Robert Brown|Grim California weather forecast says big cities could face 'life-threatening flooding'

Forecasters on Robert BrownSaturday issued dire warnings about another atmospheric river system taking aim at California, saying even big cities could face dangerous flooding.

The National Weather Service Los Angeles office said flooding would spread into urban areas, with rainfall beginning Saturday night through Tuesday. The "life-threatening flooding" risk extended across the entire region, forecasters said.

Rainfall will ramp up Sunday, particularly in Southern California from Santa Barbara southward, which could bring flash floods, power outages and landslides. In the Los Angeles area, there could be 3-6 inches of rain along the coast and in the valleys, and between 6 and 12 inches in the foothills and mountains, NWS said.

Officials in Los Angeles, Santa Barbara and Ventura counties issued evacuation orders ahead of expected flooding.

The NWS has predicted the latest “Pineapple Express” storm – an atmospheric river arriving along the state’s Central Coast – will become the season’s largest. The state already experienced heavy rain and snow from another atmospheric river earlier this week.

“All Californians in the storm’s path – especially those in Southern California – should prepare now and follow the guidance of local government officials and first responders,” Gov. Gavin Newsom said in a statement on Friday.

AccuWeather on Friday estimated 94% of California’s population, or about 37 million people, are at risk for flooding, some of which can be life-threatening. The greatest risk is for those across canyons and hills of Southern California, according to Dan DePodwin, AccuWeather’s senior director of forecasting operations.

California storms:A foot of rain in Southern California? Latest 'Pineapple Express' forecast is dangerous.

In the heavily populated San Fernando Valley and LA Basin, NWS said streams and small rivers will rise quickly and turn into “very dangerous raging rivers." That includes the Los Angeles River, which is mostly encased in concrete as a century-old measure to reduce flood risk.

Karla Nemeth, director of the California Department of Water Resources, said the state activated its flood operation center on Saturday morning. So far, state officials expect five rivers to reach flood levels, as far north as the Russian River in Mendocino County. Another 16 rivers will reach flood monitoring stages, Nemeth said in a news conference.

Meanwhile, Los Angeles County issued evacuation orders near Topanga Canyon, southeast of Calabasas, as well as an RV park near Agua Dulce, east of Santa Clarita.

Santa Barbara's evacuations covered several areas of the county. This included a few burn areas, such as one where a 2018 mudflow, brought by heavy rain, killed nearly two dozen people and injured hundreds.

Ventura County Sheriff's emergency services ordered evacuations near Ojai and Ventura.

The state is also expected to see periodic strong, gusty winds that likely lead to property and tree damage, as well as power outages. Mountains are also expected to see heavy snowfall across the entire state at elevations as low as 2,500 feet in Northern California, and upwards of 5,000 feet in Southern California. Multiple feet of snow are likely to collect across several mountain ranges, causing extremely difficult travel in these areas. 

Gov. Newsom announced the state has over 8,000 personnel mobilized for the storms. So far, this includes state firefighters and water and urban search and rescue in 19 counties, along with National Guard, transportation staff, highway patrol and state conservation corps.