Most likely pretty darn cold!
Well, it's all relative I guess. You almost can never go wrong here in jeans... I usually wear jeans, flip flops, short sleeve shirt, and sweater of some sort. Layering is key, so you can remove if it gets a warmer. Almost no one here ever wears shorts, except tourists!
They can often be seen down at the wharf freezing to death! You just don't expect it to be chilly in the summer. (However, if you are venturing outside the city, the weather can get quite warm.)
We are going to be in the 70's this week! Yeah!! Warm for us. But last week we were at the playground and I had to put on some real shoes and a coat, it was maybe in the 50's. It can be a bit unpredictable, but most likely will not be "hot".
Check the weather if you can, but if you're packing for DL, you can probably wear your short sleeves from there, bring a pair of jeans, and a light sweater or jacket. Since I see you're from PA, you won't be feeezing like folks from FL would be.
Enjoy! If you need any more SF info, just let me know!