A third atmospheric river storm is set to add to misery in California's flooded areas

Forecasters in Northern California have a sobering new-year message for people who are reeling from floods and mudslides: the situation could get worse before it gets better.

Parts of the state remained under flash flood warnings Monday morning, after a weather phenomenon known as an atmospheric river dropped historic rain levels on San Francisco, Oakland and other areas. But a second atmospheric river is predicted to arrive soon — and it will be as bad or worse than the New Year's Eve deluge, forecasters warn.

"Much like the end of 2022 storm, this will be a strong wind event along with moderate to heavy rainfall," the National Weather Service in the San Francisco Bay Area said on Monday.

It's the third atmospheric river to hit the region since Dec. 26, the office said, adding that parts of the Russian River are now at particular flood risk. The NWS also says the storm's high winds could inflict severe damage in areas where the soil is already saturated with rainwater.

Here's a rundown of where things stand, and where forecasters say they're headed:

The storm looms over a region coping with floods

As the new year arrived, a large swath of California was hit with heavy rain and snow. Floods and rock slides closed roads, and high winds knocked out electricity for tens of thousands of people.

San Francisco's downtown NWS site recorded 5.46 inches of rain on Dec. 31 — the second-wettest day at that location in more than 170 years, the NWS said.

As of midday Monday local time, around 39,000 electricity accounts were without power in California, with another 20,000 in Nevada, according to PowerOutage.us.

In the Sierra region, the storm dropped snow at a rate up to 7.5 inches per hour, according to the NWS office in Reno. The Tahoe basin saw 20 to 24 inches of snow at lake level, with roughly twice that amount above 7,000 feet.

Emergency crews rescued people from vehicles that couldn't move because of floodwaters. At least one death was linked to the weekend storm, after workers in southern Sacramento County found a person dead inside a vehicle submerged in water near Highway 99, as member station Capital Public Radio reports.

On Sunday, Sacramento's Mary Spencer-Gode and other residents gaped at the damage on their street, where the storm toppled a massive elm tree on New Year's Eve.

"The wind was just going crazy," she told Capital Public Radio. "We turned our TV off so we could hear it, and I was sitting in the kitchen, I heard a big 'woosh' and kind of the house moved."

Then came more flood worries: Sacramento County authorities on Sunday escalated an evacuation warning for the community of Point Pleasant to an evacuation order, saying flooding was imminent and would "become incredibly dangerous after sunset."

Across California, light precipitation is expected early this week — the first of two smaller systems that forecasters predict will bracket an intense storm called an atmospheric river.

Wait, what's an atmospheric river?

Atmospheric rivers are a normal part of the West Coast's weather pattern, and they're often the solution to months of warm-weather drought, bringing sorely needed rain and snowfall that packs water away high in the mountains.

"It's just a narrow area of high moisture that gets transported away from the tropics towards the higher latitudes," often before a cold front arrives, as NWS senior forecaster Bob Oravec recently told NPR.

For states along the West Coast, atmospheric rivers are "actually responsible for a good majority of the rainfall during the colder season, which is the season when they get most of their rain," Oravec said.

The precipitation can be extreme: a single atmospheric river "can carry more water than the Mississippi River at its mouth," as NPR has reported. And forecasters have long warned that the systems' winds are very dangerous. Five years ago, one of the storms toppled the legendary "Pioneer Cabin Tree" sequoia in Calaveras Big Trees State Park.

Atmospheric rivers are more likely to occur in a La Niña climate pattern like the one we're now seeing, with waters in the Pacific Ocean cooler than average. This is the third consecutive winter in which La Niña has prevailed, according to Climate.gov.

Expect to see the atmospheric river arrive by Wednesday

The strong Pacific storm system will likely start to hit California by late Tuesday and early Wednesday, according to the Bay Area NWS office, which says the storm has consistently shown "impressive numbers."

Predictions currently show the storm slowing as it nears and arrives over land, boosting the damage it could do and prompting forecasters along the California coast to raise the alarm.

On Wednesday, the atmospheric river could bring 2 to 3 inches of rain in California's Central Valley, with 3 to 6 inches in the foothills and mountains, the NWS office in Sacramento said. In addition, a strong low-level jet stream will bring wind gusts of 35-50 mph in the valley and foothills, with winds hitting 60 mph in the mountains.

Because of its expected longer duration and prodigious amount of moisture, the incoming storm "should surpass the Saturday night storm by at least an inch and likely more in the upslope areas," the NWS office in Oxnard and Los Angeles said, adding that the rain is expected to taper off Thursday night into Friday.

Along with flooding, the risk of mudslides is especially high in sites of recent wildfires, where there's no longer enough ground cover to absorb and retain moisture.

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