I’ve looked up longhorn beetles and this guy never shows up. he’s about an inch and a half long.
I live in northern Cali.
I Believe your beetle is the spotted tree borer – Synaphaeta quexi.
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?