Well first and foremost - when are you going?? If it's anytime coming up, it will rain. It's just the season that FL is in right now. The park does not close for rain, some rides will, some will still operate in the rain. It's more of a safety for thunder/lightening issue than anything else. Disney does a great job keeping everything running as long as it's safe to do so. So if it does rain, you just grab your poncho and keep going. Or take a snack break and wait for it to stop. Either way, FP+ will have your back. If you decide to cancel your FP+ that day, you will be at the mercy of what is available at that time. Just note, if you decide to change all this on the fly, the app is very user friendly as are the kiosks inside the park. So for example, if you are having a great day at Magic Kingdom and your next FP is 2:40-3:40pm, it starts pouring down rain at 2:45pm, you can remove the FP+ and select another at a later time BUT and a big BUT it will show you what's available and that will be based on openings and what tier your ride is that you just removed. You can research the whole tiered system for FP by park and see how it all works. The other thing I will say is make sure you are familiar with the FP rules as it pertains to the rides and parks you will be at, the usage, etc. It's not hard but people do get very confused.