BF and I were there last year 9/10-9/14. We were supposed to be there a week earlier but Frances decided to come to town so we had to change our plans and ended up there the following week. The crowds were pretty much non existant! It was wonderful. I don't know how much of that had to do with Frances and Ivan (on his way) but we didn't wait in line for a singe ride (even the big ones), most of the time we were the only 2 people on disney busses, the only full bus we were on was after illuminations at epcot. The pools at POR were EMPTY! the foodcourts were EMPTY! DTD was EMPTY! It was great! It seems like a good time to go because most schools are just back in session and a lot of families don't take kids out of school that soon into the year.
As for the weather...it was HOT and HUMID!! It rained every day at about 4 for about 15 minutes to cool things off but that was it. We made a daily trip back to the hotel in the heat of the afternoon to jump in the pool, cool off, rest, shower and then head back out to the parks. We loved it. I admit, there were times I thought I was going to DIE from heat exhaustion but when those times hit, we'd duck into an air conditioned shop or building for a few minutes to cool off and then move on.
We're already planning on our next trip for this year...hey, maybe we'll be there at the same time as you. My bday is 9/9 and I've decided that I want to make that our yearly "kim's bday" trip!