VENTURA, Calif.—California, its south burning weeks after blazes charred its north, is in the midst of the worst wildfire season on record, forcing residents to rethink neighborhoods built on the West’s drought-parched lands.

On Friday, a half-dozen fires blazed across Southern California in every direction—San Diego County in the south; the San Fernando Valley northwest of downtown Los Angeles; and Ventura County to the north.

The...