349
FXUS63 KGRB 042339
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
639 PM CDT Fri Jul 4 2025

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Although most of the region will remain dry, isolated
  thunderstorms are possible through early this evening. A strong
  storm is possible that could produce small hail and gusty winds.

- A better chance for widespread rain and storms will occur
  Saturday afternoon and Saturday evening. A few severe storms are
  possible that could produce damaging wind gusts. Heavy rainfall
  is also possible that could produce localized urban and low-
  lying flooding.

- Very warm and muggy conditions will result in heat indices in
  the middle to upper 90s over central and northeast Wisconsin
  through early this evening, and over eastern Wisconsin on
  Saturday. Those with outdoor plans should take precautions for
  the heat.

&&

.DISCUSSION...
Issued at 207 PM CDT Fri Jul 4 2025

Satellite imagery indicates quite a bit of clouds across the
area, which has held down temperatures across much of the region
this afternoon. Despite a mid level ridge building across the
western Great Lakes region, there are some CAMs that develop some
isolated showers and thunderstorms late this afternoon into early
this evening. Given the abundant instability (2-3 J/g) across the
region along with an uncapped atmosphere and a plethora of outflow
boundaries will keep a low chance (20 percent) of showers and
thunderstorms across the region late this afternoon and into early
this evening. Despite the small chance for showers and
thunderstorms, there is a high confidence that much of the area
will be dry for fireworks events this evening.

Dangerous Heat Potential: Clouds have not really dissipated across
much of the region today, which has kept temperatures in check
for the most part. There are some areas across central Wisconsin
and the Fox Valley which have soared with some clearing, which
could easily spread to the rest of the area if the clouds can
dissipate. Any areas that clear out will easily hit the lower to
middle 90s for heat index values and should stay below headline
criteria.

Heat indices on Saturday are expected to hit the lower to middle
90s across east-central, northeast, and portions of central
Wisconsin ahead of an approaching cold front. These hot
temperatures should then tumble with the cold front or any showers
and thunderstorms that develop ahead of the front.

Severe/Heavy Rain Potential Saturday and Saturday night: A
positively tilted shortwave trough will push a weak low pressure
system northeast across the northern Plains and western Lake
Superior on Saturday. A cold front will then slide east across
north-central Wisconsin on Saturday afternoon and through the rest
of the area Saturday night. The chance of showers and
thunderstorms will rapidly increase during the early to middle
afternoon hours over north-central WI before spreading to
northeast Wisconsin late in the afternoon or evening. An unstable
airmass (1-2 J/g of instability) will remain present ahead of the
front. Deep layer shear of 25-30 kts and precipitable water
values near the climatological max will be present ahead of the
front. Deep warm cloud depths and downdraft capes to 800 j/kg will
support a heavy rain/damaging wind threat. HREF Probability Match
Mean indicate potential for 1-3" of rain over north-central and
central Wisconsin Saturday afternoon into the early evening and
increased rainfall amounts from the NBM. There will be a marginal
threat for severe weather Saturday afternoon and evening given the
aforementioned instability and shear with damaging winds the
primary threat. The threat for severe weather will wane later in
the evening and into the overnight hours with the loss of
instability as the event transitions to mainly a heavy rain
threat.

Low MBE velocities (under 10 kts) and high warm cloud depths (up
to 13,000 ft) will lead to efficient rainfall production and
potential for training cells. While latest rainfall forecasts are
not impressive, heavy rainfall ingredients tell a different story
so will continue to advertise in the HWO. Storms will taper off to
light rain overnight before ending late Saturday night or Sunday
morning. Some localized flooding will be possible given the
potential for heavy rainfall, especially for areas with recent
heavy rainfall like portions of central Wisconsin.

Rest of the forecast: Behind the front, temperatures will return
closer to normal through the end of the next week. Despite a
reprieve from the warm and humid airmass, a shortwave will be
moving across the region during the Monday night/Tuesday
timeframe, which could interact with diurnal instability upwards
of 1000 j/kg for a chance of strong thunderstorms. This will be
the next period to watch.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 639 PM CDT Fri Jul 4 2025

Scattered daytime cumulus clouds will diminish late this
afternoon and evening, with mid and upper level convective cloud
debris continuing to spread across the region. An isolated shower
or storm will remain possible through late evening, but chances
(under 20%) are too low to include at any one TAF site. An
approaching cold front will spread scattered showers and
thunderstorms from west to east on Saturday. Best chance for
storms will be in the afternoon, with decent model agreement on
timing, so have included TEMPO groups for the most likely 4-hour
window for storms, although some spots could see more like a 6
hour window for the storms.

South to southwest winds are expected through the TAF period, with
MTW seeing a southeast wind for a time this afternoon/evening as
the lake breeze has moved inland. Gusts to ~15 kts are expected
before sunset today and up to 30 kts on Saturday. Low end low-
level wind shear over central and north-central WI is expected
tonight, then will dissipate Saturday morning as surface winds
increase.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement from Saturday afternoon through late
Saturday night for WIZ022-040-050.
&&

$$

DISCUSSION.....Kurimski
AVIATION.......Bersch