If it's bad enough, the parade and outside shows may be cancelled. In the past, they have allowed refunds or exchanges on nights that were bad where most of the events are cancelled. December 2002 had several rainy nights. Not just showers, torrentail non-stop rain.
But read the back of your ticket. It says that there are no refunds. So when they made refunds, they were really not required to do so. We have been to both parties in downpours. Life goes on.