we've done this the past two summers - once at poly and once at WL/YC.
Weather - I recall that the mornings are usually beautiful. As the day progresses, it gets very hot and eventually pours. Two years ago, I remember the rain lasting for hours. But I understand that was a bad year. The rain stops by evening. Hint: I like to carry a totes mini umbrella with me so I never get stuck.
My advice - get to the parks as early as you can. Its not that hot and you can see a lot before the crowds start. If you are not going to arrive at the gate for early-entry opening, I recommend going to a park that does not open early that day - though the new pattern of early/late hours may change this. We like to get there before opening (with a bottle of water in hand) -and go around for a while. By 12-1 or so, I can't wait to go swimming so we head back to the hotel (by then its really hot). We wear tevas on our feet (good for the heat and in case it rains), and light clothing.
If you get out early, then if it rains in the afternoon, you will still have had a great day. Its nice to spend the afternoon at the resort - as the whole trip ends up with much more of a vacation feel. Plus, we would go back to a park in the evening. The late openings may really be great here (have not tried it).
Crowds: In late august, the crowds are quite manageable as much of the country has started back to school already. We think its a good time to go.
Enjoy your trip! missy