I have gone in August and am going again this year the 20-29. This was my experience

It will rain, and that's ok

It is short lived and did not affect us negatively. Though, we find humor in anything. We are very easy going. The only unfortunate incident was being on the parking tram and a WHAMMING downpour happened. We started Epcot soaked, but you dry quick and it was a great laugh!