Temp: 60 degrees (yes, that's summer in SF)
Humidity/Precip.: Moderate (~70%)/None
Did It Work: Yes. 60 is right on the edge of t-shirt weather for me. Today's run was slow and short, so it would have been easy to never warm up. The deciding factor was the sun - on a sunny day, it's easier to warm up and having exposed skin that allows sweat to evaporate is good.
Good luck to everyone running the SF Half and Full Marathon tomorrow!