The weather forecast for San Diego throughout Comic-Con calls for it to be partly cloudy and 72° during the day and 65° at night. That is pretty much the forecast for San Diego about 85% of the time.
There are still people around who somehow think it would be a good idea for Comic-Con to move to Las Vegas. I have dozens of reasons why that would be a very bad thing, one of which is the weather. Next week in Las Vegas, it will be 110°. How would you like to cosplay as Iron Man in that climate?