I've found that weather.com is more accurate for up here than it was for down there. I have no idea why. So you might be able to trust them. I also often get the weather from
www.kptv.com they seem to be the most accurate of the local stations, but they can be off sometimes.
My best advice is to pack for everything. The other morning ds and I had to put on sweatshirts to go outside to play, by lunch we were in shorts and tank tops, and by bedtime we were cold enough to change back into pants. The temperature ranges up here way more than in the SFBA.
Hope you have a fun and safe trip!