Cold isn't the problem- heat is.
I suspect it might be too warm even for leathers. I would suggest taking the lightest fleece pullover you can find and use that in conjunction with your rain gear and/or a vented (i.e. joe rocket) jacket.
I would also suggest wearing long sleeve cotton work shirts. They allow sweat to evaporate more slowly and keep you cooler- plus they will protect you from the sun, which will be fierce. Pick up a bandana (I know- it sounds goofy), and wet it occasionally to help keep cool.
Also, I take toilet paper. In addition to the obvious, you can use it as a sunblock around your face if you start to get burnt. Think 'invisible man'.
And allow for some water. Lots more than you imagine.
I totally agree with Daniel- stay OFF the major highways. Highway 1 is beautiful, but slow. When you get into LA, try to stop by Mark Ethridge's shop "moto guzzi classics' just off the 1 in Signal Hill. He's a trip, and the bikes he has are just beautiful.