Ah, spring has arrived along with another 'Eek! I see a forecast of rain!!' thread.
First, rain often only lasts a short time in FL. Second, temps after rain are often high preferable to the temps pre-rain. Rain also clears guests from the parks.
You'll also see many folks donning plastic at the first sign of a drop. Some folks continue to wear 'em hours after it stops. We find the best approach is simply to hang in an alcove for ten minutes- unless we are already late getting some place. Often 10-15 is all you need to see sun again.
In 2013, we visited WDW in August. Almost by accident, we wore long pants on the plane. We were SO glad we did. The AUGUST nights were SOOOO cool we happily wore long pants! Multiple times we've been swimming in August where all of a sudden, late afternoon temps take a dive. Our one water park visit, 2pm temps were fine. By 3pm, we were huddled low in the pool to keep warm (water temps were still nice, but air had dropped, breezes kicked up.) another trip, we were shivering at the top of the BC pool slide!
Another important point we find is that 90 in MAY is NOT the same as 90 in August. By August, most of us are acclimated to heat. In late spring, some folks are and some are not acclimated. One of the best ways to prepare for a summer trip- is to acclimate your body to summer heat. It takes up to two weeks of exposure for your pores to open. We've learned this lesson first hand. The worst trip we ever took heat-wise was in May.
That said, if high heat and afternoon storms are an issue- one simple way to survive is afternoon breaks. Maybe hit the pool, maybe go to the room, or hang out in any resort lobby/gift shop/restaurant or show for an hour or two. After our break, we're good to go until park closing!