It's usually the daily downpour mid afternoon - soon passes and clears the crowds for half an hour, there's occasionally a heavier shower - apart from that it's usually about 30 c. (Can't wait!)
Last several trips we have gone in September. We go for about 10 days and in all honesty, it rained maybe twice each time we went. There is a few small showers here and there, but no deluge of rain at all. Most times when it starts spitting rain, we go in a ride, and when we come out its sunny again. The cloudy times are great, cuts down the heat a bit. Very humid in September. But I wouldn't worry about the rain. We are going the 12th-20th and the only fear is a hurricane coming in. But even then, it's the end of the season. So I wouldn't worry about the weather. You should be more concerned about the humidity!!
We will be there the week before and I am preparing myself for rain. We went two years ago the very same week and it stormed every night for a couple of hours. Other than that, the weather was great. It was hot and humid, but I can deal with that.