The folks at the National Weather Service are revising their forecast for San Diego from July 22 through July 25. This is when a lot of us will be there for the Comic-Con International and it looks like it'll be at least 80 degrees most days, with lows between 65 and 70. And I'll bet if those numbers change, they'll go up. So pack accordingly.
But of course, you won't have to brave the heat if you spend your afternoons attending the panels I'm moderating.