It usually will rain in the late afternoon, if it's going to rain. And it's always nice because it clears out the park and only lasts for 30 mins tops! It's usually 10 degrees cooler after the rain too.
We did get shooed out of the water parks on our two days there, due to lightning. Since it was so close to closing time, they just closed up shop. I think it was like 4:30 and they closed at 5. But we had the WPnMore tickets, so we didn't arrive until 3:30, so it was a bummer to happen to us both times! Forewarned is fairwarned!
Have fun. Definitely don't wear white due to the lovebugs, and do pack a light long sleeved shirt of some kind for the restaurants. It's just as hot as August, IMO.