Last summer I was there from the 14th to the 21st. It was really bad since it rained once for like 15 minutes one day and was extremely hot the rest of the days (heat indexes in the 110's). 95 would have been a blessing. I believe it was 115 on our Epcot day. Gets a little warm walking on the blacktop. It did torrentially downpour for several hours on our last day with thunderstorms bad enough to delay our flight out for over an hour.
The year before, it rained every single day for about 10 minutes with one day of about 4 hours of rain.
It really all depends on what week you go and how the weather is that particular week. The weather in FL is not what I call predictable and stable.