Officially, Disney won't just let you cancel if the weather is poor. But...I have often seen them offer kindness, all you have to do is ask. You're asking for a special favor, but you never know. Happened to us once, we really felt bad about asking since we knew up front exactly what our side of the bargain was, but they were great about our "special" circumstance. One of the times that has made us such loyal cruisers.
However...Not too many cold days, and rainy doesn't last long and doesn't hurt too much. Think positive and it will work out great!