Last week, central and southern California got hit with a substantial rain event, welcome news for a severely drought-stricken area that has not seen heavy rains in many months.
A high-profile element of the storm system that affected the region was a so-called “atmospheric river,” or the local variant of this feature sometimes known as the “Pineapple Express.”
These names refer to the fairly narrow (sometimes only a few hundred miles wide) streams of high atmospheric water vapor content that can result from the encroachment of a middle latitude weather system into the northern subtropics — latitudes of about 30 degrees North. The middle latitude weather system has associated with it a robust circulation that drives air northward on its eastern side and toward the equator on its western side.
This same characteristic is observed with the passage of such storms over Wisconsin at any time of year. When such a circulation finds itself as far south as 30 degrees North, the air that it drives northward on its eastern side is pure tropical air, laden with enormous amounts of water vapor. This results in narrow tongues of exceptional water vapor content rushing northward in what resemble “rivers” of water, but in the atmosphere.
On the Pacific coast of North America, the tropical air can sometimes originate near the Hawaiian Islands, hence the name Pineapple Express. When these rivers of high water vapor content are forced up the sides of the Sierra Nevada mountains in California and Nevada, the water vapor is condensed into liquid and solid form, bringing much needed rains to the area.
So, though this recent storm was powerful, it was not unprecedented and, in fact, is a rather common occurrence along the entire Pacific coast in the late autumn and early winter.