When we were at OKW last August (our first time during the summer), there was rain in the afternoons on a couple of days. The showers were scattered and didn't last long - but the clouds/lightening/winds could be pretty intense. We were at the pool one afternoon when it rained off and on and, since there was no lightening, were able to just stay in the pool - after all, you're wet anyway!!! It was actually kind of fun. On another day, we were on our way to AK when it started pouring just as we arrived. People were leaving the park in droves and we hesitated about going in. Since we had portable umbrellas with us, we decided to go (although people leaving looked at us like we were crazy). It turned out to be great. The rain didn't last long and the park was nearly deserted. We were able to do quite a bit in the two hours before the park closed - we rode Kali River Rapids seven times!!! Again, there were scattered showers, but we were wet anyway. The next afternoon that showers were forecast, we deliberately went to AK and had the same great time (and got the same weird looks as we were leaving when everyone was coming).
Bottom line, even if there is rain, you can still have a great time. Enjoy!!!