Rivers in the Sky: The Pineapple Express Phenomenon
All geographic locations have more local or regional names for weather extremes, for instance, there is the Great Norther that often hits the east coast of the USA for sub-freezing temperatures and snow. On the west coast, there is the Pineapple Express. The only snow that falls on the west coast is usually in Washington, parts of Oregon during the winter, even that is not common. In California, snow is only dropped in elevations higher than 4000+ feet on a regular basis in the winter.
More common is the confluence of weather patterns that create a "river in the sky" or as locals call it, the Pineapple Express. These are vast sub-tropical monsoon-like rain storms that originate around the Hawaiian islands that normally move along the equator-aiming at Latin America. The "Express" occurs when the river of rain parts and some of it is aimed like a funnel at California and Oregon. The band of continuous rain is narrow but concentrated. Rainfall can be as high as 20 inches in a 2-3 days to affected areas. Most of the "express" usually hits areas from Santa Cruz northward to Redding. Los Angeles receives less amounts of rain and San Diego can dodge it altogether. It just depends on how the funnel of rainstorms is aimed.
NOAA has spent $270,000 in studies trying to figure out why it occurs. They have found out that a similar condition also hits England at times. The Pineapple Express that occurs to California happens only 3% of the time but there are 42 "rivers in the sky" that do hit the state. They can be as high as 8000 ft. Usually, most of the winter weather to California originates in the Gulf of Alaska and the jet stream carries it into the area usually causing colder temps and rain. The "express" is the same but temps are much warmer, around 60F.