"The most beautiful crab spider in California" according to Robert Z. Schick, author of 1965 "The Crab Spiders of California".
At the time of writing in 1965, this was called Diaea pictilis...a species described in 1896 and considered native to coastal California from Sonoma & Napa counties south to San Diego and into northern Baja California.
Only later was it synonymized with D. livens...a species of central & southern Europe and reaching east to Iran. Presuming that synonymy is sound, it suggests this may be an introduced species in CA.
For more details on the ID here (with lots of supporting links & references) see this companion BugGuide post.
Had this down as a Conopid until someone pointed out that it was Ceriana. As far as I can tell, Physocephala exists mostly for me to mistake unrelated flies for it (Bombyliidae and Syrphidae down - I'm sure there's a Tachinid and an Asilid out there somewhere waiting for me to file them away as Physocephala, too...) but at least in this case, I do vaguely recall being perplexed at how much it refused to be forced through any key I could find to Physocephala, which in hindsight should have given me a clue.
Anyway, per F.C. Thompson's key to the Afrotropical genera of Cerioidini, This petiolate specimen should belong to Monoceromyia, which he treats as a genus but seems to flicker between genus and subgenus elsewhere.
In Curran's key (American Museum Novitates 1009) it goes quite comfortably to "Cerioides gambiana" (=Ceriana (Sphiximorpha) gambiana pictured on the Natural history museum site); I have very little accessible information on this species beyond it presumably being originally recorded in West Africa, and some details of colouration that are NOT used in Curran's key do not tally between the NHM specimen (here: https://data.nhm.ac.uk/dataset/56e711e6-c847-4f99-915a-6894bb5c5dea/resource/05ff2255-c38a-40c9-b657-4ccb55ab2feb?view_id=6ba121d1-da26-4ee1-81fa-7da11e68f68e&filters=preservative%3Adry+%28pinned%29%7CtypeStatus%3Aholotype%7Cgenus%3Aceriana, near the bottom)
Curran's key is not complete, so it is possible that this (like the other Monoceromyia from the same spot which sort of drops off half way through the key) are not in it, and I will eventually get around to digging up as many of the others as I can, but for now TENTATIVELY C gambiana.
NB - Thompson notes that the antennifer in Sphiximorpha, to which the NHM have assigned their C. gambiana, is broader at the base than it is long, and less than half the length of the scape. This is not the case in my fly or in the NHM specimen, so...