August is always an "iffy" month weatherwise. You can definitely expect it to be hot and humid as that's pretty consistent, year in year out in FL.
As far as rain, just depends. It's very common for there to be lots or t-storms and rain in FL during this month although the last few days have been drier than normal, but that can quickly change. Bring ponchos or be prepared to buy some.
Many people are used to the FL rain/t-storms lasting 15 minutes and hitting later in the day. This was pretty common in the past but in the last few years, the rain storms have been lasting longer and have been hitting earlier in the day. We were up there last month and we had 2 days when the rain started by 12:30-1 and pretty much went on all afternoon. The next day it rained later in the afternoon and lasted about an hour or so. (when it didn't rain--it was BLAZING hot!!)
Summer in FL, you never know, just have some back-up plans if you don't want to do the parks in the rain. Some rides will be shut down if there is lightening in the area & if you have a waterpark on your agenda I would get there at opening.
The first few days of August have been drier than normal but with Chris floating around in the tropics he may bring a better chance of rain to the state over the weekend and early next week. (sometimes the tropical systems suck out some of the rain in the area too).