Hi,
You can expect the temperatures to be extremely high, could be just under or over 100 degrees.
Getting to the theme parks early and generaly visiting at cooler/quieter times of the day are essential.
Many only visit in the summer holidays because it's the only time they can go.
Sorry not trying to put you off, just giving an honest appraisal, you can go in August and have a fantastic holiday if you follow the usual guidelines, but unfortunatley isn't the ideal time to go if you struggle in the heat.
Visiting theme parks at cooler/quieter parts of the day, wearing loose comfortable clothing, taking advantage of the air-conditioned indoors, drink plenty of water, have a mid day rest back at your villa/resort..are some of the standard tips for hotter times of the year.
Storms are frequent, usually they are short afternoon heavy downpours but predicting the climate these days can be more difficult as the weather pattern seems to change every year.
With Florida climate being the way it is there is always a degree of uncertainty - whatever happens you will be in for a great time I am sure.
A lot of the above to a degree depends on your tolerance to the temperatures, some love it and some cope with it as best they can.