Earlier this month, a classic atmospheric river fueled by El Niño resulted in historic rainfall levels across California. During the peak of the event, the Weather Prediction Center issued an unusual “high risk of excessive rainfall” for more than 16 million people across Southern California; strong winds knocked out power to more than 300,000 people, and three deaths were reported as a result of fallen trees as hundreds of mudslides ruined homes, businesses and roadways. These once-unfamiliar storms have become as familiar as hurricanes and tornadoes in recent years, bringing real economic hardships to the communities impacted.
What is an Atmospheric River?
Atmospheric rivers are relatively long, narrow regions in the atmosphere that transport water vapor out of the tropics. When an atmospheric river makes landfall, it typically releases the water vapor as either rain or snow. They are not a new phenomenon and are frequently relatively weak systems that can be counted on for contributing to annual precipitation. However, storms like those California is experiencing now can produce massive amounts of precipitation, creating flood risks.
Atmospheric rivers can happen almost anywhere across the globe but are most notable in the western United States. A NASA study has confirmed that while just 17% of storms in this region are caused by atmospheric rivers, up to 50% of California’s precipitation comes from these storms, including 40% of the Sierra Nevada mountain’s snowpack. In California, the atmospheric rivers are responsible for more than 80% of the state’s major floods. And in El Niño years like we are experiencing now, the research shows an increase in atmospheric river frequency.
The rainfall associated with atmospheric rivers is overwhelming to communities in its path. Los Angeles has now already seen 75% of its annual rainfall as a result of this most recent storm, with more than seven inches of rain falling in the first 48 hours of the storm. California has faced relentless rain from these back-to-back atmospheric rivers, which are storms that originate in the warm, subtropical waters around Hawaii and move toward the West coast of the U.S.
The Economic Impact of Atmospheric Rivers
Recent research has found that while atmospheric rivers pose flooding and other hydrologic challenges, there are also quantifiable economic impacts. Much of the economic damage results from the relentless and rapid succession of storms that oversaturate the soil, ultimately increasing flood risks. In 2019, these events caused flood damage totaling approximately $1.1 billion for the Western U.S. and $620 million in California alone.
Understanding the enormity of the economic impact and quantifying it is helping California’s water managers, utilities, and other public safety officials make decisions ahead of incoming storms. The water supplied by atmospheric rivers is an important part of the state’s water system, but the challenge is managing the small window of time in which all that precipitation falls. A comprehensive understanding of the storm system’s dynamics makes more informed decision-making.
The Future State of Atmospheric Rivers
Similar to other extreme weather events like hurricanes and severe thunderstorms, the atmospheric rivers impacting the West Coast are growing in intensity and volatility. One study projects that damages from atmospheric rivers will triple in the western U.S. Contributing compound events like flooding and mudslides further challenge industries and supply chains, including agriculture and utilities.
For this current storm, utilities also studied the incoming atmospheric rivers, and companies like PG&E are making the appropriate preparations for it. This includes staging crews in the areas expected to receive the most severe storm impacts. These crews follow guidance from meteorologists to forecast crew and material needs and the storm’s timing.
Municipalities also use weather data to prepare for incoming storms and minimize damage and public safety risks. The city of Sacramento, for example, had crews working ahead of the most recent storm trimming trees to keep falling trees from becoming a problem. The city crews also cleared drains as the storm could cause localized flooding if they get backed up.
As I have stated before, a critical component to be weather resilient is relevant analytics and the continued advancement of the science of weather. The more decision-makers can understand the dynamics of these intense storm systems, the better they can prepare, minimize property damage, and protect the public. This informed decision-making will also help plan, budget, and address the economic impacts of this growing storm category.