Originally Posted by chel
I second, pack for chilly evenings. Nearly frozen during the 4th fireworks in SF.
LOL!! Sorry, but the natives have a saying... You can always tell the tourists in the city from the people who live here. They're the ones wearing the tourist sweatshirts/jackets over their shorts and short sleeves.
It gets cool/cold at night year-round. There might be a handful of days/year when it's hot after dark, and that's during heat waves and low pressure zones, when there's no air movement.
September is actually the hot season here, Indian Summer. DS' first bday party (mid-September) was something like the hottest day of the year (the frosting melted right off his cake). But any time you come to N. CA, particularly the coastal areas, it's a good idea to pack/dress/think in layers. Bring shorts and tees, but also have jeans and a sweatshirt (or windbreaker) on hand. And even on days that are going to be hot, the morning may be bitterly cold until the fog burns off. If you're going to be hiking in the redwoods, the temp is typically 10 degrees cooler just because of the shade under the trees. If you're at the coast, and there's little to no wind, it's gonna be beautiful, but the wind could be anywhere from gentle breeze to staggering.
If you're coming from an area with snow, it's not that cold. But it might drop to high 50s at that time of year (at night), it rarely gets lower. During the day, expect anything from lows of 65 up to highs could be pushing 100. And you'll definitely have some climate change as you head north. The further north you go, the colder it gets, and the more exposed the coastline, the windier it gets. In the SF area, it's fairly protected because of the bay, but once you get up to the Humboldt area, the coast starts getting more exposed, more rugged and colder - although it can still get hot in areas. We did that drive at Xmas this year, and there was a noticeable difference in the severity of weather as we moved north.
Basically though, there's no predicting what the weather is going to be like. Be prepared for both heat and rain. You won't have any idea until the week before what it's liable to be.