In addition to considering wearing a jersey for the added sun protection, you should consider bringing along a wicking long sleeve.

If it *does* rain, or get cold unexpectedly, long sleeves and an extra layer are wonderful. I wore a short-sleeve jersey under a wicking REI long sleeve through a rainstorm that turned into sleet -- the outer layer got soaked, but thanks to the miracle of wicking, the underlayer and my bod were warm (relatively) and dry.