Be ready for rain
Actually September is one of the drier months, UNLESS summer hangs on.
September is fair season up here. Other than Labor Day weekend, shouldn't be hard to get a site as kids are back in school, although it seems locals are camping later each year. Weather is usually warm, dry days, w/cool nights. As others have stated, the further north you go the colder it will be. Usually! Right now there's some serious flooding going on in some areas-hopefully there won't be much damage. Still a lot of snow to melt.