Definitely, prepare for rain and/or wet, sloppy snow. Make sure you have waterproof boots. In this kind of weather, I usually bring one set of footwear for the tailgate, then change socks and boots before I head in.
I like to have one extra layer I can put on in the stadium.
Gloves, winter hat are a must.
We tailgate with a heated trailer, so we keep all our outer gear inside there and it's toasty when we head into the stadium. You can also keep it in your car and run the heat before you get dressed to head in.