Two of those occurred in April 1952 when the last four days of the month recorded high temperatures of 85, 90, 87 and 90. The real standout warm April day occurred on April 22, 1980, when the temperature soared to 94. On that same day, Milwaukee set its all-time April high record at 91, having reached 90 only one other time, on April 10, 1930.
Summer potential really arrives in Madison (and Milwaukee) in May when record highs above 90 have occurred on 17 of the 31 calendar days (15 in Milwaukee). Interestingly, from May 26 through Sept. 21 there is no calendar day on which the record high temperature in Madison is not at least 90. For Milwaukee, a similar interval stretches from May 24 through Sept. 24.
These unbroken streaks of days with the potential to exceed 90 are probably consistent with most people’s sense of the boundaries of the warm season in southern Wisconsin and so actually serve as decent operational definitions of summer for both cities.