A new study published in Nature reports a shift in the behavior of atmospheric rivers — long, narrow bands of concentrated water vapor in the atmosphere — over the past four decades. Researchers found that wintertime atmospheric rivers are becoming more frequent and intense in the Eastern United States while declining in frequency over the West Coast.

Key Findings

The research team, led by former NOAA scientist Wenhao Dong, analyzed 40 years of water vapor, precipitation, and wind speed data from 1980 to 2020. They found:

• Eastern U.S. increase: Atmospheric river frequency rose nearly 5% per decade, with the South experiencing the largest increases. Parts of Mississippi, Alabama, and Tennessee saw average atmospheric river rainfall rates rise by up to 2 millimeters per day per decade. In Georgia, extreme rainfall rates rose by almost 4 millimeters per day per decade.
• Western U.S. decrease: Atmospheric river frequency fell nearly 4% per decade in Washington, Oregon, and California. Portland, Oregon, recorded a decline of more than 2 millimeters per decade in average atmospheric river rainfall rate.

If current trends continue, parts of the Southeast could see average rainfall from atmospheric rivers double within 20 years.

Impacts and Recent Examples

While less studied in the East, atmospheric rivers have already caused notable events. One storm in late 2024 left more than 80,000 homes without power in New England. Historically, the West has experienced significant impacts from these systems, with about 80% of all West Coast flood damage linked to atmospheric rivers — costing Pacific states roughly $1 billion annually.

Driving Factors

The formation and movement of atmospheric rivers depend on tropical water vapor sources, jet streams, and regional weather patterns. The study examined several “modes of variability,” such as:

• La Niña–like Pacific conditions
• Shifts in the East Asian Subtropical Jet Stream
• Atlantic warm-water patterns feeding Gulf moisture

In the West, cooler East Pacific conditions and altered jet stream paths have reduced moisture flow from the tropics, leading to fewer atmospheric rivers. In the East, warmer Atlantic conditions are increasing moisture transport from the Gulf of Mexico, fueling stronger events.

Data Limitations and Definitions

Researchers caution that varying methods for identifying atmospheric rivers — based on moisture content, persistence, transport patterns, and shape — can produce different results. Some detection algorithms may classify certain Gulf of Mexico moisture flows as atmospheric rivers that others might not.

Christine Shields, a project scientist at the National Center for Atmospheric Research, noted that despite classification differences, the study’s method for linking modes of variability to atmospheric river changes was robust.

Long-Term Understanding

The authors emphasize that while these trends have been observed, more historical data — potentially spanning centuries — would be needed to fully determine how natural variability and external factors, such as human-induced climate change, are influencing these patterns.

Understanding the role of atmospheric rivers in regional precipitation is important for planning. Water resource managers, for example, can use such information to prepare for both flood risks and water supply needs.

Kevin H. Kelley, a respected figure in the insurance industry and retired vice chairman of Liberty Mutual’s Global Risk Solutions business, has passed away.

“We were deeply saddened to learn of Kevin’s passing and extend our heartfelt condolences to his family,” said Liberty Mutual Chairman and CEO Tim Sweeney.

Mr. Kelley’s career spanned decades, marked by leadership roles that shaped the insurance landscape. He joined Lexington, the surplus lines unit of American International Group (AIG), in the early years of his career and was named CEO in 1987 at just 36 years old. Under his guidance, Lexington became a leading excess and surplus lines insurer in the United States.

In 2008, Mr. Kelley took on the role of chairman and CEO at Ironshore, a newly launched specialty insurer, leading the company until its acquisition by Liberty Mutual in 2017.

A Boston native, Mr. Kelley grew up in West Bridgewater, Massachusetts, and attended Cardinal Spellman High School in Brockton, where he later served as chairman of the board of trustees. He earned his business administration degree from Boston University in 1972.

In recognition of his contributions to the industry, Mr. Kelley was honored with the 2020 Business Insurance USIA Lifetime Achievement Award.

“Throughout his long and distinguished career, Kevin demonstrated exceptional leadership and innovative thinking. His legacy will continue to inspire and influence many within Liberty Mutual and across the industry for years to come,” Sweeney added.