What is an atmospheric river?
These vast ribbons of moisture in the sky can deliver both beneficial rain and catastrophic flooding.
When forecasters warn of an "atmospheric river" bearing down on the West Coast, they are describing one of the most important and powerful features of the planet's weather: a vast, flowing ribbon of water vapour in the sky.
Atmospheric rivers are long, narrow corridors of concentrated moisture that transport enormous amounts of water through the atmosphere, often from the tropics toward higher latitudes. A single strong one can carry more water than the flow of the world's largest rivers, and when it reaches land and is forced upward over mountains, it releases that moisture as heavy rain or snow.
On the US West Coast, these systems are a double-edged sword. They deliver a large share of the region's annual precipitation and are vital for replenishing reservoirs and snowpack after dry periods. The famous "Pineapple Express" is one such river, drawing moisture from near Hawaii.
But when atmospheric rivers are especially strong or arrive in quick succession, they can unleash catastrophic flooding, mudslides and debris flows, overwhelming rivers and communities.
Scientists rank atmospheric rivers on a scale from beneficial to hazardous, much like hurricanes, to communicate their potential impact. As the climate warms and the atmosphere holds more moisture, researchers expect these systems to become more intense, heightening both the benefits of the water they bring and the dangers of the floods they can cause. Understanding them is key to forecasting the West's wettest, and most dangerous, weather.