The National Weather Service issued a severe thunderstorm warning for northern Cook County and northeastern DuPage County lasting until 10 p.m.
The service said a severe storm was spotted in Elk Grove Village at 9:15 p.m. and moving east at 45 mph. Wind gusts were measured at 60 mph.
The storm was expected to hit Mount Prospect, Des Plaines, Rosemont, Park Ridge, Wood Dale, Prospect Heights, Bensenville and other locations.
The service had let the heat advisory of the last couple of days expire. Just before 7 p.m. a temperature of 92 degrees was still recorded at O'Hare.
The AccuWeather hourly forecast doesn't show the air exactly getting cool tonight, with lows still in the mid-70s, but then the temperature climbs only into the 80s Saturday, with less humidity expected.
On Friday Metra officials said trains were running more slowly to ease stress on overheated tracks. And firefighters had to spray down bridges over the Calumet River because the steel was expanding in the heat.