The rain has been quite unpredictable these days. We were in WDW from last Sat. until this past Wed. We got very lucky and we only had a little light rain on Mon. morning. However as we left to drive home on Wed afternoon, the skies opened up and I know it had to make it's way over WDW, it was pretty bad driving for a while. This was due to a tropical wave moving it's way over FL.
By Dec 1st the tropical weather pretty much hangs it up until next June. Then the rains that come through are usually due to cold fronts. Sometimes they pass quickly & other times they linger.
When we were at WDW last Dec (early in the month), it rained the first night and the next day it was really chilly and drizzly! By late in the day the rain moved out but it got cold, but by the next day it was beautiful and sunny, so you really never know. In December you have to be prepared for everything.