Usually it's the closest available ballroom. But I'm sure your EM will work with you as much as possible. We had asked about the WP as a back up, and our EM actually looked into it as an option (when I woke up to rain, and was panicing) but it was booked at the time I'd of needed. I don't know where I'd of ended up as it ended up turning sunny and beautiful, thankfully. But my EM was great the whole morning, calling with weather updates, and making sure I wasn't too worried over it until it was deffinatly drying off. So don't sweat it too much. Unless it's that morning like me. Otherwise you have months and months to pray, and ask Santa for sunny weather. Plus December is the lowest rain month in Disney (one reason I picked it too)
Melissa