Though atmospheric river storms are dreaded for the damage they can cause, they are also essential to the western U.S.’s water supply, particularly in California

WHAT ARE ATMOSPHERIC RIVERS?
An atmospheric river is a narrow corridor or filament of concentrated water vapor transported in the atmosphere. It’s like a river in the sky that can be 1,000 miles long. On average, atmospheric rivers have about twice the regular flow of the Amazon River.

When atmospheric rivers run up against mountains or run into local atmospheric dynamics and are forced to ascend, the moisture they carry cools and condenses, so they can produce intense rainfall or snowfall.

Atmospheric rivers occur all over the world, most commonly in the mid-latitudes. They form when large-scale weather patterns align to create narrow channels, or filaments, of intense moisture transport. These start over warm water, typically tropical oceans, and are guided toward the coast by low-level jet streams ahead of cold fronts of extratropical cyclones.

Along the U.S. West Coast, the Pacific Ocean serves as the reservoir of moisture for the storm, and the mountain ranges act as barriers, which is why the western sides of the coastal ranges and Sierra Nevada see so much rain and snow.

WHY ARE BACK-TO-BACK ATMOSPHERIC RIVERS A HIGH FLOOD RISK?
Consecutive atmospheric rivers, known as AR families, can cause significant flooding.

HOW IS GLOBAL WARMING AFFECTING ATMOSPHERIC RIVERS?
Warmer air can hold more moisture. As global temperatures rise in the future, we can expect more intense atmospheric rivers, leading to an increase in heavy and extreme precipitation events.

My research also shows that more atmospheric rivers are likely to occur concurrently during already wet conditions. So, the chance of extreme flooding also increases. Another study, by scientists from the University of Washington, suggests that there will be a seasonal shift to more atmospheric rivers earlier in the rainy season.