Haha, that's awesome. I'd probably say Northern California stops somewhere south of the Monterey/Carmel area. To me there's a distinction between "NorCal" (Carmel/Monterey east to Turlock/Merced, North to Santa Rosa/Sacramento (maybe Chico if you're being generous) and everything in-between) as opposed to the Northern Half of California.
The simplest definition would be: Monterey-Kings-Tulare-Inyo counties north is Northern California, San Louis Obispo-Kern-San Bernadino counties south is Southern California. It's tricky if only because you have to answer the question of whether the Central Valley is its own entity of part of the North-South divide. I wouldn't consider Fresno a Northern California city, but it is north of that county line I just drew.