California could face an increased risk of heavy rainfall and flooding this winter as a strengthening El Niño raises the likelihood of wetter-than-average conditions across parts of the state. The developing weather pattern has renewed attention on the possibility of an extreme storm system known as an ARkStorm, a hypothetical disaster that could bring catastrophic flooding to California. ARkStorm, short for Atmospheric River 1,000, is a hypothetical scenario developed to examine the impact of an exceptionally severe series of atmospheric river storms in California.
The scenario was developed by experts from federal and state agencies, universities and research institutions, including NOAA, the U.S Geological Survey (USGS), Scripps Institution of Oceanography and FEMA. Their aim was to create a scientifically plausible hypothetical storm scenario to help emergency services, resource managers and the public understand the potential scale and consequences of an extreme flooding event. The scenario uses evidence from historical storms, including the Great Flood of 1861-62.
During that event, California experienced approximately 45 days of repeated storms, leaving large areas of the Central Valley underwater. It examines what could happen if a similar disaster occurred today. The USGS estimates that an event of this magnitude could cause approximately $725 billion in combined property damage and business-interruption losses.
Around 1.5 million people could also need to evacuate. El Niño is a climate pattern involving unusually warm sea surface temperatures in the central and eastern tropical Pacific Ocean. It influences atmospheric circulation and can change the position of the jet stream, affecting where storms travel.
NOAA's winter outlook for 2026-27 indicates an increased likelihood of wetter-than-average conditions across parts of the southern United States, including California. Daniel Swain, a climate scientist with the University of California Agriculture and Natural Resources, expressed concern about the months ahead. "I am concerned about this winter," Swain told USA Today.
"Due to the very strong El Niño, the risk of a severe ARkStorm-like flood resulting from weeks of heavy precipitation is more likely in a year like this one." El Niño can encourage storm systems to track further south, potentially bringing more precipitation to Southern California. However, its effects vary across the state, and the climate pattern does not guarantee a particularly wet winter. Seasonal forecasts also cannot predict the timing or severity of individual storms.
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