We've gone many times late August also. I prefer it to any other summer time to go! The crowds are very low in my opinion (my sister's kids go back to school in August, but here on the East Coast my kids also don't go back until after Labor Day). Yes it is hot, but it's hot at home! You may have storms, but they tend to last a very short time. If a rainstorm is coming (nice to have a mobile radar app on your smartphone), we just duck into somewhere!
Last time we went in August, I actually ended up going to Target to get sweatshirts. Evenings can get chilly, and I like to have a hoodie to throw on (or when the air conditioning is blasting, as it is in some places).
You won't have such late hours for parks sometimes, but that's fine with me!