(From Warblers by Dunn and Garrett, p. 159.)
Returning home after my walk, I pulled out my trusty references, since I could not recall having ever seen such a blatantly gray--and therefore O.c. celata--in California.
The Birds of Orange County, California: Status and Distribution: "Gray-headed birds believed to represent V.c. orestera and V.c.celata are uncommon fall migrants (arriving in early September), rare in winter."
San Diego County Bird Atlas: "Vermivora c. celata (Say, 1823), breeding in the trans-continental taiga zona, is even less yellow than orestera; the head is always gray, and in some females the yellowish on the underparts is reduced to irregular blotches. It reaches southern California as a rare migrant and winter visitor (Grinnell and Miller 1944)."
I believe this is a case of subspecies neglect (think Cackling Goose). Were Oreothlypis celata celata considered its own species (I propose "Goldenrod Warbler" for the common name if this ever happens), birders would probably find a lot more of them in California. But, that will probably not happen in the near future--and I hope it doesn't, since identification would be a nightmare! This bird, however, seems to be a slam-dunk.
An intriguing quandary of taxonomy and distribution five minutes from the door!