I would wait until tomorrow and see what the rain bands are doing. I think it is very unlikely it will sell out with all the trip cancellations. It might be a really great night for the party. If they cancel you will get a refund, but my fear will be that it would be rainy and they will go ahead with the party but it might be wet and no fun. If you don't mind being at the party in the rain (possibly pouring rain at times), then you could go ahead. It could be a great low crowd party if the rain holds off.
What we experienced as Matthew got closer was bands of rain. It would rain as a band moved through- then stop or really slow down for a while- and then another rain band would come through. As Matthew got closer the bands got heavier. This thing is giving me deja-vu--- right now it seems like it is close to following the same path as Matthew did. Feels like we are kind of watching the same progression as when we were at WDW last time watching coverage constantly. Of course-- that could change at any moment. Hopefully, it will just skirt everywhere and not be a direct hit anywhere.