When it comes to extreme flooding, hurricanes and majors storms are typically at the forefront of weather events. However, there is another type of weather system that’s far less understood but can be just as destructive: the atmospheric river.

Atmospheric rivers may not carry the name recognition of hurricanes, but they can deliver enormous amounts of water over short periods of time, sometimes causing equal or even greater flood damage. As these systems grow stronger and more frequent, they are becoming a major driver of extreme flooding across the United States, particularly along the West Coast.

What is an Atmospheric River?

An atmospheric river is a long, narrow band of moisture high in the atmosphere that acts like a conveyor belt, transporting huge amounts of water vapor from the tropics toward higher latitudes. These “rivers in the sky” flow in the lower atmosphere and can stretch hundreds to thousands of miles across. When this moist air hits land, especially mountain ranges, it cools and releases its water as heavy rain or snow.

The most recent atmospheric river flooding happened just a few months ago in December 2025, when Washington State experienced record-breaking rainfall and California experienced river flooding, mudslides, and widespread evacuations. Back-to-back atmospheric river systems overwhelmed already saturated ground, pushing rivers beyond their banks, damaging infrastructure, and placing millions of residents under flood warnings and evacuation orders across the West Coast.

These two events alone highlight the growing need to spread knowledge and information about atmospheric rivers, and how flooding outside of the hurricane season challenges long-held assumptions about when and where flood risk occurs. Below are some key facts that help explain what atmospheric rivers are and why they matter.

Atmospheric River Facts

They’re the Largest Fresh Water “Rivers” on Earth… in the Sky - Although they aren’t rivers of liquid water, atmospheric rivers can carry 7–15 times the amount of water that the Mississippi River discharges in a day. Because of their size and moisture content, they can be a major source of precipitation around the world.

Not All Atmospheric Rivers Are the Same - Much like hurricanes, atmospheric rivers are categorized on a scale (from Category 1 to Category 5) based on how much water vapor they contain and how long they persist. Lower-category events can be beneficial, bringing much-needed water to drought-stricken areas, while the strongest events can be extremely hazardous.

They Can Help, and They Can Harm - In some regions, atmospheric rivers are essential to water supplies. In places like California, up to half of the annual precipitation can come from these systems, helping refill reservoirs and snowpacks. But when an atmospheric river is strong or slow-moving, the rainfall can overwhelm soils and flood defenses, leading to flash floods, landslides, and widespread damage.

Wildfires and Flooding Are Connected - After a severe wildfire, the land changes. Vegetation that once absorbed rainwater is gone, and burned soils often repel water, causing rain to run off quickly instead of soaking in. When an atmospheric river brings intense rain over these burn scar areas, the risk of rapid flooding and debris flows increases dramatically.

A “Mega-Storm” Is Possible - USGS scientists have studied a hypothetical, extremely intense atmospheric river event called ARkStorm, a storm so powerful it could cause catastrophic flooding across much of California, similar in magnitude to storms that caused the Great Flood of 1862. While rare, planning for such scenarios helps guide infrastructure and emergency preparedness.

Climate Change is Making Them Stronger - Research shows that climate change is increasing the intensity of atmospheric rivers hitting the West Coast. Warmer oceans mean more water vapor in the air, which can lead to more extreme rainfall when atmospheric rivers make landfall. That doesn’t necessarily mean more storms overall, but it does mean a higher chance of bigger, wetter events.

Why This Matters for Flood Preparedness

Atmospheric rivers are a natural part of Earth’s water cycle, but as they become more intense and frequent, communities and infrastructure are increasingly at risk.

Understanding these systems is crucial for:

Improved flood planning and response

Better infrastructure design

Effective evacuation and emergency strategies

At Flood Risk America, we help communities and property owners prepare for extreme weather by assessing vulnerabilities and designing site-specific, engineered flood protection solutions. As atmospheric rivers continue to shape our weather patterns, proactive planning can be the difference between costly flood damage and a faster, safer recovery.