Breathe
You are going to WDW

You'll have a great time, rain or shine.
We have been to WDW in Aug/Sept nearly every year since the 1980s and we have weathered (pun intended) several tropical storms.
Fay was the worst (lingered for several days) and Ernesto was practically nothing (just 1 day of rain).
Part of travelling to Florida between May and November is dealing with the threat of hurricanes and tropical storms.
Personally I find the forecasting laughable now, it is almost like tabloid journalism! This "storm" is still off the coast of Africa and may not even develop into Danielle.