First of all, it depends on where you are from, as to how many heavy clothes you should bring. If you live in the South and are not used to cool weather, you should plan to bring more warm clothes than someone who is from, say, New England.
Personally, I suggest bringing layers: t-shirts (including some longsleeved), sweatshirt or warm pullover sweater, windbreaker, rain slicker or poncho, and something like a vest or jacket made of fleece. If you want to spend the money on rain pants (which are made to be worn over jeans or other slacks), they are worth the investment, not just to keep you dry, but as windbreakers for your legs. And don't forget a hat or earband, sunglasses, rain hat or hood, camera (with plenty of film or memory sticks) and binoculars! Enjoy!
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