Atmospheric rivers are relatively narrow bands of moisture that ferry precipitation across the Pacific Ocean to the West Coast and are key to California’s water supply.

They are commonly referred to as the “Pineapple Express” because of their origins in tropical regions. While atmospheric rivers are necessary to keep California’s water reservoirs full, some of them are dangerous because the extreme rainfall and wind can cause catastrophic flooding and damage. Their presence has been likened to the West Coast version of the hurricane hazard posed to the southeastern United States.

At the extreme end of atmospheric rivers is a phenomenon deemed by the U.S. Geological Survey as ARkStorm, an event that is expected to occur only once every 1,000 years but with the potential to dump massive amounts of water and cause widespread flooding.

Typical atmospheric rivers are about 250 to 375 miles wide and are present somewhere on the planet at any given time. A strong atmospheric river transports an amount of water vapor roughly equivalent to as much as 15 times the average flow of the Mississippi River. Most atmospheric rivers are not disaster events and are critical to California because they deliver about half of the annual precipitation. For operators of flood management/water supply reservoirs, atmospheric rivers are challenging because the need to maintain enough storage space for incoming runoff means sometimes releasing valuable reservoir water that communities rely on during the dry summer months.

Because of this perception of “wasted” water, efforts are underway to sharpen forecast-based decisions so that unnecessary releases can be avoided. Current forecasts of atmospheric rivers are reliable out to about five days.

Improved understanding of atmospheric rivers has come from more than a decade of scientific studies using new satellite, radar, aircraft and other observational and computer-modeling improvements.

The Center for Western Weather and Water Extremes (CW3E), part of the Scripps Institution of Oceanography, is at the forefront of atmospheric rivers research. In 2020, CW3E launched a new sub-seasonal to seasonal forecasting tool to better predict the influence atmospheric rivers will have on the Western United States.

Meanwhile, the method known as Forecast-Informed Reservoir Operations (FIRO) aims to use the latest forecast technology to plan for the arrival of atmospheric rivers. California reservoirs such as Lake Mendocino in Mendocino County, Folsom Lake northeast of Sacramento and New Bullards Bar Reservoir northeast of Yuba City are using the method to retain as much water as possible during the short storm window.