823
FXUS63 KGRB 300804
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
304 AM CDT Mon Jun 30 2025

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- Patchy areas of dense fog are possible this morning, mainly
  across central and northern WI.

- Scattered thunderstorms possible this afternoon. Gusty winds
  and large hail are main hazards.

- Temperatures will be near to above average this week.

- Round of showers and storms possible Friday into Saturday may
  impact 4th of July celebrations.

&&

.DISCUSSION...
Issued at 303 AM CDT Mon Jun 30 2025

Fog/Showers This Morning...

RAP analysis shows a weak short-wave over central WI early this
morning. This wave is causing some isolated light showers across the
region. As this wave pushes east expect the shower activity to
decrease after sunrise.

Behind the showers were skies have become clear an area of dense fog
can be seen by satellite over much of Wood and western Marathon
counties. Will continue to monitor web-cams and surface obs to see if
the fog becomes more widespread over the next few hours. If that is
the case a Dense Fog Advisory may be needed for a few hours this
morning.

Afternoon/Evening Thunderstorm Potential...

RAP analysis and WV imagery show a short-wave digging across
Minnesota early this morning. This wave will move east across
Wisconsin during the day along wind an increasing 500mb jet. CAMs
show an initial round of storms developing over east-central WI early
this afternoon just ahead of the short-wave along a surface cold
front. RAP soundings show MLCAPE values around 1000 J/kg during this
time along with 40-45 kts of 0-6km shear. These parameters could
support a few isolated stronger storms capable of producing up to
quartered sized hail and 40-60 mph winds. The wind threat will be
primarily related to any downbursts as updrafts become rain cooled as
mid-level winds are fairly weak (15-20kts). It is worth noting that
the better risk for severe weather has shifted south where better
moisture and instability will reside. CAMs also now show a right rear
jet quad further south over Milwaukee and Chicago.

A second round of showers and storms may also be possible this
evening across north-central and northeast WI behind the initial
front as cooler mid-level air overspreads warmer low-levels. Hail
appears to be the main threat as wet bulb zero heights and the
freezing leave lower in the cooler air mass. Elongated hodographs
due to the strong upper level winds also support a hail threat with
any any taller updrafts that can access the strong shear.

Rest of this week...

Persistent northwesterly flow aloft in response to a building ridge
over the northern Rockies is expected through the middle portion of
the week. This should lead to mostly dry conditions Tuesday through
Thursday, however, any subtle short-waves moving through the flow
could spark a round of showers or storms. At this point confidence
in timing out any of these short-waves is very low so kept the NBM
10-20% pops for this period.

Next period of potentially more impactful weather looks to arrive
Friday into Saturday. Global ensembles indicate Gulf moisture
returns as low-level winds become southerly and dew points increase
into the middle 60s to low 70s. Increasing instability ~ Friday 1000-
1500 J/kg and weak isentropic ascent could initiate a few isolated
storms during the afternoon, however, deep layer shear is expected
to be weak, less than 25kts, during the period. Ensembles then bring
a cold front across the region late Friday night into Saturday
morning which could set off another round of showers and storms.
Confidence in the timing of this feature is still very low, but will
be watching trends very closely given that Friday is Independence
Day.

Temperatures through the week and into next weekend should hold
steadily near to above normal with highs in the 80s most days.
Friday looks like the hottest day of the week with a few locations
in central WI and the Fox Valley possibly getting to the 90s.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1141 PM CDT Sun Jun 29 2025

Slight chance of showers and perhaps an isolated storm the rest of
the night as cold front slowly presses into the region. Greatest
chances for a shower will be over northern WI and near Lake Michigan
including at MTW. Once clouds break overnight, cooling of the still
moist airmass will result in patchy fog 07z-12z. Most sites will
see temps fall below the crossover temps, so have went with VSBY
as low as IFR in the fog. Conditions generally VFR on Monday with
cu developing late morning onward. A few pop-up showers and storms
may occur late afternoon into early evening as primary upper level
trough crosses the region. Have added PROB30 groups to the TAFs
after 20z to cover the thunderstorm potential. Strongest storms
will be isolated, but could produce gusty winds and hail. Storms
will diminish and end by late Monday evening.

&&

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

&&

$$

DISCUSSION.....GK
AVIATION.......JLA