742
FXUS63 KGRB 091817
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
117 PM CDT Thu Jul 9 2026

Forecast discussion for routine afternoon forecast issuance

.KEY MESSAGES...

- Scattered showers and a few thunderstorms will come to an end by
  early this evening over parts of central and east-central
  Wisconsin. Severe storms are not expected.

- Patchy to areas of ground fog are forecast to develop late
  tonight into Friday morning, which may result in localized
  travel impacts due to reduced visibilities.

- Building heat and humidity early next week will bring a period
  of minor heat-related impacts, with heat index values forecast
  to rise into the middle to upper 90s on Monday and Tuesday.

&&

.DISCUSSION...
Issued at 116 PM CDT Thu Jul 9 2026

A weak surface cold front is currently draped across central and
east-central Wisconsin early this afternoon, acting as the primary
focus for scattered convective activity across the region. North
of this boundary, high pressure and a much drier airmass are
already filtering into northern Wisconsin. Looking at the broader
synoptic setup, water vapor imagery reveals an intensifying mid-
level ridge centered over the western United States. This ridge
is forecast to amplify and expand northeastward toward the western
Great Lakes over the weekend and into the early part of next
week, becoming the main feature to impact the weather across
north- central and northeast Wisconsin for the remainder of the
forecast period.

The near-term forecast through this evening centers on the
scattered shower and isolated thunderstorm activity along the
slow moving front. The latest RAP analysis and satellite/radar
imagery show the boundary stretching from near Wisconsin Rapids to
Appleton and Kewaunee. Despite widespread cloud cover, a
reservoir of boundary layer moisture and surface temperatures in
the middle to upper 70s are generating mixed-layer CAPE values in
the 500 to 1000 J/kg range. This environment will continue to
spark scattered showers and isolated storms through the late
afternoon before the front fully sags south of the area. Given
weak forcing, severe weather is not anticipated; however, high
precipitable water values will allow any storm to produce heavy
downpours and lightning. Most of the shower activity will quickly
come to an end early this evening with the loss of diurnal
heating.

As the frontal boundary pushes south, focus turns toward fog
potential late tonight. Drier air settling behind the front will
mix out dewpoints into the 50s across far northern Wisconsin,
thereby lowering the risk for dense fog. However, surface
dewpoints will be slow to mix out across central and east-central
Wisconsin this afternoon. Clearing skies and light winds under
the building surface ridge will create optimal radiational cooling
conditions, favoring the development of ground fog late tonight.
The highest probability for dense fog resides across central
Wisconsin, though east- central locations will likely see patchy
fog as well. Any ground fog will burn off relatively early between
8 AM and 9 AM Friday, giving way to a fair-weather cumulus field
by late morning. Friday afternoon will feature pleasant summer
conditions across the region with highs in the low to mid 80s and
comfortable humidity levels.

For the extended period, the weather story is dominated by the
arrival of the upper-level ridge and a subsequent warming trend.
As the ridge builds closer over the weekend, a weak boundary is
forecast to sag into northern Wisconsin, bringing a low 20 to 30
percent chance for scattered afternoon showers and thunderstorms
on both Saturday and Sunday. By Monday and Tuesday, very warm
temperatures aloft, with 700 mb temperatures climbing to 13C to
14C, will establish a strong capping inversion that will shut down
precipitation chances. Projections of 925 mb and 850 mb
temperatures support surface high temperatures expanding into the
lower to middle 90s on both Monday and Tuesday. The NBM
initialization shows dewpoints in the middle to upper 60s during
this timeframe, which looks to be on the lower side of guidance.
Bumped dewpoints up slight based on non-bias corrected data. This
will yield widespread heat indices in the middle to upper 90s,
bringing a threat for minor heat- related illnesses. A pattern
shift toward the middle to end of next week is expected to break
down the ridge and bring cooler relief.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1205 PM CDT Thu Jul 9 2026

A weak cold front is dropping south from roughly Wisconsin Rapids
(ISW) to Appleton (ATW) late this morning. A mix of ceilings
ranging from IFR to VFR exist along the front and across much of
central, east-central, and far northeast WI. Skies are clearing
across far northern WI as a drier airmass moves in the low
levels.

Widely scattered showers and isolated storms will be possible
along and ahead of the boundary through 00Z. The IFR/MVFR ceilings
are expected to gradually rise, but MVFR ceilings could persist
through 20-22Z along the front. Brief downpours could drop
visibilities to IFR in the heaviest shower activity.

Skies are anticipated to clear tonight which will set up a
favorable ground fog scenario. The most favored area for ground
fog will be across central WI and parts of the Fox Valley where
dewpoints will struggle to mix out this afternoon. LIFR/IFR flight
conditions will be possible in the ground fog from 08-13Z Fri.

Fair weather clouds are expected to build after the fog burns off
later Fri morning.

Light winds are expected over the TAF period.

&&

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

&&

$$

DISCUSSION.....MPC
AVIATION.......MPC

NWS GRB Office Area Forecast Discussion