Atmospheric rivers are relatively narrow bands of moisture that ferry precipitation across the Pacific Ocean to the West Coast.
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.
Average atmospheric rivers are about 400 miles wide. They move with the weather and are present somewhere on the planet at any given time. 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. More research is planned. In 2019, for example, the California Legislature budgeted $9.25 million to study atmospheric rivers to better understand forecasts and improve flood management and reservoir storage.
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 floodwater means sometimes releasing valuable 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 discharges can be avoided. Current forecasts of atmospheric rivers are reliable out to about five days.