In 2001 we hit what all the locals called an unusually warm spell for December. We never broke out the jeans. Days were down right hot, evenings were warm enough to swim in the volcano pool while watching the EWP at the Poly! Unfortunately, I found out that right after we left (literally the next morning), the temps. dropped something like 40 degrees! Bottomline, you gotta pack it all. For my kids I pack as follows: 2 pairs of jeans, 4-5 pairs of shorts, 6-8 t-shirts, 2 tank tops, 2 pullover sweatshirts, 2 hooded zip sweatshirts, lined windbreaker, swimwear and the sweat suit/fleece set they wear on the plane. I had planned on wearing shorts by day, jeans at night. WRONG. We were so hot, on some days we'd go back and change into a fresh pair of shorts. I ended up having to do laundry, which I had never done during a vacation. It wasn't that bad and ended up making things work out pretty well. Plan for anything!