Weather is very unpredictable. My kid graduated outdoors in a place where the temps are moderate all the time…and usually it was in the 70’s at the time of year for the graduation. It was in the morning…so even cooler.
Well…graduation day it hit 90, sun. No shade.
If you get a sleeveless or short sleeve something with a sweater, you should be ok.
OTOH, at DSs graduations…end of May…it was cold and raining. I never took my coat off. I could have worn pajamas.