Hi, we have been in Sept 4 times (anywhere from the 15th through the end of the month). From our experience it is still very hot and humid (not unusual for 90's with 80+ humidity. But, like everyone says, drink, drink, drink, take breaks and if you are on the dining plan that really seems to help get you out of the heat and rest. Last year when we went, I can remember at least 3 days of our trip feeling unbearable but it is Disney so you make the best of it.
Have fun.