Last week, the National Weather Service warned Northern Californians of a “brutal” storm system approaching the coast, threatening floods, landslides, power outages, and potential loss of life. San Francisco Mayor London Breed even said the city was “preparing for war” with sandbags, shelters, and emergency supplies, and urging people to stay indoors during the storm.

California Gov. Gavin Newsom said Sunday that 12 people in the state have died so far, asking people not to “test fate” as “the worst of it is still ahead of us.” With nearly 130,000 households without power as of Monday afternoon according to tracking website Poweroutage.us, Californians may have to continue hunkering down for several more days.

The National Weather Service warned in a news bulletin Monday of a “parade of cyclones” over California, as storms threaten to disrupt daily life across the state. On Sunday evening, President Joe Biden declared a state of emergency in California over the storms, ordering Federal assistance to local response efforts.

The NWS said “two major episodes” of heavy rain and snowfall are nearing the state that will exacerbate current conditions. The first hit coastal areas on Monday with heavy precipitation while the second that is expected on Tuesday will lead to higher rain and snowfall in the south and inland. The news bulletin warned of “rapid water rises, mudslides, and the potential for major river flooding” across the entire state. 

Heavy snow in California’s mountainous inland areas could create dangerous travel conditions and lead to avalanches. Coastal regions meanwhile are bracing for intense wind gusts, downed trees, and even more power outages. The risks are highest in areas that were hit by the state’s record-breaking wildfires last summer, where the soil is looser and more prone to deadly mudslides and debris flows. 

The NWS warned of thunderstorms and a greater than 50% chance that flash flood warning thresholds will be exceeded across the state over the next few days. Forecasters are expecting these conditions to persist until Wednesday.

The storm’s intensity is being caused by an “atmospheric river” originating from the tropics and carrying so much water vapor that the National Oceanic and Atmospheric Association says it rivals the average flow at the mouth of the Mississippi River. These atmospheric rivers dump huge amounts of rain or snow, and are often accompanied by powerful winds. 

The atmospheric rivers have been overlapped since last week with low-pressure systems known as “bomb cyclones,” sudden drops in air pressure that form when cold and warm air collide and can turbocharge existing storms.

The Sacramento Valley is currently under a flood advisory, while some cities have asked residents to leave before the worst of the storm hits. Officials in Wilton, a town in Sacramento County of around 5,000, issued an evacuation order for residents Sunday evening, citing the high risk of flooding cutting off routes leaving the area. Evacuation advisories were also issued Sunday evening in parts of Alameda and Santa Clara counties.

As of Monday, more than 34 million people in California are under a flood warning, nearly 90% of the state’s population.

The heavy rainfall could help refill California’s parched reservoirs, which have been at critically low levels after a multiyear drought. Officials have said the state’s largest reservoirs have enough capacity to absorb the precipitation, although meteorologists have warned that reservoirs and ecosystems need a steady flow of rainfall to replenish, while the fast and heavy precipitations of the last few days could oversaturate the ground with water and lead to overflowing reservoirs and even higher flood risks.