Ditto the above posts. Layering is the key. Even if it's cold outside, wear a jacket or sweatshirt over a t-shirt because the attractions will be VERY warm. If you can't take off your outer layer, it'll feel like a sauna!
We had similar experiences to the above. We spent 6 days the first week of Dec (this was several years ago). First day was rainy in the morning and early afternoon, temp in the 70's-80's. The next 4 days were absolutely beautiful, but COLD (like 30's-40's). The last day, back up to the 70's, nice during the day, thunderstorm at night.