938
FXUS63 KGRB 271149
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
649 AM CDT Sun Jul 27 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- 20-40% chance for thunderstorms this evening over far northern
  Wisconsin. Another round of storms is possible late this evening
  and overnight mainly over central WI. Some storms may become
  severe, with damaging winds as the primary hazard. Additional
  thunderstorms are possible Monday through Tuesday night, some
  of which could be strong.

- Hot and humid today and Monday. A few locations will see heat
  indices reach 100 today. More widespread heat indices around 100
  are expected Monday, when a Heat Advisory may be needed.

- Drier, cooler, and less humid conditions return later Wednesday
  through next weekend.

&&

.DISCUSSION...
Issued at 428 AM CDT Sun Jul 27 2025

Short Term...Today...Tonight...and Monday

Thunderstorm chances and heat are the primary focuses in what is
shaping up to be a complicated short term.

In the near term it is a warm, muggy morning with patchy fog. Fog
has not been widespread as our area missed out on showers Saturday.
Smoke aloft moves in through the day which likely will hold down
max temps from what they could be with full mixing. No surface
smoke impacts are expected. Incorporated slightly cooler guidance
to account for this. High temps this afternoon mid 80s to lower
90s with expected dewpoints in the lower 70s resulted in heat
indices mainly below 100F. Held off on any Heat Advisory headline
today as a result. In terms of storms today, we`ll have to watch
far northwest WI late today as convection may redevelop on tail of
shortwave that is causing robust storms from North Dakota into
northwest MN this morning. Not agreement here though as some
models, including CAMs, restrict this to late evening at the
earliest.

Next question is how convection evolves that forms on the edge of
mid-level capping and extreme instability (MUCAPE of 4000-5000
j/kg from MO/IL into much of MN) this evening. Low-level jet
aimed into MN will result in initiation of storms, but it is VERY
unclear how far east these storms will track late evening into the
overnight hours. Generally favored a NAM/HREF idea (supported by
06z HRRR) which rides the CAPE gradient over western WI, but
other guidance keeps complex of storms farther southwest and/or
well to the north tied more to the shortwave crossing MN/Lake
Superior and less tied to the higher instability. Effective shear
of 30-40 kts and ample instability could certainly lead to severe
storms. 0-1km shear seems sufficient to maintain cold pool balance,
allowing an even elevated complex of storms to produce damaging
winds at the sfc, especially given the extent of instability
present. Not all models show that though. Some just completely
dampen out the complex of storms all-together while other guidance
track it all the way into IL on Monday morning, so there is that.
Ultimately, this still looks like mainly an issue for north-central
to central WI and less of issue farther east. At least that idea
is holding right now, but given the pattern we are in, even that
could be subject to change.

Whatever convection is around first thing on Monday should quickly
shift south/southeast out of the area. Most models are then
benign with pops/thunder chances most of Monday morning. Afternoon
development may occur due to building instability with very warm
and humid airmass in place as MLCAPEs rise to 2000-3000j/kg. Also
could stay capped in subsidence behind the late night storms.
Maybe a bit better signal that tail of shortwave along with lake
breeze convergence may lead to higher pops over northern Bay of
Green Bay/far northeast WI mid to late afternoon. Some of this
activity if it develops could try to build more toward Green Bay
area given the instability that will be available. If storms
develop on Monday afternoon, they could be severe with effective
shear over 40 kts and more than sufficient instability.

Unless convection and associated cloud cover is much more widespread,
Monday will see highs reach the upper 80s to lower 90s again, this
after an adjustment downward from full mixing potential due to more
elevated wildfire smoke. No surface smoke, impacts are expected.
Dewpoints are slightly higher, though not looking to be as high
as what was shown a few days ago. Regardless, looks like more of
the area (especially Waushara to Waupaca counties into parts of
the Fox Valley, far northeast WI) will see heat indices reaching
upper 90s to around 100. Heat Advisory could be needed, but there
is still some uncertainty regarding convective trends as outlined
above. Will let later shifts issue if it still looks needed.
Through the next two days to beat the heat, be sure to stay
hydrated by drinking water, even if you don`t feel thirsty, and
take frequent breaks in the shade or in air-conditioned locations
if spending time outdoors.

Long Term...Monday Night Through Saturday

Precipitation/Thunderstorm Chances...A zonal upper-level flow will
bring embedded shortwaves to the region through Wednesday before
evolving into a northwest flow midweek. Precip chances will be
focused with any embedded shortwave energy interacting with a
lingering warm/moist air mass and surface features.

The first embedded shortwave of interest slides east across southern
MN Monday evening into WI late Monday night into Tuesday, with an
eventual MCS developing. The better forcing and severe parameters
will be focused further to the west of the forecast area; however,
there is potential the MCS will still have some fuel (sufficient
elevated instability, steep mid-level lapse rates, and deep-layer
shear) as it enters central WI late Monday night/early Tuesday
morning. While there are some models indicating the MCS will track
eastward across the forecast area through Tuesday morning resulting
in showers and storms across almost the entire area, other models
indicate the MCS will drop southeastward instead, which would leave
the majority of the forecast area dry, except along the far southern
portions of the CWA. Either solution, the MCS would be following
along the CAPE gradient. Determining when and where convective
initiation develops and where the CAPE gradient sets up will be the
main features to watch. Unfortunately, this is another case of
predictability being low until observed trends are apparent.
Additional showers and storms may develop Tuesday afternoon and
evening, but coverage would be more isolated/scattered.

The second embedded shortwave of interest appears to be slightly
further south than the first, tracking east/southeast across IA late
Tuesday night into the WI/IL area Wednesday morning/afternoon.
Trends continue to indicate the MCS that develops with this
shortwave should remain mainly south of the forecast area, including
the severe potential. However, portions of central and east-central
WI could see some rain from being on the northern periphery of the
thunderstorm complex on Wednesday.

Finally, the upper-level pattern becomes much less active after
Wednesday, evolving into a northwest flow with surface high pressure
spreading over the region from the north. The high pressure will
dominate over the region through next weekend, resulting in dry
weather and plenty of sunshine.

Temperatures...The warm/humid air mass will gradually exit the
region Tuesday with the departure of the morning MCS. However,
temperatures will remain on the warmer side with highs in the low to
mid 80s, but with less humid conditions. As the high pressure
gradually slides into the region on Wednesday, temperatures will
lower even more with highs in the mid 70s to low 80s. These
temperatures will continue through the remainder of the week and
into the weekend. Beginning Wednesday night, overnight low
temperatures will also be much cooler with readings mainly in the
50s, but a few spots in far north-central WI could see values in the
40s.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 648 AM CDT Sun Jul 27 2025

Shallow fog, locally dense, will diminish by 13-14z. VFR today
with just some scattered cu. Smoke aloft will make the sky hazy.
Winds will be light yet again. Main concern late today will be
if storms developing over northwest WI make it to RHI. It is
uncertain, but close enough to include PROB30 group. Another
potential for thunderstorms will occur overnight from north-
central to central WI. Have addressed this low-end potenital
with PROB30 groups at RHI, AUW and CWA. At this point, it does not
appear that showers and storms will make it farther east through
end of the TAF period. Some more fog may form late tonight at
GRB/ATW/MTW and eventually RHI.

.OSH...Fog has not developed yet and think this risk is done.
Balance of day will feature scattered cu this afternoon and
increasing high clouds. Smoke aloft will result in a hazy sky.
Light winds through tonight. Possible that light fog (MVFR vsby)
may develop late tonight. Any showers and storms that occur are at
this point expected to remain west of OSH through end of TAF
period, but this will have to be monitored.

&&

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

&&

$$

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