According to this geographically noteworthy Quora answer, the diving line between Northern California and Southern California has been declared. The line, it seems, goes right through San Luis Obispo. Lucas J Meeker, who created the dividing line, notes: "[T]here is an X factor here which is that the coast from Lompoc to Carmel is generally referred to as the Central Coast, and as a result could be considered somewhat of a floating barrier. You might see the occasional 'bro' in this region repping a 'CenCal' vinyl decal on his lifted truck. He is most likely from the Fresno area, if not from Paso Robles/San Luis Obispo."
But who takes Fresno? Meeker goes on to point out, "Bakersfield is definitely SoCal and Modesto is definitely NorCal, and nobody wants to lay claim to Fresno, the apparent regional culture DMZ." Which, sure. Many claim Fresno is a less than a wasteland. But we sort of like the place. Sort of like it. It's got a heavy Armenian population, raisins grow there, and national treasure Audra McDonald hails from Fresno. So, why not, let's say it's part of NorCal. Sounds good.
Hey, at least it's not Riverside.
(Thanks for the tip, Don Otvos!)