Cheyne
Believe your beetle is the spotted tree borer - Synaphaeta quexi. See if this looks like your longhorned beetle - http://bugguide.net/node/view/49819/bgimage
There is not much info on this longhorn. Did see a comment that said southern California populations are darker and smaller, while those in the Sierra Nevadas are larger and more silvery-white.
These are found in CA from April to July.
Let us know if this is it?
