751
FXUS63 KGRB 251200
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
700 AM CDT Fri Jul 25 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Small chance (15-25%) of pop-up showers and storms on Saturday.
  Greater chances for showers and storms late Sunday through
  Monday, with the potential for strong storms.

- Turning hot and humid again Sunday and Monday. Heat Advisories
  may eventually be needed.

&&

.DISCUSSION...
Issued at 331 AM CDT Fri Jul 25 2025

Short Term...Today...Tonight...and Saturday

Frontal boundary has settled south of the area early this morning
with high pressure drifting across northern WI/Upper Michigan.
The clearing skies and light winds with nighttime cooling has
resulted in fog and stratus forming early this morning. Pockets of
dense fog are observed. If the dense fog becomes more widespread,
will need to consider issuing a short term Dense Fog Advisory. Fog
will not stick around long, diminishing by 8-9am.

Rest of today will be mostly sunny and less humid (especially north
where dewpoints will be in the comfortable 50s), though still warm
as highs reach the low to mid 80s. Canadian smoke that is advecting
into the north already will continue to spread over the rest of
the area through the day. Based on upstream obs, likely will see
some reduction to visibility and some locations could smell the
smoke as has occurred in recent interludes with the smoke. Air
Quality Advisory remains in effect through 11 PM tonight. For
tonight, high pressure shifts over the northeast, but still will
extend a ridge back to the western Great Lakes. Dry weather now
looks to hold through the night. Lows may drop into the 50s again
over the north. Patchy fog is possible again and though not as
widespread as today, areas of smoke will occur in some locations.

Next wave lifting north (emerging out of the convection over the
central plains this morning) has certainly trended farther south,
with some models not even bringing QPF to even the southern forecast
areas on Saturday. Suppose that could be a little too aggressive
as it seems keeping a mention of very isolated pop-up shower or
storm during peak heating is prudent with the wave moving through
given MLCAPEs around 1000J/kg and convective temps in the mid
80s. Highs on Saturday will be past that, well into the 80s with
even some upper 80s expected in the corridor of central WI to far
northeast WI (Rapids to Waupaca to Shawano and Wausaukee). Heat
index values may reach around 90 in these areas as dewpoints will
quickly be rebounding once the Canadian high overhead today is out
of the area. Still dry Saturday night with lows struggling to
fall much below 70 for the Fox Valley with mid to upper 60s
elsewhere. This will set stage for a very warm Sunday.

Long Term...Saturday Night Through Thursday

Temperatures/Heat (Sunday-Monday)...Shortwave ridging will
progress across the western Great Lakes on Sunday, with
accompanying southwesterly low level winds, drawing a very warm
and humid air mass back into NE WI. The combination of heat,
humidity, and periods of at least partly cloudy skies will result
in elevated heat indices the 90s both Sunday and Monday afternoon.
With low level warm advection ongoing from Saturday night through
Monday, expect the aforementioned heat indices will gradually
build Sunday through Monday, resulting in the most oppressive
readings of the period on Monday. Current forecast shows some
potential for advisory level heat indices (100 or greater) on
Sunday and Monday, particularly in the Fox Cities, with Monday
having the higher potential. Uncertainty remains if these values
will materialize due to cloud cover and a potential passing
system. Regardless of whether or not any heat headlines are
issued, the Sunday-Monday heat & humidity will have the potential
to be impactful, particularly for those planning to be outdoors
for prolonged periods of time. Plan to drink plenty of water, wear
light-colored & loose-fitting clothing, and take frequent breaks
in shaded or air-conditioned areas if heading outside Sunday
through Monday.

Precipitation/Thunderstorm Chances (Sunday-Monday)...An embedded
shortwave riding a nearly mean zonal flow is progged to track east
across southern Ontario Sunday into Monday night. With a hot and
humid air mass entrenched, plenty of instability will be in place
ahead of the approaching shortwave and associated surface
boundary, supporting the return of showers and storms sometime
between Sunday night through Monday. As a result, models are
showing signs of a potential MCS developing somewhere over the
Upper Mississippi Valley. However, the precise location and timing
of the MCS development remains uncertain as models differ
significantly. Regardless of where it develops, strong to severe
storms will likely be associated with the MCS given sufficient
deep layer shear and instability, and steep mid-level lapse rates.
Any storms will also be capable of producing heavy downpours
given 1.25-1.75"+ PWAT values pooled ahead of the system. Will
continue to monitor trends and make adjustments in the coming
forecasts.

Rest of the extended...Models hint at additional chances for
showers and storms from another embedded shortwave sometime
between Tuesday and Wednesday, but timing and location greatly
differs. Following this feature, quite the pattern change sets up
with high pressure spreading into the area from the north
associated with an upper-level northwest flow. This will result
in a dry forecast and much more comfortable conditions to end the
week. Anticipate highs in the mid-upper 70s, with overnight lows
generally in the low-mid 50s. A few locations could dip into the
40s near the Wisconsin-Michigan border.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 649 AM CDT Fri Jul 25 2025

Smoke and fog will be the main impacts during this TAF period.
Fog across the area is already starting to lift since 10z. Expect
improvement of VSBY to be followed by improvement of CIGS. Any
lingering bkn stratus deck will scatter out at all sites 14z-15z.
Few to Sct cumulus will develop into the afternoon with skies
becoming mostly clear tonight with increasing high clouds from
the south late. Winds will be light through the period.

Surface based smoke over northern WI will expand to the rest of
the forecast area this morning. VSBY will be reduced to 3-5sm
through much of the day/evening. Some hint in modeling that smoke
will diminish later tonight over central and north-central WI.

.OSH...
IFR/low MVFR stratus persists through 13-14z before scattering
out to cumulus by 15z. A brief period of VFR vsbys may occur this
morning, but smoke and associated MVFR vsbys should arrive in the
late morning and remain for the rest of today through tonight.
Winds will be light northeast and turn to the east by late morning
at no more than 10 kts. Winds become light again tonight.

&&

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

&&

$$

DISCUSSION.....JLA/Kruk/QUIGLEY
AVIATION.......JLA