CNN
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California is bracing for another powerful atmospheric storm this week, continuing the onslaught of a major weather boost after a historic year-long mega-drought.
Many hailed the heavy rains and snow this winter because they were so desperately needed to replenish the state’s badly drained reservoirs and depleted groundwater.
But the storms kept coming. California is now facing its 12th significant atmospheric river since the start of the parade of strong storms in late December.
“This is an unusually high number of storms this winter in California,” said Daniel Swain, a climatologist at the University of California, Los Angeles. “No matter how you slice it, no matter how you make these formal definitions, it’s unusually many.”
Determining how to count atmospheric rivers is an ongoing conversation in the scientific community.
While Swain said California has seen 12 significant atmospheric rivers so far this season, Chad Hecht, meteorologist at the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes (CW3E), said the center has. counted 29 so far, including some of the weaker systems and systems that only hit California. Six of the storms fell into the category of what Hecht described as severe or extreme.
“These numbers are probably higher than most numbers you see in the media, because their criteria for an atmospheric river are probably more stringent and take into account the impacts they bring,” Hecht said. “There is [also] instances where an atmospheric river primarily targets the Pacific Northwest, but cuts through portions of extreme northwest California, e.g. Del Norte County, with weak conditions.
Weak, low-end moderate thunderstorms tend to have less impact and mostly bring beneficial precipitation to the state, he said. Meanwhile, high-end atmospheric rivers are the big producers of rain and snow, leading to more severe impacts.
Hecht said this year has already exceeded the state’s average annual atmospheric river count. Many of them came in a rapid series of storms in early January.
“We usually call these successive types of atmospheric rivers ‘AR families,'” Hecht told CNN. “While AR families aren’t that rare, we don’t see them every year and the run of nine we had at the start of the new year was a more active family than we usually see.”
While the barrage of rain and snow eased the drought, the storms were also destructive and deadly.
An atmospheric river is like a conveyor belt of moisture that originates in the tropical waters of the Pacific Ocean. They can carry more than 20 times the amount of water in the Mississippi River, but in the form of steam. As these storms hit the state in quick succession, the ground became oversaturated and vulnerable to flooding and landslides.
It’s unclear how the climate crisis could play a role in the number of storms that hit the West Coast. But climatologists have linked the climate crisis to an increase in the amount of humidity in the atmosphere, which means that storms – such as the hurricanes and atmospheric rivers currently affecting the west coast – will be able to bring more humidity. inland than they would without the climate. change, which in turn results in increased rainfall rates and flash floods.
Swain said this week’s storm will be another rapidly strengthening event and could even approach what’s called a “bomb cyclone” – a rapidly intensifying storm at a rate of at least 24 millibars of pressure in 24 hours. The storm is expected to affect much of the California coast, from San Francisco Bay to Southern California.
“This [storm] is going to bring a whole litany of concerns that are probably bigger than what we originally anticipated a few days ago,” Swain said. “Frankly, even widespread moderate rain at this point is going to make flooding conditions worse in some places – so that’s not the best news.”
Despite those dangers, Swain said California is lucky to have breaks between storm cycles. And forecasters for the National Oceanic and Atmospheric Administration announced last week that they expect the western rain and snow spigot to likely turn off in April.
“Even though people feel like the winter has been relentless, we actually had breaks of at least a week, and in some cases breaks of weeks between those streaks,” he said. he declares. “Had we had this winter and everything going on without any interruptions during the storm cycles, the flood level and damage level in California would have been significantly higher.”
The storms also erased the disastrous consequences of the drought, especially the mandatory water cuts in parts of the state.
Severe drought now covers just 8% of the state, with just over a third of the state remaining in some level of drought — the lowest amount in nearly three years, according to the latest US Drought Monitor. years.
