709
FXUS63 KGRB 210628
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
128 AM CDT Mon Sep 21 2026

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Hazardous boating and swimming conditions are expected on the
  Bay and Lake through Tuesday afternoon.

- A quieter and drier weather pattern will settle into the region
  for the next week. Patchy frost will be possible at times
  across the northwoods.

&&

.DISCUSSION...
Issued at 128 AM CDT Mon Sep 21 2026

The latest RAP analysis and satellite and radar imagery show a
split flow pattern across North America, characterized by
shortwave energy traversing the central Plains that is
undercutting a broad area of ridging positioned over Canada.
Closer to home, a sprawling area of strong surface high pressure
centered over northern Ontario continues to supply a cool and dry
airmass into the western Great Lakes. Precipitable water values
hover around 70 percent of normal as a result. Temperatures aloft
are sufficiently cold to support the development of lake clouds
over northeast Wisconsin courtesy of persistent northeast winds,
but regional radar imagery remains free of precipitation echoes
this morning. Upstream, an upper-level low pressure system
situated over the Plains is expected to migrate eastward toward
the Great Lakes from late tonight into Tuesday, bringing periods
of mid and high-level clouds over the local area.

An large layer of dry air situated between 875mb and 700mb will
exist across the region today and tonight. While some high-
resolution guidance attempts to bring light sprinkles or showers
into central Wisconsin late this afternoon and evening in
association with the approaching upper low, confidence remains
very low that any precipitation will reach the surface.
Daytime high temperatures will remain in the low to mid 60s today
and Tuesday.

With the cool Canadian airmass entrenched and persistent
northeasterly flow, overnight low temperatures are forecast to
drop into the mid to upper 30s across sheltered and low-lying
areas of far northern Wisconsin late tonight and again on Tuesday
night. Widespread mid and high- level cloud cover streaming in
ahead of the approaching upper- level low creates some uncertainty
regarding optimal radiational cooling conditions.

Quiet weather will remain across the region as the upper-level
low slowly drifts from the central Great Lakes toward the East
Coast Wednesday through Friday. In its wake, surface high pressure
will remain present over the western Great Lakes and effectively
suppress any weak shortwave energy attempting to advance into the
region on Thursday and Saturday. Precipitation chances will remain under
10 percent through Saturday. The next potential for precipitation
arrives late Saturday night into Sunday as a broad trough begins
to slide toward the central United States, providing a slight 20
percent chance of rain showers to close out the forecast period.
Temps will undergo a steady warming trend late in the week, with
high temperatures rebounding from the 60s into the lower 70s by
the weekend.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1045 PM CDT Sun Sep 20 2026

A few pockets of high-end MVFR and low-end clouds are expected
late this evening and overnight. Additional high-end MVFR and
low-end stratocumulus clouds are expected on Monday. Low chance
(10-20%) of some lower clouds overnight, but will not include. A
sprinkle is possible in any of the lower clouds on Monday, but
will not have any impacts to aviation. Otherwise, high clouds will
spread across the region through the TAF period.

Northeast winds will gust to 15 kts overnight, locally higher in
eastern WI. East to northeast winds are expected on Monday,
gusting up to 20 kts.

&&

.MARINE...
Issued at 128 AM CDT Mon Sep 21 2026

The primary immediate weather hazard involves elevated winds and
high waves on the Bay and Lake. The pressure gradient between the
robust high pressure over northern Ontario and the advancing low
pressure to our west is producing onshore winds to 25 kts and
wave heights to 6 feet on Lake Michigan, and 3 to 4 feet on the
Bay of Green Bay early this morning.

The gusty northeast winds today will veer to the east on Tuesday
and remain gusty in the 20-25 kt range. As a result, Small Craft
Advisories and a Beach Hazards Statement look to remain on track.
Winds and waves might subside enough for an early cancellation on
the Bay late tonight or Tuesday, but conditions will undoubtedly
remain choppy with gusts to 20 kts. Will relay the trends to the
next shift. Therefore no changes planned to the current marine
headlines at this time. Winds and waves will gradually subside
late Tuesday into Wednesday as the pressure gradient begins to
relax.

&&

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

$$

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