897
FXUS63 KGRB 211101
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
601 AM CDT Tue Jul 21 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Cooler and less humid today with gusty northwest winds reaching
  20 to 30 mph. Small craft advisories are in effect through early
  Wednesday morning.

- Scattered showers and an isolated thunderstorm will spread across
  northeast and east-central Wisconsin late this morning through
  this afternoon. No severe weather is expected.

- Comfortable and quiet conditions through Friday, followed by
  increasing heat, humidity, and thunderstorm chances (30 to 50
  percent) over the weekend.

&&

.DISCUSSION...
Issued at 140 AM CDT Tue Jul 21 2026

The latest surface analysis and satellite/radar imagery show a
primary cold front exiting northeast Wisconsin early this morning,
while a secondary cold front extends from western Lake Superior
into central Minnesota. Upper-level troughing is settling across
the Upper Midwest and Great Lakes while an amplified ridge builds
over the Intermountain West. This pattern leaves the region under
cool, cyclonic northwest flow aloft today into midweek. Upstream,
a broad area of stratocumulus with bases between 1500 and 3500
feet exists behind the secondary front, with moderately strong
cold air advection dropping 850mb temperatures into the lower
teens Celsius. A high-pressure system across the central Plains
will gradually shift eastward toward the Mid-Mississippi Valley
through Wednesday, eventually anchoring stable and comfortable
weather over the Western Great Lakes through the end of the work
week.

As the secondary cold front drops southeast across northern
Wisconsin today, mid-level height falls will cool temperatures
aloft into the tenth percentile. Strong daytime heating will
steepen low-level lapse rates, producing diurnal instability in
the 300 to 600 J/kg range across northeast and east-central
Wisconsin. Scattered showers currently confined to far north-
central Wisconsin will expand southward through late morning and
afternoon. An isolated rumble of thunder cannot be ruled out
within the stronger convective cores, but severe weather is not
expected due to weak instability. In addition, deep boundary-
layer mixing will tap into stronger winds aloft, generating gusty
northwest winds of 20 to 30 mph across the forecast area this
afternoon. These winds will generate hazardous waves in the
nearshore waters, prompting Small Craft Advisories on the Bay of
Green Bay and nearshore waters of Lake Michigan into early
Wednesday morning. Showers will quickly wane late this afternoon
and evening with loss of heating. Clearing skies tonight will
clear the way for a sunny and comfortable Wednesday with high
temperatures in the mid 70s.

Models show minimal potential for near-surface smoke or
visibility reductions, so smoke has been excluded from the
forecast.

For the long-term period from Thursday through Monday, the western
Great Lakes will remain positioned downstream of the broad ridge
building over the Intermountain West, maintaining northwest flow
aloft through Friday. This persistent northwest flow will keep the
main reservoir of deep instability suppressed well south of the
region, ensuring seasonable temperatures and pleasant humidity
levels. Highs through Friday will reside in the upper 70s to low
80s under mostly clear to partly cloudy skies. By Saturday and
Sunday, the upper-level flow pattern begins to flatten, allowing
Gulf moisture and an unstable airmass to surge northward. High
temperatures will climb into the upper 80s to near 90 in central
Wisconsin on Sunday, accompanied by rising dewpoints in the upper
60s. Precipitation chances will gradually increase this weekend
(30 to 50 percent chance) as shortwave impulses embedded within
the flattening flow interact with the returning instability. While
machine learning guidance indicates only a subtle severe weather
threat at this time, details on timing and mesoscale organization
will need to be monitored in upcoming forecast updates.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 600 AM CDT Tue Jul 21 2026

Upper troughing will settle over the western Great Lakes today as
a secondary cold front drops southeast across the area. Beneath
this cyclonic flow, convective clouds are expected to become more
widespread with daytime heating this morning. Cloud bases will
start out in the MVFR range over north-central WI, and in the high
MVFR to low VFR elsewhere. As instability builds, scattered
showers and perhaps an isolated thunderstorm are expected to pass
across the area from late morning through the end of the
afternoon. Added a tempo group for showers at RHI and a prob30 at
GRB, ATW, MTW.

Clearing skies and diminishing winds are forecast this evening.
VFR conditions are therefore expected from about 00-03Z through
the end of the taf period.

Northwest winds will become breezy today with gusts of 20 to 25
kts at the TAF sites. These winds should subside during the
evening.

.KOSH...

Clear skies will start out the period, but expect clouds to build
with daytime heating this morning. Brief MVFR ceilings will be
possible, but should quickly rise to VFR by 16Z. Scattered showers
are likely to develop across the area so added a prob30 group for
the most likely time period. There is a very low chance of a
thunderstorm and left it out of the forecast. Clearing skies are
forecast this evening.

Northwest winds under 10 kts at the start of the taf period will
increase by 14-15Z. Gusts up to 25 kts will be possible from late
morning until early evening before subsiding.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion