Biggest concern is hurricanes. We went last year right after Irma. Were lucky to not have our trip cancelled. Crowds were still heavy enough to need FPs and ADRs. We talked to several locals who took advantage of the hurricane to plan a last minute trip. Some were without power at home, so they came to Disney on AP discount. As stated above, WDW is never NOT crowded. But, September is about as good as it gets. Temps are typically in upper 80s, humid. We are going again this year 9/5- 9/9. Enjoy!