Millions of Californians already hammered by ferocious snowfall were preparing Thursday for a new storm, with torrential rain threatening to cause dangerous flooding and the Weather Prediction Center increasing its excessive rainfall outlook for parts of the state Friday to a level 4 of 4.

“And then in the higher elevations, it will wash away some of that snowfall. So, rain on snow will begin to fill up parts of the San Joaquin Valley.”

About 17 million people are under flood watches in California and slices of Nevada. Heavy rain is underway with the worst rainfall and most significant impacts expected to occur from late Thursday afternoon through the day Friday.

The level 4 excessive rainfall warning is targeted to two sections in central California – the coast from Salinas southward to San Luis Obispo and areas in the foothills of the Sierras near Fresno. The last time the Bay Area and Central Coast were in “high risk” was in 2010, the National Weather Service office in San Francisco said.

Much of the state is under some risk of excessive rainfall Thursday and Friday.

“An atmospheric river will bring anomalous moisture to California Thursday and Friday. The combination of heavy precipitation and rapid snow melt below 5,000 feet will result in flooding,” the prediction center said Wednesday, adding that “numerous” floods are likely for millions.

The most vulnerable areas for flooding from rain and snowmelt are creeks and streams in the foothills of the Sierra Nevada, the prediction center said.

Higher elevations will see heavy wet snow. “This will lead to difficult travel, and combined with an already deep snowpack, may lead to increasing impacts from the depth and weight of the snow,” the prediction center said.

The bleak forecast spurred officials across central and Northern California to urge residents to prepare, with residents in one area advised to stock up on essentials for two weeks. Others were asked to use sandbags to protect their properties and clear their waterways to lessen any flooding impacts.

Widespread rain, strong winds, and potential thunderstorms arrive this afternoon. Taking a look at potential rainfall totals through Friday night, the Santa Cruz Mountains and the Santa Lucia Range will see the most rain from this system. #CAwx pic.twitter.com/5qPXdt18VB

— NWS Bay Area (@NWSBayArea) March 9, 2023

Here’s what the storm could bring:

• Heavy rainfall: Most urban areas could get 1.5 to 3 inches of total rainfall, the National Weather Service in the San Francisco Bay Area said. The threat heightens for coastal ranges and inland hills, which could get deluged with 3 to 6 inches of rain, the weather service said. The Santa Cruz Mountains could see up to 8 inches of rain, and local areas with higher terrain could eventually be inundated by up to 10 inches of rain over a prolonged period of time. “The abnormally warm and wet conditions moving in are expected to cause rapid snowmelt. Combine this snowmelt with as much as 10 inches of rain in the 24 hours [from Thursday evening to Friday evening], and the potential for widespread flooding is considerable, especially in the High Risk areas,” the prediction center noted.

• Ferocious winds: More than 15 million people across central and Northern California, northern Nevada and southwestern Idaho are under high wind alerts. Wind gusts could reach up to 55 mph across lower elevations and up to 70 mph across peaks and mountains. Strong winds could knock down power lines and trees – exacerbating thousands of existing power outages from previous storms that dumped heavy snow, particularly in higher elevations.

• More intense snow: Parts of the Sierra Nevada above 8,000 feet could get hit with 8 feet of snow. And some higher elevations across southern Oregon and the Rocky Mountains in Idaho, Montana and Wyoming could get pounded by 2 feet of snowfall between Thursday and Friday.

Already, 34 of California’s 58 counties are under a state of emergency issued by the governor’s office due to previous storms and this week’s severe weather.

Many of the areas preparing for Thursday’s storm have not had a chance to recover from the multiple rounds of fierce snow that buried some neighborhoods and made roads inaccessible as residents ran low on essential supplies.

How local officials are preparing

As the storm hits central California, some urban flooding along with flooding from the smaller creeks and streams is likely. Eventually, more roads are expected to flood as the main rivers rise, said Katrina Hand, a meteorologist at the weather service’s Sacramento office.

San Francisco officials urged small businesses to clear storm drains, stock up on inventory, use sandbags and ensure equipment is properly stored. They also suggested employers consider adjusting their work schedules for workers’ safety.

In Merced, crews tried to clear storm drains and fortify creek banks ahead of the storm.

City officials said flooding from previous, deadly rounds of atmospheric rivers that battered much of the state in January has made the city’s water ways unsafe.

Atmospheric rivers are long, narrow bands of moisture in the atmosphere that carry warm air and water vapor from the tropics.

“The city urges all residents to avoid these waterways and walking paths,” Merced officials said. “Because of ground saturation and erosion from prior storms, expect to see more debris in creek flows.”

In Monterey County, parks will close Thursday and Friday due to the storm’s threat, officials tweeted Wednesday.

In the Big Sur area, officials urged residents to have enough food and other essentials for at least two weeks. The Big Sur area, a roughly 90-mile stretch of California’s central coast, is one of the area’s renowned tourist attractions with rugged cliffs, mountains and hidden beaches along the Pacific Coast Highway.

In Kern County, home to Bakersfield, fire officials urged residents to create emergency kits and to be aware of escape routes and safe areas to seek shelter if needed. Officials also encouraged the use of sandbags to protect properties.

In San Luis Obispo, city officials said residents should be informed on flood insurance policies and be prepared to protect their homes and for possible evacuations.

And in Sacramento, city officials said they intend to open overnight warming centers beginning Friday in preparation for the expected heavy rainfall and low temperatures.