Greetings from Florida and welcome to the sunshine state!!!
My question for you is, where are you from and what kind of weather are you accustomed to?
I really dislike the “what is Florida weather like” questions, because they are almost impossible to answer. As I always say, “Florida is the only place you can watch three different weather forecasts and they all can be wrong.” Currently, outside my window it is sunny, about 90 degrees, very bright with not many clouds. I have no clue why I am at the computer and not the pool or beach!
Truth is September is still hurricane season and part of our rainy season. Which means our weather can change rather quickly. September is still a very hot month. But, if there is any tropical activity it can be very rainy, dark and humid. Overall, the average Sept day at Disney is going to run you about 90 degrees with not much wind in the parks and about 50% humidity during the day. Later on it may get as low as mid to low 80’s in the evenings. If we get a cold front dropping through we may see mid-80’s in the day.
Plan for it to be very warm in September. Depending on where you are from, more than likely you’ll never need pants. You’ll probably want to wear lots of light colors, comfy shoes and as light of clothing as possible. Even if it does rain, and the weather cools off a bit… During the day the humidity will be worse than the actual temperature. So, I’d say dress light, bring a pair of pants in case you do a dinner or a restaurant that calls for them. But, more than likely, it’ll be warm here.
My best advice is to pack for it to be warm. Keep an eye on weather.com and the ten day forecasts starting a week or so before your trip and feel things out for yourself.