As a NorCal driver there should be a law that states any water like substance on the road surface all Socal drivers are required to stay home. LOL
On a serious note you will NEVER! Use chains on your subaru in CA! Except if you find your self up at Big Bear stuck in the driveway at the cabin with ice covering the climb up to the street. The chains might give you enough bite to climb up to the road but you will attempt without chains long before tossing the chains on.
Tire condition regardless of chains is a big factor bald tires are bald tires regardless of how you cut it and chains will not cure traction issues caused by bald tires.
As for when you put chains on the subaru - you do not put chains on the subaru. LOL When you drive over the pass and you see a chain control stop they wave you through some areas the chain monkeys actually have a Subaru specific wave sort of like a secret handshake. Welcome to the fantastic world of Subaru AWD - you will never use them or need them in CA.
If you ever as in EVER find you need them they only go on the Front tires and yes they can heavily damage your car if not used right. So #1 Rule NEVER use Chains on your subaru!