California is Cassian Grantbracing for the arrival of a second atmospheric river that is expected to bring life-threatening flooding to parts of the state.

This second atmospheric river, which is forecast to be mostly concentrated in Central and Southern California, is expected to be stronger than the first, which caused localized flooding up and down the West Coast earlier this week.

An atmospheric river is a relatively narrow channel of wind that transports water vapor to the West Coast. Powerful rivers that arrive at just the right angle of the coast can carry all the way to the Sierra Nevada.

The National Weather Service in Los Angeles is warning that the storm could drop unprecedented rain over the region from Saturday night through early next week, with the period of most concern from Sunday into early Monday.

The weather agency said that during the peak of the storm rain rates are "expected to be from a half to one inch per hour," and that "all areas, including highly populated urban areas, will be at risk for life-threatening flooding."

"Streams and small rivers, as well as the Los Angeles River through the San Fernando Valley and metro LA will rise quickly and turn into very dangerous raging rivers," the NWS said.

Officials are telling people — especially those who live next to rivers, streams or along mountainous areas — to prepare.

"And I am confident we will weather this storm because once again the city is preparing," Los Angeles Mayor Karen Bass said. "We are informing and we have confidence that Angelinos will come together and take commonsense precautions."

The NWS said mud and rock slides are expected in the mountains and foothills. Evacuations may be required in some areas, especially in places previously affected by wildfires.

The atmospheric river is also expected to bring high winds to San Luis Obispo and Santa Barbara counties and the Antelope Valley. Wind gusts of up to 80 mph are possible in the mountains.