How ARkStorm Could Threaten California This Winter

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. What Is An ARkStorm? 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.

Read More on News 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. ... How Could El Niño Affect California? 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. Atmospheric Rivers Could Increase Flooding Risks Another important factor is atmospheric rivers, which are long, narrow bands of moisture that transport large quantities of water vapor from the tropics toward the West Coast. When these systems reach California, they can produce intense rainfall and snowfall.

Several arriving in quick succession can saturate the ground, overwhelm rivers and drainage systems, and increase the risk of flash flooding and landslides. Atmospheric rivers have contributed to some of California's most significant flooding events. However, El Niño does not automatically mean more atmospheric rivers will hit the state. Research suggests their frequency does not necessarily increase significantly during El Niño events, although the wider climate pattern can influence where precipitation falls.

Coastal Flooding and Climate Change Add to the Threat California's coastal communities could face additional risks from high tides, powerful ocean swells and rising sea levels. Recent events have provided an example of the coastal risks facing the state. In October, T ropical Storm Rachel brought powerful waves and flooding to Southern California , damaging beachfront homes in Long Beach and Newport Beach, and prompting evacuations in some areas.

The storm's effects were intensified by unusually high tides, elevated ocean levels and El Niño-related conditions. Experts have also highlighted the influence of exceptionally warm Pacific waters and climate change. El Niño can raise sea levels along the California coastline, while king tides—exceptionally high natural tides caused by the alignment of the Moon and Sun—can increase the potential for coastal flooding. When these conditions coincide with winter storms, flooding and coastal erosion can become more severe.

Long-term sea level rise linked to climate change adds another layer of vulnerability by raising the baseline water level. Climate change could also increase the intensity of extreme rainfall. Warmer air can hold more moisture, potentially allowing storms to release greater quantities of precipitation. The USGS has considered climate-change impacts in its ARkStorm research, highlighting how changing conditions could worsen the consequences of an extreme storm.

Could California Experience an ARkStorm This Winter? "An ARkStorm is plausible, perhaps inevitable," the USGS stated. "The geologic record shows six megastorms more severe than 1861-1862 in California in the last 1800 years, and there is no reason to believe similar events won't occur again." While there is no forecast that an ARkStorm will hit California this winter, the combination of a strong El Niño, atmospheric river activity and rising sea levels could increase the state's exposure to severe flooding.

For now, scientists will continue monitoring ocean temperatures and atmospheric conditions as winter approaches. The ARkStorm scenario serves as a reminder of how vulnerable California could be to prolonged extreme rainfall. Newsweek’s reporters and editors used Martyn, our AI assistant, to produce this story. Learn more about Martyn here . Contact Newsweek editors for this story: Robert Greenall and Ben Kelly