What is an atmospheric river? Why the Bay Area is under a flood watch
Winds to accelerate throughout evening hours
High wind warnings are in effect for parts of the Bay Area until 4 a.m. Wednesday. There could be reports of downed trees and power outages as winds increase. The National Weather Service is also warning of possible thunderstorms.
SAN JOSE, Calif. - Another atmospheric river is forecast to drench the Bay Area beginning Tuesday evening and lasting through Christmas, bringing heavy rain and triggering a flood watch. As the storm intensifies, plumes of moisture from a Pacific cyclone are pushing toward the region, raising the threat of damaging winds.
Atmospheric rivers are long, narrow bands of moisture in the atmosphere that can deliver heavy rain or snow when they move over land.
Here’s a look at the phenomenon:
Where do atmospheric rivers come from?
What they're saying:
Atmospheric rivers are massive plumes of moisture carried across the sky that can dump heavy rains or snow over land.
Atmospheric rivers generally form in tropical regions, where warm temperatures can cause water vapor to rise into the atmosphere, according to the National Oceanic and Atmospheric Administration. The winds aloft then carry that moisture to northern and southern latitudes.
They occur globally but are especially significant on the West Coast of the United States, where they create 30% to 50% of annual precipitation and are vital to water supplies but also can cause storms that produce flooding and mudslides, according to NOAA.
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Timeline: When will it rain in the San Francisco Bay Area?
Santa is going to have a heck of a time delivering his toys this Christmas.
Formed by winds associated with bomb cyclones, atmospheric rivers typically range from 250 miles to 375 miles in width and move under the influence of other weather.
Many atmospheric river events are weak. But the powerful ones can transport extraordinary amounts of moisture. Studies have shown they can carry seven to 15 times the average amount of water discharged daily by the Mississippi River, according to the U.S. Geological Survey.
They’re also getting bigger, wetter and more frequent as Earth’s atmosphere warms, according to a 2025 study.
Atmospheric river, cold front to collide bringing intense rain and wind
A major storm is headed for the Bay Area bringing the threat severe thunderstorm activity.
What happens when an atmospheric river reaches land?
Dig deeper:
When the moisture-laden air moves over mountain ranges such as the Sierra Nevada along the California-Nevada line, the water vapor rises and cools, becoming heavy precipitation that falls as rain or snow, according to NOAA.
While traditional cold winter storms out of the north Pacific build the Sierra snowpack, atmospheric rivers tend to be warm. Snow may still fall at the highest elevations but rain usually falls on the snowpack at lower elevations. That can quickly prompt melting, runoff and flooding and decrease the snowpack needed for California’s water supply.
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Bay Area storm to intensify into powerful bomb cyclone
The Bay Area saw heavy rain over the weekend, with showers lingering into early this week. However, a bomb cyclone developing in the Pacific now brings the threat of damaging winds to the region.
What is a pineapple express?
What we know:
It is a nickname for a strong atmospheric river that originates in the tropical Pacific near Hawaii.
Where did the term atmospheric river come from?
The name came from research published in the 1990s by scientists Yong Zhu and Reginald E. Newell of the Massachusetts Institute of Technology. Atmospheric rivers are often referred to as ARs.