I am from Denver, where the weather is predictably unpredictable. It can snow in the morning and be 70 degrees by 2:00 (this has happened. A lot.). The rule in these sorts of situations is LAYERS. If you know it is going to be warmer during the day and cool at night, wear a t-shirt, a long-sleeve shirt or sweater, and a hooded, zip-up sweatshirt or jacket. When it's warm, you can remove the zip-up sweatshirt/jacket and tie it around your waist while you walk around or wait in line. You can easily throw it on long enough to ride Dueling Dragons or Rip Ride Rockit without protest. And then you can wear it when it gets cool. If you think it's going to be warmer, skip the t-shirt or long-sleeve/sweater, and if you think it's going to be cooler, add both the long-sleeve and the sweater.