CNN
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Millions of Californians already hammered by ferocious snowfall were hit Thursday by 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 to a level 4 of 4.

“If you have feet of snow on your roof, all of a sudden that’s going to get very, very heavy. That snow is going to absorb the rainfall,” CNN Meteorologist Chad Myers warned Thursday.

“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 16.7 million people are under flood watches in California and slices of Nevada. Hourly rainfall rates will steadily increase in intensity across California from Thursday overnight through Friday morning, potentially reaching 1 inch per hour.

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 – Thursday overnight into Friday. 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.

“We are asking people to watch their news, stay informed, have a full tank of gas in case they need to evacuate, get snow off of their roof if they can, if it’s safe,” Lt. Gov. Eleni Kounalakis told CNN on Thursday. “And just be very vigilant and prepared, because we are in the era of extreme weather, and that’s what we are seeing this week.”

Here’s what the storm could bring:

• Heavy rainfall: The National Weather Service in San Francisco forecasts rainfall totals through Sunday morning will be from 1.5-3 inches for most urban areas with 3-6 inches in some hilly areas. As many as 8 inches could fall on the Santa Cruz Mountains and locally up to 12 inches over some peaks and higher terrain of the Santa Lucia Mountains. The National Weather Service in Los Angeles is forecasting 2-4 inches across Santa Barbara and San Luis Obispo counties, with some areas in the latter receiving as many as 10 inches through late Friday night. The Weather Prediction Center said: “The abnormally warm and wet conditions moving in are expected to cause rapid snowmelt.”

• 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. The state activated its flood operations center Thursday morning.

The forecast also led some ski resorts to announce closings. Kirkwood Mountain Resort said it would not open Friday, as did the Northstar California resort and the Heavenly resort in South Lake Tahoe, on Nevada’s border with California.

Meanwhile, the Eastern Sierra Avalanche Center issued a backcountry avalanche warning for sections of Mono County, according to the National Weather Service in Reno, Nevada.

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.

In hard-hit San Bernardino County, one of the recent storms claimed the life of a resident in a car crash, the sheriff’s department told CNN on Wednesday.

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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 San Luis Obispo, city officials on Wednesday said residents should be informed on flood insurance policies and be prepared to protect their homes. On Thursday, they issued an evacuation order for residents south of the Arroyo Grande Creek Levee.

Evacuation warnings were also issued for residents in low-lying areas of Santa Cruz County and for people in Tulare County.

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.

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.