I agree with other posts that if the forcast is not for today or tomorrow, ignore it. Go and have fun and don't worry about the rain. It normally rains every day but clears out pretty quickly.
This will be our 12th trip to Disney World and we normally go during hurricane season. We have only experienced HORRIBLE rain that canceled a parade once and that was last year and it was the last 2 1/2 days of our trip and our
MNSSHP 
.
It had started raining around the time the party started at 7, by 7:30 it was raining really heavy, they announced that the 8:35 parade was canceled due to heavy rain and lightning, about the same time they announced the cancellation, the skies opened up and we were in a torrential downpour with horrible lightning, it was so bad that we were wading in water about ankle deep trying to leave the park. They ended up canceling the party at 9:20pm and as everyone was trying to leave the CM's were telling everyone to see their concierge at their resort and replacement tickets would be given. It was so gracious that Disney replaced the tickets, unfortunately, there wasn't another party during our trip. When I explained that to the concierge, she asked if we were coming back in 2015, I told her that we were coming back in 2015 but in November, so instead of MNSSHP tickets, she gave us replacement tickets good for the
MVMCP for this year

The next day, it POURED down rain again, we spent the day at Epcot at the F&W festival and it was empty but hard to get to the booths and then find a dry place to eat. That night we had O'hana reservations and I wish you could have seen us trying to get there. We left our resort and headed to MK, we couldn't get out from under the awnings to get to the monorail for about 45 minutes and by the time we tried to get to the monorail to the Poly, in that area that is really low by the boat dock outside MK, we had to take off our shoes and we were wading in water midway up our thighs.

It was crazy!
But this is definitely not the norm.