El Nino or not, no one can predict Thanksgiving weather this early. Period.
Go prepared with rain suits and boonie hats and if it rains, stay dry and comfortable and enjoy the low crowds and short wait times a steady rain brings Disneyland guests.
We went last year, November 30th to December 7th. It rained almost non-stop for two days but we were dry to the skin in our Frogg Toggs Ultra-Lite2 suits
and waterproof Keens. I think we had at least 100 people soaked under torn, thin ponchos ask us where we bought them! These lightweight suits run $20-25 apiece and are made for adults and kids, plus you can usually find them at any hunting and fishing outfitter. Seek out a retailer with a return policy so you can buy them before the trip and return them (new/unused) if it doesn't rain while you're there.
http://www.froggtoggs.com/mens/all-weather-suits/the-frogg-toggsr-ultra-lite2tm.html
http://www.froggtoggs.com/kids/youth/yout-ultra-lite-rain-suit.html
Don't worry if rain will cancel fireworks. High winds are common in the fall/winter and will cancel fireworks on otherwise pretty days. You just gotta go with the flow!