Try not to let the weather forcasts get you down. Florida weather is really so unpredictable, the forcasts are usually so vague or inaccurate. Unless we have a tropical depression of some kind hanging over the state the rains usually don't last all day. (we're not into the tropical season just yet).
Actually, you probably don't want to hear this but it's better that there is rain than none. This time last year there were a lot of wildfires in central FL which really had a bad effect at WDW, really bad smoke in the air. I thought we were going to have to cancel our trip then but it luckily cleared before we got there.
The afternoon storms do start about this time of year. Just bring some ponchos so you don't get caught in it, but really the best place to be is in the parks (with your poncho), when it rains. There are many places to take cover and really the rains don't usually last all day long. September is usually the wetest month and sometimes the rains then can last several hours but it's still not an "all day" affair (unless it's a tropical system).
So GO AND ENJOY YOUR TRIP!! HAVE A GREAT TIME!