I would be prepared for afternoon showers. That is the tail end of the rainy season, but best to be prepared than caught unaware. This year has been wetter than unsual (Tampa is about 16" ahead on rain for the year thus far). The showers are caused by the East Coast Seabreeze and the West Coast Seabreeze meeting in the middle. Usually that is the later part of the day, but depending on if there is a front coming thru or soemthing like that, the pattern could be reversed and rain in the morning. Unless something else is going on with the weather (a front, tropical system, etc.), the regular afternoon thunderstorms are not an all day thing. By late September they have tapered off and you should be fine. When they say scattered or isolated showers here, they mean it. It could pour on you, and not be raining at all literally across the street. Just bring a poncho and enjoy yourself. And be aware of the weather because the rain isn't the problem, it is the lightning that you need to worry about. That is a great time to come down here. We are going to be at WDW the 1st 2 weeks of Sept. and aren't worried about the rain (but then again we live here and are used to being aware of it). The good thing is, unless there is a front or tropical system, it doesn't rain all day here (like it does in other parts of the country), maybe a few hours and it clears up. Have fun!