713
FXUS63 KGRB 010115
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
815 PM CDT Mon Aug 31 2026

New Information added to update section

.UPDATE...
Issued at 814 PM CDT Mon Aug 31 2026

Despite the CAMs insisting that thunderstorms will fire in just
one more hour, which they have done pretty much all afternoon, the
loss of daytime heating and substantial cap in place, as evidenced
by the 00Z KGRB RAOB sounding, tell another story. It now appears
unlikely that thunderstorms will develop given instability should
now be on the downward trend and there isn`t another form of
forcing available for convection like a low level jet or mid level
shortwave. Therefore, will tone down wording in the other products
for the severe potential.

Given the lack of precipitation this evening, the potential for
fog may also be at risk. However, not quite ready to give up on
this completely given the abundant low level moisture in place, as
evidenced by dew points in the 70s, and partial clearing making
its way through the region.

&&

.KEY MESSAGES...

- Isolated to scattered (20-40% coverage) showers/storms this
  afternoon and evening. Some could be strong or severe, capable
  of producing hail, damaging winds, and torrential downpours.

- Warm, humid conditions continue into Tuesday. Heat indices in
  the 90s expected this afternoon, and again for some locations on
  Tuesday.

- Warm temperatures along with intermittent shower and storms
  chances mid-late week into the weekend.

&&

.DISCUSSION...
Issued at 135 PM CDT Mon Aug 31 2026

Precipitation/Storm Trends This Week...

For this afternoon and evening...dry conditions will continue
early this afternoon as the cap is holding. Ingredients still
coming together for a conditional severe weather threat later this
afternoon into this evening, peak threat time between 4-10pm.
Instability is building with SBCAPEs between 2000-3500 J/kg over
much of the area, but clouds have limited insolation in some
spots. As a weak mid-level shortwave and surface frontal boundary
traverse the area this afternoon, the combination of the eroding
cap, building instability, deep-layer shear of 35-45 kts, and steep
mid-level lapse rates should allow the cap to erode enough for
isolated to scattered shower/storm development. The prime spot for
development looks to be the corridor between Rhinelander and
Shawano between 4-6pm, where the cap will be the weakest and the
forcing will be the greatest. Some lake/bay boundary interaction
could also help initiate/focus convection near the MI border. The
clouds could also create a differential temp gradient, which could
serve as a initiation spot for storms. The shower/storm activity
will shift east/southeast through the early evening, likely ending
by 10-11pm as capping strengthens and instability weakens. The
most intense storms will be capable of producing damaging winds
and hail. Impressive PWATs of 1.7-1.9" will support locally heavy
rainfall in any storm as well.

Overnight into early Wednesday...as the frontal boundary
sags/hovers to our south, mainly dry conditions are expected
into Tuesday afternoon. The next period of concern will be late
Tuesday night into early Wednesday, as another shortwave/MCV is
forecast to ride along the boundary, likely setting off an area of
convection which will race east, arriving in the early morning
hours of Wednesday. Still some uncertainty on the exact track of
this feature, but many CAMs are showing it impacting at least the
southern half of the CWA. HRRR Neural Network really starting to
highlight southern MN and northern IA Tuesday evening then
spreading the highlighted threat region into parts of WI. SPC
kept the Marginal Risk hugging the southern CWA, but this area may
need to be expanded.

Late This Week and Weekend...the frontal boundary will linger in
the western Great Lakes for most of the period, with a pool of
moisture/unstable air either just to our southwest or in the area.
This will create intermittent shower and storm chances. Severe
clusters of storms, possibly MCSs, will be possible as we remain
on the northern edge of the heat dome over the central Plains.

Temperatures/Humidity...heat index readings will top out in the
90s for much of the area this afternoon. Despite the cold front
moving through this evening, temps on Tuesday will be close or
just slightly cooler than today. It will also still be humid on
Tuesday, as dew points remain in the 60s to low 70s across east-
central WI, but locations in north-central WI will see a little
relief with dew points forecast in the 50s to low 60s.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 557 PM CDT Mon Aug 31 2026

Abundant clouds continue across the area late this afternoon with
mid and high clouds across central and east-central Wisconsin, and
MVFR clouds across north-central Wisconsin.

Isolated to scattered showers and storms are expected into this
evening, mainly through 11pm, initiating between RHI and AIG,
then tracking east/southeast. Confidence is not high in these
showers or storms moving directly over any one terminal, and if
they will form over or just east of AUW/CWA/RHI, so will continue
with PROB30 groups for now and add TEMPO groups if the storms do
form near the TAF sites. If one of these showers or storms move
over a TAF site, a brief reduction in visibility to MVFR/IFR is
expected with the potential for strong wind gusts and hail.

Some question if we can get more low clouds over northern WI
tonight (MVFR/IFR) as flow is not as favorable as last night. But
with some recent rain, light winds, and some clearing skies,
ground fog will be possible as well. Recent models and MOS
Guidance bear this out as well that the potential for fog will be
there tonight. Continued to use TEMPO groups at all TAF sites for
the lower cloud/fog potential.

West to southwest winds will continue early this evening gusting
to around 10 kts, turning westerly behind the cold front, then
becoming light and variable overnight. Winds turn to the southwest
Tuesday morning, with gusts to 15-20 kts by around noon.

&&

.MARINE...
Issued at 135 PM CDT Mon Aug 31 2026

The persistent south winds will continue to produce 2-3.5 ft
waves on Lake Michigan into early this evening and a moderate
risk of rip currents along Lake Michigan beaches. With dew points
in the upper 60s to low 70s through Tuesday and water temps in
the upper 50s and 60s, some dense fog will be possible at times.
Any fog threat should end as winds turn offshore Tuesday night.

&&

.FIRE WEATHER...
Issued at 135 PM CDT Mon Aug 31 2026

Drier air, deep mixing, winds gusting to 25 mph and warm temps may
lead to elevated fire weather conditions in north-central WI,
especially Vilas Co., Tuesday afternoon. RHs should dip into the
upper 20s and low 30s during peak heating. After coordination with
surrounding offices and the DNR, no SPS will be issued as just
enough enough moisture from previous rains should keep any fire
danger subdued, especially with the latest fire danger in the low
category.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE.........Kurimski
DISCUSSION.....Bersch
AVIATION.......Kurimski
MARINE.........Bersch
FIRE WEATHER...Bersch

NWS GRB Office Area Forecast Discussion