If it's more than a drizzle you can do a lot in the park!! It doesn't empty but seeing as us Californians have a deep distrust of anything falling from the sky that is not rays of sunshine a decent amount of people will be in hiding. (Now, the outside rides will close in a good rain so keep that in mind!)
You're still rather far out to have a halfway decent weather read, but at least you won't find yourself dashing inside to buy $10 plastic bags, uh- "ponchos", like I did on my trip to WDW a couple weeks ago!