And to supplement Buenchico's post, you'll find San Francisco always on the cool side with the wind off the bay. Daytime temps in September will be in the 50 to 65 F range (occasionally much higher)... night time down to low 50's to high 40's... should be dry with little fog that time of year. Yosemite will be nice, but cool. Expect low temps at night to reach down to high 30's or low 40's F. Should be dry, clear and cool. Come prepared though. It has been known to snow lightly in late September, early October in Yosemite... altitude there is about 5,000 feet above sea level on the valley floor up to almost 9,000 feet in surround ridges.
Welcome to the U.S.!