Why does northern California have palm trees?

Low elevations in Northern California do have palm trees. Mainly snowless/ frostless areas but in inland areas that are prone to occasional winter frosts, some of the more tender ones will not grow well. Most of the popular varieties of palms, however, are ones that are hardy to the climate, and which are mostly the same as you see in SoCal. This is because they are fast growing and abundant. There are some palm growing farms in the Central Valley too.