Atmospheric River: Floods and mudslides plague California

Atmospheric rivers are rolling across California, bringing heavy rain, high winds and mountain snow.

California is renowned for its sunny weather. But for weeks, the state has been hit by a deluge of rain and snow. Many rivers, such as the Tule River in Springville, burst their banks and submerged surrounding areas.

At least two people have already died as a result of the severe floods. There were also numerous power outages and road closures, with a state of emergency declared in 34 of California's 58 counties.

In some flood-prone areas, residents have been asked to leave their homes and evacuate to higher ground for safety.

A notorious weather phenomenon on the west coast of the US is responsible for this torrential rain: the "Pineapple Express".

The Pineapple Express is what we refer to as an atmospheric river; long, narrow air flows, like rivers in the sky, of enhanced water vapour transport. Typically several hundred kilometres long, they transport warm, humid air from the tropics to higher latitudes.

During a Pineapple Express, a low-pressure system over the North Pacific directs air from Hawaii to the west coast of the US. About as much water can be transported as what flows through the mouth of the Mississippi.

Above the warm ocean, the air absorbs a lot of moisture. Arriving at the coast, the moisture is released again as persistent rain or snow in the mountains, such as the Sierras.