Hot and humid, with sharp showers frequent from mid-afternoon. If you have a choice, there are better times of year to visit for the weather. But if you don't have a choice - school holidays, etc - then don't worry as the heat and humidity isn't unbearable. I'm not a great fan of very hot weather (it's often mid or high 90's) but I manage OK. The trick is to make sure that you take frequent breaks (the weather can be energy-sapping) and kids in particular should drink lots to avoid dehydration.
There are also advantages to August: the parks are often open later into the evenings, and there is usually a full roster of parades, entertainment and fireworks.
Regards
Rob