The main issue that could affect firework shows is the actual initial loading of the fireworks into the launch tubes during the daytime. If the weather is so bad that the show cant be loaded, it wont run. It's my understanding from talking to IllumiNations crews, that as long as the show is loaded, it will run, regardless of the weather (expect of course real extreme weather hitting during the show). Illuminations had a record of something like 15 years of uninterrupted shows, all the way through until the wild fires cancelled shows.