It can get warm in Chicago in September, but not usually this warm. The forecast today calls for 34°C with a dewpoint above 21°C, the epitome of the worst July weather we get here.
The culprits are the tropical systems currently destroying islands in the Atlantic. A dome of unseasonably warm air has stalled over the eastern US and Canada, because Jose and Maria are dumping energy into the air right off shore.
So, today's temperatures will be 12°C above normal and will likely surpass the record 33°C set in 1970. Same tomorrow. Saturday and Sunday will also be hot but probably not quite as hot—though, I have to say, 31°C still really sucks the day after the September equinox.
The gross weather pattern should clear out by Wednesday. I really hope so; I want autumn.
Update: Yesterday's 33.3°C high also broke a record (32.7°C set in 1933).