California receives much of its winter rain through relatively few, very large and sustained storms that come off the Pacific Ocean. Some of these storms are known as “Atmospheric River Events” because they transport a long band of moisture that resembles a river in the sky on weather maps. A well-known example of this type of storm is the so-called “Pineapple Express,” a storm named for the moisture it brings to the west coast from the tropics near Hawaii.
These types of storms typically deliver between 30 to 50 percent of California’s winter precipitation. While these storms are an important source of water supply for the State, they can also bring flooding and other impacts.
The correlation between strong El Niño conditions and the number of winter Atmospheric River Events is not well documented. El Niño does not guarantee an increase in Atmospheric River Events. However, an above average likelihood of increased seasonal precipitation due to El Niño, combined with the annual prevalence of Atmospheric River Events, means that Californians should prepare for the potential of major winter storm impacts. Here are some websites and videos to help you prepare: