We had this happen to us. We were there for Halloween 1997...the weather was unbelievable---rain, rain, rain, and the thunder and lightning that picked up around 3pm was CRAZY!! We left the park for safety reasons, and ditched the party, even though we (7 of us) had tickets to the party.
We called disney ticketing, they told us they would not be refunding any money for bad weather, but they also offered us an address, I cleverly wrote a letter to Mickey and Minnie, apologizing for missing their party, realizing we RSVP'd and all, but because mom and dad felt the weather was just too dangerous for us to attend....blah blah blah, pass the kleenex.....you get the picture. Asked for the refund, enclosed the tickets, wished them well, and looked forward to being invited next time we came to see them! We got the refund for all the tickets.....I guess its all in how you approach it.
We are finally going to the party this year.....praying for good weather.....