Monday into Tuesday. Additional Small Craft
Advisory or Gale Warning and Beach Hazard Statements will likely
be needed.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 1 PM CDT this afternoon for WIZ022.

Beach Hazards Statement until 10 AM CDT this morning for WIZ040-
050.
&&

$$

DISCUSSION.....GK
AVIATION.......GK
MARINE.........GK

NWS GRB Office Area Forecast Discussion