NWS-Green Bay Forecast Discussion

						FXUS63 KGRB 092345
AFDGRB

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

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- 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 00Z TAF Issuance
Issued at 644 PM CDT Thu Jul 9 2026

VFR daytime cumulus clouds and other MVFR/VFR clouds associated
with a weak cold front will slowly fade / shift south through the
late afternoon and early evening hours, with mainly only a few
high clouds remaining across the region into tonight. A stray
shower can't be ruled out south of Hwy 29 before sunset as the
front drifts to our south, but the dry air looks to be winning
out, so most spots will stay dry.

A favorable set up for fog overnight as skies clear this evening,
winds remain light, and recent rain will provide some moisture
near the surface. 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/evening. LAV/MET/MAV hitting
AUW/CWA the hardest, with SREF/HREF/REFS also highlighting this
area across central WI. VLIFR/LIFR/IFR flight conditions will be
possible in the ground fog from ~08-14Z Fri.

On Friday, look for any fog to lift in the morning hours, with
daytime fair weather cumulus clouds developing in the late morning
and afternoon. Another batch of high clouds will also spread
across parts of the region. A stray shower will be possible,
mainly in the afternoon, but coverage looks to be under 15% so
will not include any mention. Winds will generally remain under
10 kts, with a few higher gusts possible closer to Lake Michigan.

&&

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

&&

$$

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