Q/A: Atmospheric Rivers and Their Impact on California's Reservoirs


image of Elissa Linn

Elissa Lynn, DWR climate change program section chief. Courtesy

“Pineapple Express” or “Atmospheric River” are terms you may hear often. But what do they mean, really? DWR Climate Change Program Section Chief, Elissa Lynn, gave a presentation on DWR’s Water Wednesdays live educational series where she discussed these storm systems, what they mean for California, and their impact on the state’s water reservoirs.

Watch her talk on DWR’s YouTube channel to learn more about these rivers in the sky.

What is your role with DWR?

I am the Program Manager for climate change adaptation, supervising 10 full-time climate staff across the Department. They are engineers and scientists who help assess the vulnerability of DWR and the State’s water sector to changing hydrology, including reduced snowpack, a transition from snow to rain, higher flood and drought risk, and warmer temperatures.

What is your education and background?

I was the Chief Meteorologist at KXTV (ABC, Sacramento) for 10 years prior to coming to work for the State. I have an Emmy in Science Communication and 20 years on-air experience. I came to the Department of Water Resources’ (DWR) Flood Operations Center in 2006, where I did storm forecasting, then promoted to Climate Change Program Section Chief (in the Division of Planning) in 2010. My education includes a Bachelor’s in Physics and a Masters in Atmospheric Sciences.

What is an Atmospheric River? Why are they important?

Atmospheric Rivers, called ARs for short, are air currents that carry huge amounts of water vapor from the tropics to the West Coast. On average, each can transport as much water as 20 Mississippi Rivers. ARs are important because they are a key feature in the California water cycle.

Why are Atmospheric Rivers important for DWR and water storage facilities?  

Half of California’s water supply comes from ARs, often in powerful downpours that can hit in rapid succession. On the flip side, we can gradually progress into droughts when we get fewer and fewer ARs and don’t often get out of a drought until we get an AR. Seven of the last 12 droughts ended with ARs.

On average, how many Atmospheric Rivers does California get in a rainy season?

This varies. During an average season we usually see about five or six, but we can have some years with only one or two. However, during the last water year, 19 ARs made landfall, but only one was in the strong category, which is when flood impacts become more of an issue. Researchers are continuing to study why some years have so many, and others, so few. 

Is a Pineapple Express different than an Atmospheric River?

A Pineapple Express, the name California used to give these storms, are just one type of Atmospheric River. When scientists first began to study this type of storm, they believed that the band of moisture which forms over the tropics near Hawaii and makes its way to California was unique to the West Coast. Since then, scientists have found that these storms form in many places across the world and have started using “Atmospheric River” as a more general term for this type of water vapor transport.

What information does DWR gather from Atmospheric Rivers? How does this help the State’s water resources?

Studying ARs makes California better prepared for critical water supply and flooding situations. Nuclei centers in rain droplets can form from either ocean salt or man-made particles, such as the burning of fossil fuels. Once these particles form, they help determine how much rain or snow an AR will bring. One AR that looks like another on satellite could yield 40 percent less precipitation if the source of the seed particles is man-made. DWR’s research partner, Scripps Institution of Oceanography, hope to study enough storms to where they can isolate storms that are affected mostly by Mother Nature versus those that also have the human influence.

Studying ARs helps us understand the mechanics of these storm systems, which improves our ability to forecast, communicate warnings and develop response capabilities. With the AR-Recon study DWR was involved in, we were collecting the first-ever comprehensive data set on the strength and structure of ARs offshore.