A Pineapple Express is a non-technical term used to describe a weather pattern that originates in the tropical Pacific and impacts the west coast of the United States. More specifically, it is a type of atmospheric river.
According to NOAA’s Earth System Research Laboratory, atmospheric rivers are “relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics.” Coming in a variety of shapes and sizes around the globe, they form when strong winds associated with storms draw moisture into a thin area ahead of a cold front. The strongest atmospheric rivers can carry as much as 15 times the amount of water that flows through the mouth of the Mississippi River.
Directed by the jet stream, a Pineapple Express transports moisture from the Hawaiian tropics to the west coast of the US. They are famous for bringing warm, moist air and heavy rain to California, Oregon, and Washington. Just a few of these events, according to NOAA, can supply the region with 30% to 50% of its annual precipitation. While beneficial on one hand, they can also be dangerous. If these systems stall over an area, they can cause major flooding and landslides.
The term Pineapple Express is named for the tropical Pacific’s popular fruit.