Please don't sweat it (figuratively or literally). If the worst happens and you end up visiting during a week of record heat or torrential rains -- and my family as done both -- that doesn't have to stop you from having a great time. Pack your cooling towels and your "good" ponchos and umbrellas (and quick-dry footwear), and your sense of adventure, and plan your touring so that you're hitting rides with outdoor queues during the coolest/dryest parts of the day. The vast majority of WDW's attractions and transportation are covered and climate-controlled, anyway.