Go early. It usually rains in the afternoon. The hot sun heats up the ground, moisture goes up into the athmosphere, and it rains. I'm down here in South Florida, and it just started raining at 01:00pm. From the early morning, to this time, there was no rain.
What happens, is that anytime between 2-6pm, it can pour, but it usually lets up after a few hours, and the rides open again. If there is no lightning, the rides will continue to run.