I have been to WDW quite a few times now and have spent 69 days there in my life. All of those days being from the 2nd week of July to the 1st week of August. In all those times, I would say it has been totally dry for maybe 15 of those days, rained for less than an hour another 35 days, rained for 1-3 hours for 15 days and only rained the entire day maybe 5 days. I do remember going in mid-July of '95 and it literally rained and poured non-stop for 3 days straight. It was the only time I have been when the sun never actually came out the whole day. The highs were also super cool for that time of year, highs only hit the low 80's. And the lows actually dipped down to the high 60s.
Typical Summer weather for central Florida, is for an afternoon/evening thunderstorms to pop up. They last about an hour or so (normally).
Dump a lot of rain and move on or dissipate.
Often the percentage given on the weather forecast (30%, 40%) is coverage rather than a chance of water falling from the sky.
Its going to rain, but on 30% of us. The storms move around differently from day to day depending on the sea breezes. You can look at a dark sky on the horizon and never feel a drop of rain, or you could be getting wet in 10 minutes.
Put a radar app on your phone, keep an eye on the clouds and bring a poncho or two. (extra shoes and socks or quick drying shoes are good too)
The rain helps cool things down a bit, but will make it very steamy.
We do occasionally get a more rainy week (this coming week looks to be that). In this case, its because of extra tropical weather that has come our way. Today we have a tropical storm dumping extra wet on us.
Just got back. Was there from 8/5-8/10. It rained twice. Once on 8/5 for about an hour, once on 8/8 for about an hour. Once the rain stopped it was good and steamy. The rain really didn't effect our plans at all.