There may be a geographic component. If you drive along the Pacific Coast Highway, the coastal topography changes dramatically a few miles north of San Simeon/Morro Bay. It becomes more mountainous and more forested.
This continues to the Oregon border, and beyond. The next place you arrive after Morro Bay that's anywhere is the Big Sur, Carmel, Monterey Bay area. Beyond that the San Francisco Bay area, Bodega Bay, etc., up the coast. The weather and scene up there is much different than down coast. In a sense "Northern California" begins near latitude 36 along the coast. In the Central Valley, which is geographically more uniform along its north-south length, probably at Stockton near latitude 38.