Severe weather including tornadoes form when cold dry continental polar air (from the Canadian shield in northern Canada) abuts warm tropical air from the Caribbean.

The air masses are very different in temperature and humidity and where the two distinct air masses meet, instability develops. This instability is ultimately what gives rise to intense weather.

California weather is not influenced by the Caribbean tropical air, because we are far too west and because weather systems move west to east at our latitude. For this same reason we get almost no thunderstorm activity (a weak form of severe weather) in CA.