Rain in California seldom lasts long and it’s usually quite predictable. So if you are careful, with reasonably cheap equipment you can join the ranks of the fearless rain riders.
First of all, cargo. Many of us commute with important computing materials, paper artifacts, non soluble foods and spare clothing that we’d prefer to keep dry. You probably have nice waterproof panniers, but, as my kayak instructor used to say, there is no such thing as waterproof, only different degrees of wetness. To keep wetness away as much as possible from precious cargo, I usually pack year round plastic bags (trash bags are just fine) and keep them at the bottom of the panniers. Before leaving under the rain, I make sure to shove all my important stuff in the bag. Zip up the panniers, and you are reasonably sure everything will be dry as bread on arrival.
Clothing varies. In general, in California, I’d advise to layer wisely. It’s usually not that cold when it rains, and it’s easy to overdo it. In other words, don’t pedal in a parka.
Upper body: I use a standard light waterproof jacket with a hood. It’s a really cheap one I picked up in an outlet somewhere, one size bigger than what I would normally wear, so I can layer anything underneath, and have better lower-body protection. Some people, especially those of the short hair/no hair persuasion use hoodless jackets, of the kind that roadies use. However, I found out that unfortunately on many of those cold streams of water easily find their way down the collar area, and you might want to prevent that. I usually wear my helmet on top of the hood. It might look odd, but it keeps everything in place, even in relatively strong winds.
Lower body: there are divergent school of thoughts. Some people have found that there is something more waterproof and breathable than Gore-TexTM: human skin. Those people pedal in the rain in shorts and sandals, dry up on arrival and wear dry clothes (see above for how to make sure you have those at destination). Others prefer a standard waterproof over-pants on tough hiking or mountain biking shoes. What you choose will probably depend on your attitude and even more on your latitude.
Lights: I feel for those poor car drivers, especially those of the breeder variety. Many of them are juggling frantic schedules, fielding phone calls, failing to discipline their kids on the back sear, and often all at once! Plus they get bored a lot, I mean, wouldn’t you be bored if you were locked in one of those metal and glass cages? So I see more and more of them texting while driving. Even with sunshine, it’s not surprising that so few of them seem to notice us or to make the effort to use those turning lights to let us know which way they intend to go. So imagine when the weather turns bad (and those wipers are so much difficult to use than the blinkers!). So try to be as visible as possible. I have reflectors on my rain gears, and of course on the bike, and if it’s pouring I keep my lights on even in daylight, switched on blinking mode.
Don’t let the occasional shower ruin an otherwise perfectly good riding day!
