Many areas of the US state of California are under threat from flooding and avalanches as a large area braces for days of intense rain and snow.
Forecasters predict that beginning on Thursday, back-to-back "atmospheric rivers" will bring significant precipitation to central and northern California while also dropping snow at higher elevations.
According to the National Oceanic and Atmospheric Administration, atmospheric rivers are plumes of tropical moisture that dump huge amounts of rain. They are relatively long, narrow regions in the atmosphere-like rivers in the sky-that transport most of the water vapor outside of the tropics.
After a week of unusually bad weather on the West Coast, the atmospheric river could dump as much as 3 inches (8 cm) of rain in the San Francisco Bay area and across the Central Coast from Thursday through the weekend, the National Weather Service (NWS) said in its forecast.
Hilly inland areas and some coastal ranges inundated by snow in recent weeks could get 6 inches of rain. Some 8 inches were expected in the Santa Cruz Mountains, south of San Jose, and up to 10 inches in the Santa Lucia Mountains, along the coast near Big Sur, the service added.
"Most of the flooding concerns are for the lower-lying areas susceptible to rapid river and stream rises," NWS Weather Prediction Center meteorologist William Churchill said.
"It's really a combination of all this heavy rainfall coming and also rapidly melting snow."
More than 15 million people in the metropolitan areas around San Francisco Bay and Sacramento and in parts of the region were under excessive-rainfall and flood advisories from the weather service on Thursday. Some of the watches and warnings in California were to remain in effect until Sunday.
Some waterfront communities along major rivers and their tributaries also braced for the possibility of overflowing streams swollen by heavy showers and runoff of melting snow.
(With inputs from agencies)
