118
FXUS63 KGRB 301837
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
137 PM CDT Sun Aug 30 2026

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Isolated showers/storms this afternoon into tonight. Small
  chance (10-20%) for showers and storms on Monday, but if storms
  develop, they may become strong or severe.

- Hazardous boating and swimming conditions on Lake Michigan
  through this afternoon and could return on Monday. Dense fog
  possible on Lake Michigan on Monday.

- Warm, humid conditions continue into Monday, with heat indices
  in the 90s expected Monday afternoon.

&&

.DISCUSSION...
Issued at 137 PM CDT Sun Aug 30 2026

Precipitation/Storm Trends...A few showers and perhaps a storm
(only ~500 J/kg of CAPE to work with) will continue across
northern WI this afternoon as a 850mb warm front/low-level WAA
gradient gradually lift northeast across the region. Can`t rule
out some isolated activity along/near the warm front further
south, but the capping inversion will likely prohibit much
activity from occurring. The area of showers/storms over
northeast Iowa associated with a shortwave and LLJ could also clip
the far southeast later this afternoon. For tonight, the majority
of the shower/storm activity looks to focus just north of the
area, mainly associated with the nose of the LLJ and MUCAPE
gradient as it shifts northward. That area could clip far northern
WI overnight. Locally heavy downpours (impressive PWATs near 2")
and small hail would be the main concern of any elevated storm.

On Monday, there will be a conditional threat for strong to severe
storms as plenty of instability/shear (MUCAPE 2000-3000 J/kg and
deep-layer shear of 30-40 kts) in place, a weak frontal boundary
and surface low sagging across the area, and a weak shortwave/jet
overhead. However, the capping inversion looks to still be in
place for much of the day. Question will be whether it will break
during peak heating to allow for the storms to form. CAMs not much
help due to the cap, with models flip-flopping around each run.
HRRR Neural Network showing some probabilities near 10% now across
central and east central WI, signaling the scenario if the cap
can break. Other ML/AI guidance paints a small threat for severe
storms across central/eastern WI. SPC has expanded the Marginal
Risk into much of the area due to this threat, which is
reasonable. Main threats would be damaging winds and hail, along
with torrential downpours. Some elevated convection could linger
into Monday night, but coverage looks limited. Tuesday should
start off dry, but chances for additional showers and storms
returns later in the day into Tuesday night as the boundary will
be hovering to our south. A couple shortwaves then look to move
over the region through the remainder of the work week, bringing
intermittent chances for showers and storms at times.

Temperatures/Humidity...After a warm and somewhat humid day today,
temps will be even warmer on Monday and Tuesday with highs in the
middle 80s to lower 90s. The persistent southerly flow will
continue to usher higher dewpoints as well, increasing into the
upper 60s and low 70s. Heat indices in the 90s are expected on
Monday, with a few of the hotter spots getting close to 100, if
temps or dewpoints slightly over perform.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1237 PM CDT Sun Aug 30 2026

The low stratus deck over far northern WI continues to slowly lift
or mix out late this morning, with some IFR ceilings still being
reported. Expect this cloud deck to continue to rise this
afternoon, with RHI becoming VFR by ~21/22z for a short time,
before low clouds (IFR/LIFR) return later this evening and remain
into Monday morning. Elsewhere, look for VFR and a few MVFR
conditions to continue into this evening with a mix of cumulus
clouds and some mid-level clouds, with the lower clouds (mainly
MFVR/IFR) slowly shifting south to AUW/CWA this evening. The MVFR
cloud deck could sneak as far south as GRB/ATW overnight, but
confidence not high enough to include. The threat for some
showers will continue mainly across northern WI this afternoon
into tonight, but chances will be under 30%, so will use PROB30
groups when/where confidence is highest. Fog is expected across
parts of northern and central WI overnight into early Monday as
well. Look for CIGs and VSBYs to slowly improve Monday morning.

Surface winds will stay under 15 kts this afternoon into Monday
morning with winds mainly from the south/southeast in east-central
WI and east in central and north-central WI. Winds will become
east/southeast at all TAF sites overnight. Winds will shift to the
southwest Monday morning, with gusts to ~15 kts by noon. LLWS is
expected across central and east-central late this evening in to
early Monday morning, as southwest winds increase to 35-40 kts at
2000 ft.

&&

.MARINE...
Issued at 137 PM CDT Sun Aug 30 2026

Marginal Small Craft Advisory and Beach Hazards Statement
conditions continue on Lake Michigan with 10-20 kt south winds and
waves around 4. SPOT buoy off of Manitowoc briefly spiked to 5
ft, but has generally been between 3-4 ft with the mid-lake buoy
off of Manitowoc reaching 5.3 ft for a time this morning. Won`t
make any changes to the current Small Craft Advisories and Beach
Hazards Statement as waves slowly subside slightly this afternoon.

Waves look to generally stay under 3 ft tonight. On Monday, south
winds pick up again, gusting to around 20 kts, which will build
waves to around 4-5 ft and generate a moderate to possibly high
risk of rip currents along Lake Michigan beaches. But a slight
offshore component to the wind may keep the higher waves just
offshore. As dewpoints increase through the 60s on Monday into
Monday evening, some dense fog will be possible, as water temps
remain in the upper 50s and 60s. Any fog threat should end as
winds turn offshore Monday night.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 5 PM CDT this afternoon for WIZ022-
040-050.
&&

$$

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

NWS GRB Office Area Forecast Discussion