LOS ANGELES — Millions of California residents recovering from a deluge that killed at least one person over the weekend and contributed to flooding, mudslides and power outages are bracing for the arrival of another atmospheric river, a swath of long, narrow stream of moisture drawn from the tropics. .

Flood watches are in effect for 12 million people in central and northern California. Many of these communities were hit hard over the weekend by strong winds and rain, and will be hit again starting Wednesday.

In Los Angeles, where temperatures topped 70 degrees less than two weeks ago, a cold front prompted local authorities to declare a cold weather watch Tuesday as wind chills are expected to drop below 32 degrees in some communities.

Clouds over the Los Angeles Harbor on Friday.Carolyn Cole/Los Angeles Times via Getty Images

The looming storm is the latest extreme winter weather to hit the US. In recent weeks, storms have set records and left hundreds of thousands of people without power across the country.

Atmospheric rivers, plumes of concentrated moisture that flow into the sky, can generate up to 50% of the precipitation that reaches some parts of California. They are often described as fire hoses because on weather radar, they are often seen as jets of water vapor shooting up from the tropics.

The Center for Western Weather and Extremes, a research agency that lists atmospheric rivers on a scale of 1 to 5 based on how much water vapor they carry and how long they will linger, called the upcoming storm a Category 3 event.

“This is a serious situation to monitor the forecast and take action,” said Marty Ralph, the agency’s director.

CalTrans workers clear a fallen tree blocking traffic on both lanes of State Highway 68 in Monterey, California on December 31, 2022.
State Department of Transportation workers clear a downed tree blocking traffic on both lanes of State Highway 68 in Monterey, California, on Saturday. Nic Coury/AP

This atmospheric river is accompanied and intensified by an offshore ocean surface low pressure pattern that is expected to rapidly “pump” and strengthen over the Pacific.

“It will become a weather bomb cyclone,” Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research, said in a statement. YouTube summary of the weather patterns behind the storm.

A bomb cyclone is a rapidly intensifying storm. A strong one contributed to an arctic blast last month that brought deadly winter weather to much of the country.

A cable car on Nob Hill, San Francisco, as heavy rain hits the area.
A cable car on Nob Hill in San Francisco as heavy rain hit the area on Thursday.Tayfun Coskun/Anadolu Agency via Getty Images

The bomb cyclone is not likely to make landfall in California, but it will bring widespread strong winds and heavy rainfall likely to trigger flooding and mudslides, especially in northern California, but also as far south as Los Angeles, Swain said in a statement. sent by email.

The cyclone will send a pair of warm and cold fronts over Northern California, which is already drenched in rain. And more storms could come.

“We are stuck in this prolonged wet pattern. It looks like it’s going to continue for a while, in fact, really for the foreseeable future now,» Swain said, with modeling hinting at the possibility of more atmospheric storms on rivers this weekend and next week.

atmospheric rivers cause $1.1 billion in annual flood damage on averageaccording to research by Ralph of the Center for Western Weather and Extremes. About 84% of flood damage in the western states it is associated with atmospheric rivers.

Flood water in a house Monday after heavy rain in San Francisco.
Flood water in a house Monday after heavy rain in San Francisco.David Paul Morris/Bloomberg via Getty Images

Because a warmer atmosphere can absorb more water vapor, atmospheric rivers are expected to become stronger as the climate changes.

“When you have more water vapor in the air because the air is warmer, you can transport more water vapor quickly,” Ralph said.

Flood damage from atmospheric rivers in the West could double or triple by the end of the century, according to Ralph’s researchwhich modeled the results in scenarios of moderate and high greenhouse gas emissions.

This week’s storm will arrive early Wednesday and is likely to bring even more rain and wind than a weekend storm that caused flooding and power outages from the Bay Area to the Sierra Nevada foothills.

In Sacramento, a New Year’s Eve deluge killed one person, prompted the evacuation of more than 1,000 inmates at a county jail and washed away a section of a levee system that protects mostly rural farmland.

Winter storm shakes US travel season with cold and snow
A worker stands between Alaska Airlines planes during a snow storm at Seattle-Tacoma International Airport on December 20.David Ryder/Bloomberg via Getty Images

Southerly winds are expected to gain strength during Wednesday, giving way to a cold front and associated atmospheric river. Gusts are expected to reach 40 to 60 mph on the ridges and higher mountain peaks.

«These winds, combined with already saturated soils, are likely to cause downed trees and branches causing isolated or scattered power outages, as well as possible property damage,» the National Weather Service in San Francisco said in its report. daily forecast.

Urban areas in Northern California could receive up to 5 inches of rain, while coastal regions could receive up to 8 inches through Thursday.

The state’s severe drought could offset some of the impacts of the flooding, Swain said. Although some smaller reservoirs in Northern California have filled up, the larger reservoirs still have the capacity to absorb more.

Heavy rains are expected to help reduce some impacts of the drought.

«I think we will have greatly alleviated the near-term drought in Northern California,» Swain said. «From a surface soil moisture perspective, a water flow perspective, it’s all going to look pretty good.»

He warned that longer-term impacts such as groundwater deficits, deep soil moisture and forest health will not be corrected in a single season.

Alicia Victoria Lozano reported from Los Angeles and Evan Bush from Seattle.