September is hot; however lower crowds make it bearable. Expect rain, but on a good note, rain does help clear out the parks even more as someone else stated. On our last September trip, I think we ran into rain twice during the entire week and it didn't slow us down a bit. Our first day in the parks was an MK day and it rained steadily all day, but we put ponchos on and kept on going and it was one of our best days ever. MK was basically deserted!!! I find that Disney is more bearable than other theme parks in the heat because many attractions are inside..so that helps. Just come prepared. Pack sunscreen, hats for shade, plenty of water, ponchos, extra shoes and socks (incase in rains and gets your other's wet), etc. You'll be fine.