HI Michael,
Rain, August and Disney seem to go hand and hand! We routinely vacation in Disney in August and my husband and I can always tell the time at Blizzard Beach without a watch. If there is a thunderstorm, it is 2:00 pm.
The two water parks, Blizzard Beach and Typhoon Lagoon, are more luckily to close than other rides in the four major theme parks. Next is Animal Kingdom because the animals are the attraction and they need to be safe. (One tip-visit this park after the rain because there are no crowds and the animals are out and playing.)
Rides such as Big Thunder Mountain, Splash Mountain, Tower of Terror and Test Track will close if there is thunder. Many of the other rides will operate so pack your poncho or umbrella and keep on touring.
Unfortunately, the rain rarely cools things off for long so drink plenty of water and eat a cool snack to help you stay hydrated in the parks.
Wishing you a magical vacation!
Beth