619
FXUS63 KGRB 271058
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
558 AM CDT Mon Jul 27 2026

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Severe thunderstorms capable of producing damaging winds and
  isolated large hail are possible this morning, and again across
  northeast Wisconsin this afternoon. A Severe Thunderstorm Watch
  is in effect until 9 AM.

- Heat indices are forecast into the middle to upper 90s across
  central Wisconsin this afternoon.

- Gusty northerly winds will build waves from 3 to 5 feet on Lake
  Michigan south of Two Rivers this evening into Tuesday
  afternoon.

- A cold front passing through the region will bring a shift to
  northerly winds and relief from the high humidity for Tuesday
  through mid-week, with lower thunderstorm chances until late in
  the work week.

&&

.DISCUSSION...
Issued at 245 AM CDT Mon Jul 27 2026

The latest RAP analysis and satellite/radar imagery show a surface
low pressure centered over northwest Minnesota. A cold front extends
southwest across eastern South Dakota, while a warm front extends
southeast and across far northern Wisconsin. Elevated convection
continues across parts of northern Minnesota and northern Wisconsin
early this morning, situated on the cool side of the warm front.
The environment remains conditionally favorable for severe weather
across the region, with MUCAPE in 2000-3000 J/kg range and strong
deep-layer shear of 40 to 50 knots. While storms have struggled
to intensify over northern Wisconsin so far, a Severe Thunderstorm
Watch has been issued until 9 AM.

Low-level warm advection ahead of the southeast sliding surface
low is expected to generate additional storm development early
this morning. Most guidance continues to depict an increase in
storm coverage and eventual clustering or organization into one or
more squall lines, capable of producing damaging winds and
isolated large hail. While a very low probability, a weak spin up
along a more organized line segment is possible given favorable
0-3km wind shear. Confidence remains low with the evolution, as
the CAMS have a relatively poor track record over the past 24-36
hours.  These storms will likely exit northeast Wisconsin by mid
to late morning.

A lull in thunderstorm potential is expected late this morning
through early afternoon following the departure of the morning
convective cluster. Rapid destabilization is then forecast during
the afternoon under strong heating. Upwards of 2000 to 3000 J/kg
of CAPE could build near peak heating, primarily over east-central
Wisconsin. Deep-layer shear of 50 knots will support rapid storm
intensification if storms redevelop. The most probable window for
severe storms is from 2 PM to 6 PM.

Very warm and humid conditions will again push heat index values
into the middle to upper 90s across Wood, Portage, Waupaca, and
Waushara counties this afternoon. Heat indices in these areas may
briefly flirt with 100 degrees but duration will be short (1-2
hours). Therefore no Heat Advisory will be issued.

Behind the departing surface low, north winds will intensify
tonight with gusts of 20 to 25 mph. Skies will clear from north to
south, leading to a mostly sunny Tuesday. While temperatures will
remain near to slightly above normal on Tuesday, a much drier
airmass will bring welcome relief as humidity levels fall into the
lower 40s in the afternoon.

Long Term...Wednesday Through Sunday
High pressure will stick around into Wednesday, keeping conditions
dry and quiet across north-central and northeast Wisconsin.
Numerical models have trended slightly quicker with warm advection
returning late Wednesday, resulting in more ambitious shower
development for the second half of Wednesday through Thursday. The
next chance for widespread rain and thunderstorm activity will
arrive on Thursday night and linger at times into Saturday. Upper-
level winds are forecast to remain relatively light, which should
keep the severe weather risk low. Temperatures will remain at or
slightly above normal next weekend with high pressure building
back into the western Great Lakes.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 558 AM CDT Mon Jul 27 2026

A warm front and surface low over northern Minnesota will bring
potential for thunderstorms today. Clusters of showers and
thunderstorms extending from the Arrowhead of Minnesota into
northeast Wisconsin continue to move southeast along the warm
front. The primary threat through 17Z will be across far
northeast Wisconsin, particularly along the Upper Peninsula
border. If storms continue to organize, potential will increase
for hail and strong wind gusts north of a RHI to SUE line. Other
storms over northeast WI should exit by shortly after TAF
issuance. MVFR ceilings expanding along the front over northwest
Wisconsin could drift into KRHI and AUW for a few hours this
morning. Otherwise, predominant VFR conditions are expected
across central and northeast Wisconsin terminals.

A general lull of the thunderstorm activity is expected from late
morning through early afternoon before additional isolated to
scattered storms redevelop between 19Z and 23Z along and ahead of
the surface low. The highest probability for afternoon convection
will focus over east- central Wisconsin, potentially impacting
KGRB, KATW, and KMTW. Confidence on timing and spatial coverage of
showers and storms remains low, so PROB30 groups were added.
Behind the low, winds shift north to northeast with gusts to 20
kts. A period of MVFR ceilings is possible at all terminals this
evening before clearing to VFR after 08Z.

Outlook: High pressure brings widespread VFR conditions and
lighter winds Tuesday into Wednesday.

.KOSH...
Scattered to broken clouds with ceilings 3.5 to 4.5 kft could
impact OSH this morning, which eventually evolve into a cumulus
field with daytime heating. Low confidence that any storms will
impact OSH this morning.

As the surface low moves across northern WI this afternoon,
concern increases for thunderstorms development. Forecast
soundings show ample dry air that should keep coverage isolated.
Added a prob30 group for the most favored time frame for showers,
but held off adding thunder. Confidence is low due to the dry air.

Behind the low, winds will shift to the north or northeast and
become gusty for a time. Gusts to 15-20 kts seem reasonable. The
north winds could also bring a brief period of MVFR ceilings
before skies clear overnight.

&&

.MARINE...
Issued at 245 AM CDT Mon Jul 27 2026

A cold front sagging south across the region will bring a risk of
severe thunderstorms to Green Bay and Lake Michigan this morning
and again this afternoon, posing a hazard to mariners. Behind the
front, a tightening pressure gradient will cause northerly winds
to increase to 15 to 25 knots tonight into Tuesday. Waves will
build to 3 to 5 feet on the Lake Michigan nearshore waters south
of Two Rivers, and a Small Craft Advisory will be issued. Winds
and waves will gradually subside Tuesday afternoon as high
pressure builds in from the north.

&&

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

&&

$$

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

NWS GRB Office Area Forecast Discussion