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Yes it is hurricain season but ships are given warning and head off away from them! Blyth is also correct in their answer.
A problems associated with San Francisco is fog. We were heading there having passed through the Panama Canal and when we woke in the morning we could hear the ships foghorn going then all the way in and off the ship, it wasn't until we got clear of the city centre did the fog disappear but it is a lovely city to see. As you are obviouly going down the west coast of the USA then it is typhoons which will be a problem, but on the past 4 years less problamatic than hurricanes. Others may contibute further.
I have done the west coast Mexico cruise/ Vegas in September & had no bad weather encounters at all. It's down to luck!