California flooding: New storm threatens to bring heavy rain to places still covered in snow

Millions of Californians already hammered by ferocious snowfall are about to get hit with a new storm starting Thursday, with torrential rain threatening to cause dangerous flooding.

"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."

More than 17 million people are under flood watches in California and slices of Nevada. Much of California is under a significant risk of excessive rainfall that could inundate places already devastated by severe flooding earlier this year.

"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 Weather Prediction Center said, 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. "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.

Northern California can expect to see rain and snow beginning late Thursday morning, with rainfall set to steadily intensify over the central and northern parts of the state through early Friday morning.

"Rain will fall on snow cover, potentially making the flood situation even worse as the snow melts and the rain accumulates," CNN meteorologist Dave Hennen 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 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:

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.

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.

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.

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