California residents near Big Sur have been told to stock up on enough supplies to last them at least two weeks ahead of a new storm.
Intense flooding and snow is expected in the state, which is still recovering from extreme weather that left mountain communities under several feet of snow.
Yosemite National Park, which has already broken snow records, could get 6ft (1.8m) of snow.
Experts say the combination of rain and snowmelt could lead to flooding.
California has already had one of its wettest winters on record with the state hit by back-to-back storms in recent weeks.
Nine atmospheric rivers – which occur when water evaporates into the air and is carried along by the wind, forming long currents that the sky that can bring severe rains and mountain snow – came through the state in early January.
Up to 7ft (2.1m) of snowfall earlier this month left residents stranded in the San Bernardino Mountains.
The San Bernardino County Sheriff said the department has rescued 100 people in the region so far, but at least one person has died.
Volunteer helicopters are bringing in supplies for those still trapped in the mountains without enough food and water.
Northern and central California are expected to bear the brunt of this storm though, with 16 million people under flood watches as of Wednesday.
Noah Diffenbaugh, earth system science professor at Stanford University, says the context of this storm is important – while the current forecast may not look too severe, the extreme weather that preceded it makes it more risky.
“We have had a lot of moisture accumulation, saturating soils and so the potential for landslides, debris flows, is enhanced,” he told the BBC.
California Governor Gavin Newsom declared a storm state of emergency in 21 additional counties ahead of the atmospheric river event in addition to the 13 counties with a state of emergency already in place from the last storm.
On Twitter, the California Governor’s Office of Emergency Services told residents to “check in on loved ones to make sure they have a plan, get your go bag ready with essential items and be prepared to evacuate if told to do so”.
Incoming rain could rapidly melt existing snow, putting lower elevation areas at high risk of flooding.
Coastal areas could see as much as 5in (12.7cm) of rain.
Big Sur, south of San Francisco, as well as the central coast and Bay Area, are under a flood watch until Sunday.
Monterey County is offering residents sandbags to protect their property ahead of the storm. Earlier this year, storms caused landslides near Big Sur, closing off a major highway and cutting residents off from supplies.
Fearing a repeat, Big Sur residents have been urged to gather essentials now.
In San Francisco, businesses were advised to secure outdoor furniture, adjust staffing for employees and pick up 10 free sandbags in preparation of the storm.
The city of San Luis Obispo has warned locals to prepare.
In case of a power outage, residents were told to “locate your flashlights, charge your devices, and remember to only use generators outside”.
They were also advised to “move important documents and keepsakes into waterproof containers and store them at higher levels in your home” especially if they experienced flooding from the storm in January.
Meanwhile, residents in high elevation areas have been told to try to clear their roofs of snow as rainfall could weigh down existing snow leading to potential roof cave-ins.
Central parts of the state with elevation over 8,000ft (2,438m) – including Yosemite National Park – could expect to get 6ft (1.8m) of snow.
Yosemite National Park, closed since the last winter storm, said it will remain closed until March 12 – possibly longer.
At elevations over 5,000ft (1524m) the National Weather Service said there is potential for “significant avalanche danger”.
In terms of how this storm affects California’s drought, Dr. Diffenbaugh says the overall “outlook is certainly a lot better now than it was in October” but there remain “accumulated water deficits”.
About 30% of the state’s water supply is dependent on snowpack, which is accumulated snow on the ground, according to the California Department of Water Resources.
“The larger amount of snow accumulation is great news from a water resources point of view” but it “does create some elevated risk of spring flooding”, Dr. Diffenbaugh said.