Atmospheric river expected to cause ‘bomb cyclone’ and more floods in California

LOS ANGELES — Millions of Californians recovering from a flood that killed at least one person over the weekend and caused flooding, landslides and power outages are bracing for another atmospheric river — a long, narrow swath of moisture from the tropics. .

Flood Watch operates for 12 million people in Central and Northern California. Many of these communities were hit hard over the weekend by high winds and rain, and will be hit again from Wednesday.

In Los Angeles, where temperatures topped the 70-degree mark less than two weeks ago, a cold front prompted local officials to issue a cold weather warning on Tuesday as cold wind temperatures are expected to drop below 32-degree temperatures in some communities.

Clouds over the harbor of Los Angeles, December 30, 2022.
Clouds over Los Angeles harbor on Friday.Caroline Cole/Los Angeles Times via Getty Images

The coming storm is the latest extreme winter weather to hit the US. In recent weeks, storms have set records and cut power to hundreds of thousands across the country.

Atmospheric rivers, streams of concentrated moisture flowing in the sky, can carry up to 50% of the precipitation that falls in some areas of California. They are often referred to as fire hoses because they often appear on weather radar as jets of water vapor erupting from the tropics.

The Center for Western Weather and Extreme Events, a research agency that lists atmospheric rivers on a scale of 1 to 5 based on the amount of water vapor they carry and how long they will linger, has rated the impending storm as a Category 3 event.

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

CalTrans workers clear a fallen tree blocking traffic on both lanes of State Route 68 in Monterey, California, December 31, 2022.
State Department of Transportation workers clear a fallen tree blocking traffic on both lanes of State Route 68 in Monterey, California on Saturday. Nick Kouri / AP

This atmospheric river is accompanied and amplified by a low-pressure model at the surface of the ocean in the coastal zone, which is expected to “explode” and rapidly intensify over the Pacific Ocean.

“This is going to be a weather bomb cyclone,” Daniel Swain, a climatologist at UCLA and the National Center for Atmospheric Research, said in a YouTube briefing on the weather conditions behind the storm.

A bomb cyclone is a storm that grows rapidly. A strong one contributed to last month’s Arctic explosion that brought deadly winter weather to much of the country.

The cable car in Nob Hill, San Francisco, as heavy rain hit the area.
Cable car at Nob Hill in San Francisco due to heavy rain on Thursday.Typhoon Coskun/Anadolu Agency via Getty Images

The bomb cyclone is unlikely to make landfall in California, but it will bring widespread high winds and heavy rainfall that could lead to flooding and landslides, especially in Northern California and as far south as Los Angeles, Swain said. application by email.

The cyclone will send a pair of warm and cold fronts over Northern California, which is already wet from rain. And there may be more storms.

“We are stuck in this prolonged wet pattern. It looks like this will continue for some time, in fact, for the foreseeable future,” said Swain, whose models hint at the possibility of more atmospheric river storms this weekend and next week.

Atmospheric rivers cause an average of $1.1 billion in flood damage annually, according to a study by Ralph of the Center for Western Weather and Extreme Events. About 84% of flood damage in the western states is due to atmospheric rivers.

Flooding in the house on Monday after heavy rain in San Francisco.
Flooding in the house on 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 get stronger as the climate changes.

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

According to Ralph’s study, which modeled results for both moderate and high greenhouse gas emission scenarios, flood damage from atmospheric rivers in the West could double or triple by the end of the century.

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

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

Winter storm rattles busy U.S. travel season with cold and snow
A worker between Alaska Airlines planes during a snowstorm at Seattle-Tacoma International Airport Dec. 20.David Ryder/Bloomberg via Getty Images

Southerly winds are expected to increase during Wednesday, leading to a cold front and associated atmospheric river. In mountain ranges and peaks, wind gusts are expected to reach 40 to 60 mph.

“These winds, combined with already saturated soils, are likely to result in downed trees and branches, leading to isolated or scattered power outages, as well as potential property damage,” the San Francisco National Weather Service said in a daily forecast.

Up to 5 inches of rain could fall in urban areas of Northern California, and up to 8 inches could fall in coastal areas on Thursday.

Severe drought in the state could offset some of the effects of flooding, Swain said. Although some of the smaller reservoirs in Northern California are already full, larger reservoirs are still able to absorb more water.

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

“I think we’ve pretty much mitigated the short-term drought in Northern California,” Swain said. “In terms of soil surface moisture, in terms of river flow, everything will look pretty good.”

He warned that long-term effects such as groundwater scarcity, deep soil moisture and forest health would not be corrected in one season.

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