Hi Penny,
Went 8th March last year and it was raining the day we arrived in San Francisco. After then there was no rain at all the entire trip.
San Francisco can be quite cool. So I`d recomend a fleece or a hoodie ( the mornings are the coolest )
After that the holiday got progressivly warmer the further south in California we went.
Las Vegas was very warm and sunny.
I`d take clothes you can wear in layers so then you can take off accordingly.
Took pairs of shorts with me but didnt wear them too often.
Las Vegas is superb and is one of the main reasons I want to go again. San Francisco is truly wonderful too!!!
Hope I was of some help? Look forward to meeting you.
Darren