California atmospheric river: West Coast braces for life-threatening floods

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.

Pineapple Express is a nickname for a strong atmospheric river in the tropical Pacific near Hawaii.

Atmospheric rivers are long and relatively narrow bands of water vapor that form over an ocean and flow through the sky, transporting much of the moisture from the tropics to northern 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 the National Oceanic and Atmospheric Administration.

Formed by winds associated with cyclones, atmospheric rivers typically range from 250 miles to 375 miles (400 to 600 kilometers) 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 that 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.

Forty-six atmospheric rivers made landfall on the U.S. West Coast during water year 2023, according to the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes. Nine were categorized as strong, two were extreme and one was exceptional. California experienced extensive flooding and massive snowfall.

According to the National Oceanic and Atmospheric Adminstration (NOAA), atmospheric rivers are narrow regions in the atmosphere that transport much of the moisture from the tropics to northern latitudes. The weather systems are part of the Earth’s ocean water cycle, and are tied closely to both water supply and flood risks.

A well-known example of a strong atmospheric river is the “Pineapple Express” – which hit California earlier this week – so-called because moisture builds up in the tropical Pacific around Hawaii and can wallop the U.S. and Canada’s West Coasts with heavy rainfall and snow.

A second atmospheric river is set to hit California this weekend.

Prevailing winds cross over warm bands of tropical water vapour to form a “river,” which travels across the Pacific as part of the global conveyor belt. When it reaches the West Coast, the Pineapple Express can dump as much as five inches of rain on California in one day.

According to NOAA, atmospheric rivers come in all shapes and sizes, but those that contain the largest amounts of water vapour and the strongest winds are responsible for extreme rainfall and subsequent flooding. These events can affect the entire West Coast, often disrupting travel and damaging property in the process.

Despite the strong winds and heavy rain in Los Angeles brought on by the Pineapple Express storm, surfers braved the conditions to hit the waves on Thursday morning.

Famous tourist hotspot Venice Beach, which usually sees thousands of people and vendors passing through, was shuttered and quiet, as gusts battered the boardwalk.

Only a handful of thrill-seekers remained, delighting in the waves – which reached up to six feet. Among them was Ian Dell, who described the outing as “a regular Tuesday”.

A giant redwood tree fell on a Saratoga, California home and car on Wednesday, trapping a young girl inside the vehicle. She was transported to the hospital with only minor injuries, NBC Bay Area reported.

Despite the warm, tropical air that the Pineapple Express brought to much of the West Coast this week, the atmospheric river also dumped several inches of snow on the Sierra Nevada Mountains.

The Mammoth Mountain ski resort, pictured below, got 12-14 inches of snowfall Thursday night into Friday, the Associated Press reports.