When I went in 2002, they were experiencing a cold snap (wore sweatshirt, pants, & glovesin the morning & evening). Being from NE, I already had these items, but I found many people standing around freezing in shorts & t-shirts, then dashing to the stores to buy warmer clothing for their entire family.
When I went in 2003, it was cool in the morning & evening but during the day I was in a short-sleeved shirt & shorts and swam every day.
I'd go prepared for either side of the weather coin. Bring some lighter clothing, but also bring (or wear) a long sleeved shirt, sweatshirt & gloves. You just never know & better safe than sorry.