Try not to worry too much about the weather...it changes so quickly.
That being said, to say it rained on our party last year is the understatement of the year. We got there at about 6, got to our LTT ADR just as it opened up and the ocean fell from the sky. Figured we'd wait it out while we ate...nope. Rained til 10:00. We waited 45 mins for them to cancel the 1st parade. They cancelled the Villian's mix n mingle. They DID NOT cancel the fireworks. The 2nd parade ran, w/out the headless horseman.
Did we get drenched? Yep. Did we still have a fantastic time? Yep. After the fireworks we worked our way through the park...spent 20 minutes just hanging with the Mad Hatter as we were the only people in that area of the park. Rode the last ride of HM of the night with no one else on the ride.
They were offering refunds for the tickets that night, or you could go another night. We got enough out of the party so that I didn't feel justified getting a refund.
My goodness I got wordy...my short point is go, have fun...quit worrying.