We were there a few weeks ago...there was a thunderstorm that evening. They delayed the fireworks for about a half hour. I was talking with a cast member and they mentioned that in the 17 years they had fireworks at the Magic Kingdom, they only cancelled them once due to weather.
The park was open till 1am on the evening i was there, and they would have postponed up till that time...according to the CM anyways. She said the dessert party will stay open until the fireworks have ended.
If anyone has heard or experienced anything differently, please let me know so i can revise what i heard.