158
FXUS63 KGRB 210245
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
945 PM CDT Thu Aug 20 2026

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- There is a 50-80% chance of showers and storms from late
  afternoon through late tonight.  No severe weather is expected.

- Scattered thunderstorms (40-60% chance) are expected to
  redevelop over eastern WI between 3 and 9 PM Friday. Isolated
  strong to severe storms are possible (level 1 of 5 risk).

- Scattered showers and thunderstorms (40-60% chance) are
  expected over north- central to eastern Wisconsin on Saturday.

- Through Saturday, most areas will have potential to see at least
  a quarter inch of rainfall (60-80% chance).

&&

.DISCUSSION...
Issued at 124 PM CDT Thu Aug 20 2026

The latest RAP analysis and satellite/radar imagery show an area
of low pressure moving across northwest Minnesota with an
associated cold front extending south across the eastern Dakotas.
Warm advection and shortwave energy ahead of this system are
generating showers across northwest Wisconsin. These showers have
been weakening as they move into high pressure and a dry airmass
over eastern Wisconsin, where precipitable water values remain
around 80 percent of normal.

Warm advection ahead of the advancing cold front will continue to
spread east into the forecast area through tonight. Dry air in
the lower levels below 7 kft will delay rain reaching the ground
across north-central Wisconsin until late afternoon or early
evening. The main axis of instability is expected to remain west
of the region through peak daytime heating. Thunderstorms are
anticipated to develop in this instability axis later this
afternoon over Minnesota and western Wisconsin which will move
into north- central Wisconsin by mid to late evening while
gradually weakening. An isolated strong storm could reach central
Wisconsin late in the evening, but severe weather is not
anticipated tonight as instability wanes.

Shower and thunderstorm clusters will continue to move east across
the region overnight. On Friday, the cold front will traverse
eastern Wisconsin during peak heating. Following a lull in
precipitation late Friday morning to early Friday afternoon,
destabilization of 1000 to 1500 J/kg and surface temperatures
approaching 80 degrees will foster increasing storm coverage after
3 PM. Modest deep- layer shear of 20 to 25 knots will limit
organized severe weather, but DCAPE values up to 800 J/kg will
create a localized threat for strong downburst winds in the
strongest cells (hail will also be possible). This threat will
diminish by 8 to 9 PM over eastern Wisconsin as the front exits.

A brief amplification of an upper-level trough is forecast across
the Great Lakes region over the weekend. As a shortwave trough
moves through the cyclonic flow on Saturday, there is a chance of showers
and thunderstorms across north-central and northeast Wisconsin.
However, weak deep-layer shear will prevent significant storm
organization, keeping the severe weather risk low.

Through Saturday, most areas will have potential to see at least a
quarter inch of rainfall (60-80% chance). Scattered amounts
greater than 1/2 inch are possible (30-50% chance).

By Tuesday and Wednesday, another shortwave trough riding along
the northern stream is projected to move across the region,
bringing the next chance for showers and thunderstorms.
Instability appears modest with this mid-week feature, keeping
severe probabilities low.

Temperatures throughout the 7-day period will stay close to late-
summer norms, with daily highs mostly ranging from the mid 70s to
lower 80s and overnight lows in the upper 40s to lower 60s.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 945 PM CDT Thu Aug 20 2026

Main impacts to aviation during this TAF cycle will revolve around
rain/storm potential this evening through Friday afternoon. Rain
chances continue to increase across the central and north-central
sites late this evening, becoming more widespread overnight. Have
included prevailing -SHRA at RHI, AUW, and CWA where rain chances
are greatest, and have followed up with PROB30 group for -TSRA
for late tonight/early Friday morning. SCT showers should then
reach the east-central terminals by late Friday morning. Brief
lull occurs late Friday morning through early afternoon, with TSRA
re-developing (30 to 35% chance) along the cold front over east-
central Wisconsin during peak heating late Friday afternoon.

Cigs lower and thicken to MVFR, potentially IFR, late tonight
with the arrival of the showers. Conditions then improve to VFR
behind the front Friday afternoon.

South winds veer to north/northwesterly by Friday afternoon,
gusting between 15 and 20 knots at times.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion