NOAA Outlook Flags California Flood Risk as El Niño Builds

NOAA’s 2026-27 outlook points to wetter-than-average conditions across parts of California, raising the risk of heavier rain and flooding this winter. If extreme storms arrive, communities—not the agencies forecasting danger from a distance—would face floods, damaged homes and possible evacuations.

Daniel Swain, a climate scientist with the University of California Agriculture and Natural Resources, told USA Today: “I am concerned about this winter.” He added: “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.”

The people facing the water

A flood comparable to California’s Great Flood of 1861-62 could leave about 1.5 million people needing evacuation, according to the USGS estimate. The same estimate puts combined property damage and business-interruption losses at approximately $725 billion. These figures show what communities and businesses could lose when extreme weather overwhelms the systems they rely on.

During the historical flood, repeated storms battered California for approximately 45 days, leaving large areas of the Central Valley underwater. ARkStorm—short for Atmospheric River 1,000—is a hypothetical scenario designed to examine the consequences of an exceptionally severe series of atmospheric-river storms. Experts from federal and state agencies, universities and research institutions, including NOAA, the USGS, Scripps Institution of Oceanography and FEMA, developed it to help emergency services, resource managers and the public understand the possible scale of extreme flooding.

What the forecast can—and can’t—say

El Niño involves unusually warm sea-surface temperatures in the central and eastern tropical Pacific Ocean. It affects atmospheric circulation and can shift the jet stream, changing where storms travel. El Niño can push storms farther south, potentially bringing more precipitation to Southern California, though its effects vary across the state.

A wetter outlook isn’t a timetable for disaster. El Niño doesn’t guarantee a particularly wet winter, and seasonal forecasts can’t predict when individual storms will arrive or how severe they’ll be. Atmospheric rivers are long, narrow bands of moisture that carry large quantities of water vapor from the tropics toward the West Coast. In California, they can bring intense rain and snow. When several arrive in quick succession, they can saturate the ground, overwhelm rivers and drainage systems, and increase the risks of flash flooding and landslides.

Atmospheric rivers have contributed to some of California’s most significant floods, but El Niño doesn’t automatically mean more will hit the state. Research suggests their frequency doesn’t necessarily increase significantly during El Niño, although the broader climate pattern can influence where precipitation falls. The account describes no grassroots response, mutual-aid effort or direct action; scientists monitor conditions, while emergency services and resource managers plan.

Coastal risks compound

California’s coastal communities also face high tides, powerful ocean swells and rising sea levels. In October, Tropical 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. Unusually high tides, elevated ocean levels and El Niño-related conditions intensified the storm’s effects. Experts have also pointed to exceptionally warm Pacific waters and climate change.

El Niño can raise sea levels along the California coast. King tides—exceptionally high natural tides caused by the alignment of the Moon and Sun—can increase the potential for coastal flooding. When they coincide with winter storms, flooding and coastal erosion can become more severe. Climate change raises the baseline water level through long-term sea-level rise and could intensify extreme rainfall: warmer air can hold more moisture, potentially letting storms release more precipitation.

There’s no forecast that an ARkStorm will hit California this winter. As winter approaches, scientists will keep monitoring ocean temperatures and atmospheric conditions. The USGS stated: “An ARkStorm is plausible, perhaps inevitable.” It added: “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.”