The state has an average of a dozen tornadoes per year, most of them. Most California tornadoes originate in the Pacific as winter thunderstorms. As some storms approach the coast, cold air over the ocean mixes with the warmer air on earth, resulting in the kind of climate instability that can cause tornadoes. Northern California saw unusual weather for this time of year.
It was a storm system with rain, snow and even a couple of tornadoes. This “stunning” cloud would be impressive in Tornado Alley, but a Californian witnessed the meteorological wonder. Under the right circumstances, tornadoes can occasionally develop under simple California rain if it has enough twist. A lot of people move to California because of its perfect weather, and most of them probably don't expect tornadoes to be part of the deal.
Any of these supercell storms that develop in California are usually shallow and shallow surface, significantly smaller than their Great Plains counterparts. California's tornado season occurs mainly during winter and spring, although summer tornadoes sometimes occur (especially in Southern California) due to monsoon humidity.