What to wear when meeting my boyfriends parents?

My boyfriend invited me to a family gathering on July 4th and I have no idea what to wear! The forecast for that day is 99 degrees so I will have to dress appropriate for the weather, appropriate for my age (going on 17 this week), and appropriate for meeting his parents. From what my boyfriend says they are really laid back so I am wondering if I could pull off just a pair of shorts and a nice tee shirt for the ocassion or if I should go buy a casual dress or skirt? Any other suggestions?