Back-to-back atmospheric rivers are plowing through California

After a series of unrelenting winter storms, California is forecast to see another slate of grueling weather, including heavy rain and snow through the weekend.

The severe downpour will be a result of back-to-back atmospheric rivers, the first of which arrived on Thursday. The second is expected to make landfall on Monday afternoon, according to Brian Ochs, a meteorologist for the National Weather Service based in Hanford, Calif.

The coming storm is expected to be warmer, creating a dangerous combination of excessive rain and snowmelt in some areas that could pose serious risk of flooding.

"Lately we've had snow levels as low as 1500 feet, but since we have warmer air, we're seeing snow in elevation as high as 7,000 feet and above," Ochs told NPR.

Earlier this week, California Gov. Gavin Newsom proclaimed a state of emergency in 21 counties. Newsom also has requested federal assistance in anticipation of the state's need to respond to the extreme storms.

An atmospheric river moves into the West Coast

Also known as "rivers in the sky," these meteorological phenomena develop when a relatively long, narrow channel of wind transports water vapor from the tropics. The current atmospheric river pummeling California is called a "Pineapple Express," because the moisture arrived from the tropics near Hawaii.

The columns of highly concentrated moisture move with the weather, and when they make landfall, they can produce heavy rain or snow. Atmospheric rivers with the largest amounts of water vapor and strongest winds can lead to severe rainfall, causing mudslides and property damage.

A warmer, wetter storm will hit central California the hardest

Central California is forecast to receive between 4-9 inches of rain on Friday. Counties on the central coast like Monterey are at high risk of severe, widespread flash flooding.

The downpour is expected to lighten by Sunday in the central region, giving some respite, before a second atmospheric river brings heavy rain on Monday and Tuesday, with lingering showers on Wednesday, said Ochs from the National Weather Service.

In northern California, areas with higher elevation are expected to see heavy, wet snow. Meanwhile, creeks and streams in the foothills of the Sierra Nevada are forecast to be the most vulnerable to flooding from rain and snowmelt.

Southern California is expected to see minor flooding on roads and along small creeks. In Santa Barbara, there is a moderate threat of significant road and stream flooding, particularly in the northern part of the county.

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