textproduct: Milwaukee/Sullivan

KEY MESSAGES

- Expecting hot and humid conditions through much of next week leading to hazardous/dangerous heat related concerns due to a combination of daytime heat indices in the 90s to above 100F at times, the first stretch of heat spanning multiple days of the summer beginning Monday, and not much overnight relief with overnight lows only in the 70s each night.

- Any heat headlines for southern WI for the start of the week will be taken day-by-day as factors such as daily storm chances and eastern shift in the upper-level pattern all will play a role in meeting heat thresholds.

- Chance for showers and storms for late Sunday afternoon into the evening with additional daily chances Tuesday through Friday.

SHORT TERM

Issued 232 PM CDT Sat Jun 27 2026

Tonight through Sunday night:

High pressure over the Upper Great Lakes this afternoon will continue to slide eastward overnight with east winds turning more southeasterly into Sunday. Tonight expect one more night of lows in the upper 50s to lower 60s. Will begin see the upper- level ridge axis begin to build to our west over the Upper Mississippi River Valley. As a result, expect warmer temps and higher dewpoints begin to advect into southern WI through the day Sunday. Southeasterly flow off of cooler Lake Michigan will still keep things milder along the lakeshore areas with highs in the mid to upper 70s, while areas inland and west of the Kettle Moraine look to warm into the lower to mid 80s by the afternoon.

Accompanying the start of the warm up will be a potential for a few showers and storms to develop in the afternoon and evening. Overall, much of the 12z CAMs hint at a decaying MCV from upstream convection to track across the Upper Midwest later in the morning through the afternoon and bring only some light and scattered shower to southern WI. There is enough of a signal with this activity to warrant lower PoP and thunderstorm chances (15-40%). However, will need to keep an eye on how things develop upstream and evolve overnight as cannot rule out that we could see a few more developed and stronger storms develop. The 12z models continue to hint that this decaying system will be moving into an environment with synoptic forcing as a mid- level ripple of vorticity treks across the area while low-level WAA builds northward along with a swath of 700 mb frontogenesis swings through. Pair with the increasing moisture and building instability approaching 750-1200 J/kg of SBCAPE toward southwestern CWA.

While there are synoptic ingredients favoring development, there remains a few factors that limit much of the development. The first being the strong capping inversion as seen on the model soundings where the strongest instability is progged to set up. The other factor will be the alignment of these ingredients as they may stay offset and not come together to actually develop much. While the aforementioned limiting factors paired with the CAMs is the most likely scenario, cannot completely rule out the MCV influence to spur additional development through the afternoon, especially if we see the upstream activity develop further south as hinted by the 12z NAMnest as well as the Canadian.

Wagner

LONG TERM

Issued 232 PM CDT Sat Jun 27 2026

Monday through Saturday:

The continued focus for the extended period continues to be the hot and humid pattern setting up over the region through the end of the week. Still seeing trends of daytime temps in the 90s with dewpoints in the 70s resulting in heat indices of 100F or more at times Monday through at least Wednesday. Nightly lows will also be a concern as models and ensembles prog 70s for much of the week. The combo of these conditions paired with the first hotter stretch of the season will likely lead heat related issues and concerns.

While the signal for this heat continues, there remains caveats with this pattern still being a few days out. First of these will be the upper-level ridge placement. Still expecting the upper-level heights to continue to build over the Upper Midwest for the start of the week, however the 00z-12z mid-range global models and ensembles have been trending the axis of this ridge a bit further east compared to previous days models. As a result, southern WI could become placed more along the peripherals or even within the southwesterly upper- level jet. Thus contributing to increased potential for impulses of upper-level vorticity maxes to over run the top of ridge and pair with the warm and humid conditions to bring daily shower/storm chances to area as hinted by the 12z GFS, ECMWF, and CMC model runs. Given the eastern shift in upper-level pattern and increased potential for daily shower/storm chances Tuesday-Friday, may ultimately influence temps/heat indices each day, especially for the later half of the week.

Overall, will be best to handle things day-by-day for next week as influence from previous day's activity will play a role as well, especially with heat headlines. Monday still looks favorable for over 100F heat indices as the ridge begins to build into the region and likely see the Heat Advisory threshold be met, but at this time looks to stay below 105F threshold for a warning. Tuesday continues to trend as the hottest day of the week with heat indices teetering between advisory and warning thresholds at this time, but this is when models suggest the eastward shift in the upper-level jet and ridge axis come more into play. These influences will then continue through the middle of the week with uncertainty growing each day Wednesday onward.

Regardless of the heat headlines/thresholds and storm chances, still looking at periods of daytime heat indices in the 90s and above each day along with warm nightly lows for multiple days through next week. Even if the not as hot side of the forecast pans out, these factors will be enough to create hazardous heat related issues across southern WI, especially for vulnerable populations without easy access to stay cool and those spending extended periods outside.

Wagner

AVIATION

Issued 232 PM CDT Sat Jun 27 2026

VFR conditions prevail through the evening and into Sunday as high pressure over the Upper Great Lakes slides farther east. Mainly looking at a wind forecast with breezy easterly winds this afternoon ease and shifting more southeasterly into Sunday. Will continue to monitor for the potential shower and storm development Sunday afternoon, but not enough confidence to add into the TAFs at this time.

Wagner

MARINE

Issued 232 PM CDT Sat Jun 27 2026

Lighter winds over the lake continue as high pressure over the Upper Great Lakes gradually slides east through the day Sunday. Will see winds begin to pick up as low pressure develops and deepens over the Plains. Will see more southerly winds over Lake Michigan for the start of week with occasional gusts of 25-30 knots as the hot pattern setups across the region through midweek. With these gusty winds, small craft conditions for southeastern and east-central WI nearshore zones will be possible overnight Sunday into Monday.

Wagner

MKX WATCHES/WARNINGS/ADVISORIES

WI...None. LM...None.