By Alicia Victoria Lozano and Evan Bush -
NBC News
LOS ANGELES — Twelve million people are under alert in central and northern California for the arrival Wednesday of a new atmospheric river, a long, narrow band of moisture from the tropics, just days after a great deluge triggered in the same area floods, landslides, power outages and will leave at least one person dead.
In Los Angeles, where temperatures reached 70 degrees Fahrenheit (21 degrees Celsius) less than two weeks ago, a cold front prompted local authorities to declare a cold weather alert Tuesday as wind chills are expected to drop by below 32 degrees (0 degrees Celsius) in some communities.
Vehicles submerged on Dillard Road west of Highway 99 in Wilton, Calif., on Sunday after heavy rains on New Year's Eve caused levee breaches.Hector Amezcua/The Sacramento Bee via AP
In recent weeks, storms have broken records, knocking out power to hundreds of thousands of people across the country.
Atmospheric rivers, plumes of concentrated moisture that flow across the sky, can cause up to 50% of the precipitation that reaches some parts of California.
They are often described as
firehoses
because, on weather radar, they are often seen as jets of water vapor shooting out of the tropics.
The Center for Western Weather and Extremes, a research organization that ranks atmospheric rivers on a scale of 1 to 5 based on the amount of water vapor they carry and how long they last, has rated the approaching storm as category 3.
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“This is a serious situation where you have to watch the forecasts and take action,” said Marty Ralph, director of the agency.
This atmospheric river is accompanied and intensified by a low pressure pattern at the ocean surface offshore, which is expected to “bombard” and rapidly strengthen over the Pacific.
Workers clear logs from a roadblock in Monterey, California, on Dec. 31, 2022. Nic Coury / AP
“It will become a weather bomb cyclone,” Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research, warned in a YouTube briefing on the weather patterns behind the storm.
A bomb cyclone is a rapidly intensifying storm.
Last month, one of them triggered an arctic storm that brought a deadly winter to much of the country.
The bomb cyclone is unlikely to make landfall in California, however, but will bring widespread strong winds and heavy rainfall, likely to leave behind flooding and landslides, especially in northern California, but also as far south as Los Angeles Swain explained in an emailed statement.
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The cyclone will send a pair of warm and cold fronts over Northern California, which is already drenched from rain in recent weeks.
And more storms could come.
“We are locked into 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 models hinting that there may be more atmospheric rivers this weekend and into next week.
From north to south and from east to west the force of nature strikes many communities in the US.
Jan 4, 202301:52
Atmospheric rivers cause an annual average of $1.1 billion in flood damage, according to research by Ralph of the Center for Meteorology and Western Extremes.
About 84% of flood damage in the western states is associated with atmospheric rivers.
Since a warmer atmosphere can absorb more water vapor, atmospheric rivers are expected to grow stronger as the climate changes.
“When there is more water vapor in the air, because the air is warmer, more water vapor can be transported quickly,” Ralph said.
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Flood damage from atmospheric rivers in the West could double or triple by the end of the century, according to Ralph's research, which modeled the results under moderate and high greenhouse gas emissions scenarios.
This week's storm will arrive early Wednesday, and is likely to bring even more rain and wind than the weekend storm, which caused flooding and power outages from the Bay Area to the Sierra Nevada foothills.
In Sacramento, the New Year's Eve deluge killed one person, forced the evacuation of more than 1,000 inmates from a county jail and washed away a section of a levee system that protects mostly rural farmland.
Traffic moves through flooded lanes on the 101 Freeway in south San Francisco, Calif., Saturday, Dec. 31, 2022. Jeff Chiu / AP
Southerly winds are expected to gain strength throughout Wednesday, giving way to a cold front and associated atmospheric river.
Gusts are expected to reach 40 to 60 miles per hour on the ridges and higher mountain peaks.
“These winds, combined with already saturated soils, will likely cause downed trees and branches, causing isolated or scattered power outages, as well as possible property damage,” the San Francisco National Weather Service says in its daily forecast.
Urban areas of Northern California could receive up to 5 inches of rain, while coastal regions could receive up to 8 inches through Thursday.
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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.
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Jan 2, 202301:43
“I think we will have greatly alleviated the near-term drought in Northern California,” Swain said.
"From the standpoint of surface soil moisture, stream flow, it's all going to look pretty good."
He warned that longer-term impacts, such as deficits in groundwater, deep soil moisture and forest health, will not be corrected in a single season.