I think the fact that a lot of days have been upper 80s to low 90s is causing wild disparity from place to place, but that Milwaukee number is crazy low. It's been a hot summer so there have been a lot less cooldown days then you would expect in a typical summer, but also, the amount of extreme hot days has been way less than most of our hot summers, especially the ones the first half of the 20th century.
Detroit (DTW) has had 14 days of 90+ this year. The average in a year is 12, so that is nothing impressive consider how warm its been. Certainly another month until we're out of the woods, but I don't really see any heat building.
Detroit city airport, which sits closer to the water, has only had 11 days this year. It was the official weather station during those extremely hot summers of the 1930s-50s. So it really makes me wonder, those years when they were getting 30, 35 days of 90+, how many would inland present day DTW have gotten!!!???
The DTX NWS office, in Detroit's far northern suburbs, rural and high in elevation, has only had 4 days. Yet Flint, which is North of there but actually closer to the NWS office than DTW, has logged 18 days. Nearby to FNT is Saginaw, which only has 13 days. And then Toledo, OH has a whopping 27 days.