My first trip to WDW was in September... it was hot, but about the same as August in Toronto (if that's any help to you)... say low to mid 30s and humid. ; Plus there was the rain (it didn't rain all the time we were there, but when it rained, you knew it meant it!).
But, the parks were relatively empty... hardly any lines to speak of! ; And from what I've read over the years, WDW is a peach to deal with if a hurricane hits Orlando in terms of courtesy offerings and compensation.
I will say this... we had some tense moments on the way back. ; This was 1998, while Georges was wandering between South Florida and Mississippi (second worst storm that season... although luckily it didn't hit central Florida with more than the outer edges)... our flight had to make an unscheduled 3 hour layover and then re-route to try to get around the storm... we ended up getting home about 6 hours later than we expected. ; The seasons have gotten significantly worse since then, and the experts are predicting this season will be worse and longer than last season... so although Disney's a peach to deal with, and folks in Florida are well versed in hurricane prep, and airlines make every effort to ensure that their passengers and planes are safe, you might want to take it into consideration, since it may mean delays and/or a shortened or extended trip.