It was a really hot summer in Cleveland that year also. A few years back, I was getting acquainted with new co-workers and discovered that several of us had children that year and we all remembered being pregnant during that hot summer. It was brutal.

From IMDb trivia
Most of the filming was done during the summer of 1988, which was one of the hottest summers on record in Milwaukee. This is easily apparent in the final playoff game, where most of the players are wearing long sleeves and jackets to indicate a cool fall night, whereas almost all the fans in the stands are wearing short sleeve shirts and shorts.

So you're saying Milwaukee is warmer than Cleveland at the end of September?

And it was filmed in Milwaukee.

The average temperature in September is in the 70's. People in the midwest hold on to summer as long as possible so yes many people would still have shorts and other summer clothes on.

Maybe they were having a bout of Indian Summer.

A journey into the realm of the obscure:
http://saturdayshowcase.blogspot.com/

I went to Indians / Yankees playoff game in October 2007 and wore short sleeves. It was a night game and really warm.

hopefully it is during the world series!
The food I've liked in my time is American country cookin'-Colonel Sanders

It can be pretty warm in Cleveland in September.