I"m taking summer classes right now in Pittsburgh. The weather thus far has been in the mid to upper 70s, with a few days in the mid to low 80s. It has rained a few time this past month, but nothing too crazy.
Bring clothing for warm weather, but do pack somehting in case the weather cools down. Pitt is known for being 60 degrees one day and then snowing the next. For summer its not that extreme, but just have some clothing in case it cools down, or it gets windy/cool.
Have fun and enjoy da 'burgh.