272
FXUS63 KGRB 282315
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
615 PM CDT Fri Aug 28 2026

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Rain probabilities range from 60 to 90 percent on Saturday, with
  widespread rainfall amounts generally between 0.20 and 0.40
  inches possible.

- Hazardous boating and swimming conditions on Lake Michigan from
  late Saturday night through Sunday evening.

- Becoming warmer and more humid late in the weekend into early
  next week with thunderstorms possible at times. While severe
  weather is not anticipated on Saturday, it is too early to
  determine if any of the storms would become severe from Sunday
  through Tuesday.

&&

.DISCUSSION...
Issued at 124 PM CDT Fri Aug 28 2026

The latest RAP analysis and satellite/radar imagery show surface
high pressure centered over the Great Lakes region early this
afternoon, maintaining quiet and dry weather. Observations across
northern and central Wisconsin indicate partly to mostly sunny
skies with scattered fair weather cumulus, light and variable
winds, and temperatures generally in the 70s to lower 80s. Looking
upstream, satellite imagery reveals mid and high clouds streaming
across the Dakotas and Minnesota within a region of stronger warm
air advection and mid-level shortwave energy. As the high
pressure shifts east toward the New England coast tonight, the
upstream shortwave trough and associated warm front will track
into the Upper Great Lakes, bringing the next chance for
precipitation by Saturday.

The initial forecast focus is on a progressive shortwave trough
and weak surface warm advection pushing into the region on
Saturday. High pressure shifting east tonight will allow mid and
high clouds to gradually thicken and lower from west to east
overnight. Patchy fog may develop late across far northeast
Wisconsin if skies remain clear long enough, but no widespread
travel impacts are expected.

By Saturday morning, increasing isentropic ascent will push a
band of rain into north-central and central Wisconsin, spreading
into eastern Wisconsin by the late morning and afternoon. Forecast
soundings indicate a relatively saturated column, but most-
unstable CAPE values generally under 100 J/kg will limit
widespread thunderstorm coverage. While deep- layer shear is
adequate, the lack of robust instability keeps the severe weather
threat very low, though an isolated rumble of thunder cannot be
ruled out. In general, widespread rainfall amounts of 0.20 to 0.40
inches are anticipated. There is a 30-50% chance of rainfall
greater than 0.50 inches over central and east-central WI (lower
probabilities further north). The widespread precipitation and
cloud cover will hold daytime high temperatures in the lower to
middle 70s. Most of the steady rain is expected to exit to the
east by late Saturday night.

Heading into the latter half of the weekend and early next week,
the synoptic pattern transitions as upper-level ridging builds
across the eastern CONUS and the surface warm front slowly lifts
north of the area. This setup will place northeast Wisconsin
firmly in the warm sector by Monday, with a considerably warmer
and more humid airmass in place. High temperatures Sunday through
Tuesday will return into the 80s, with surface dewpoints rising
into the upper 60s to lower 70s.

The increasing heat and moisture will result in a notable jump in
instability, with MLCAPE values forecast to build between 2000
and 3000 J/kg during peak afternoon heating on Monday. Despite the
robust instability, precipitation chances appear relatively
limited Sunday and Monday as significant mid- level capping and
weak large-scale forcing will largely suppress widespread
convective initiation. However, if the capping inversion can be
breached by subtle shortwave energy riding over the ridge,
isolated to scattered strong thunderstorms would be possible given
the thermodynamic environment. Trends in thunderstorm potential
will need to be monitored in the coming shifts.  A slowly
approaching cold front from the northwest will bring slightly
higher probabilities for showers and thunderstorms toward the
Tuesday and Wednesday timeframe, before a drier airmass and more
seasonal temperatures return to the region later in the week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 615 PM CDT Fri Aug 28 2026

High pressure will maintain VFR conditions with light winds under
10 kts through tonight. Fair-weather clouds above 5 kft will
dissipate early this evening, followed by thickening mid and high
clouds overnight. Patchy fog is possible over far northeast WI
late tonight, with minimal impacts to the terminals.

Rain chances increase Saturday morning as a weak shortwave and
warm advection move in. Rain arrives at KRHI/KAUW/KCWA between
10Z-12Z Sat, reaching KGRB/KATW/KMTW around 14Z-16Z Sat. MVFR
ceilings and visibilities will develop in steady rain with IFR
ceilings possible across north-central Wisconsin, all highlighted
with TEMPO groups. Thunder chance remains 

NWS GRB Office Area Forecast Discussion