LOS ANGELES, CA — What are the odds of a thousand-year storm hitting California twice in one month?
In this era of rapid climate change, such devastating storms aren’t really so rare, the Golden State discovered recently when storms unexpectedly flooded San Diego homes under six feet of water and washed away parked cars weeks after a similar downpour dropped a month’s worth of rain in an hour on Ventura County. Both storms were catastrophic and caught residents, businesses and community leaders by surprise.
The powerful downpours were dubbed “thousand-year” storms because the chance of such intense rainfall occurring in the same place twice is about once in a millennium, said Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography at UC San Diego.
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But as a series of looming atmospheric river-fueled storms may soon prove, increasingly more corners of the state can expect their own thousand-year storms as global warming makes our droughts and our storms more extreme.
Ralph and his team created the AR scale, which tracks and forecasts atmospheric rivers, assigning a numeric value to the storms that reflect the duration and intensity of the precipitation much in the same way categories assigned to hurricanes denote their destructive power. In the coming years, Californians are likely to become as alert to the AR scale as Floridians are to hurricane categories.
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“We do expect the wettest days to get wetter in the future and the dry periods in between the wet days to be longer and drier,” Ralph said. “We are seeing this in the climate models.”
Ralph joined the Center for Western Weather and Water Extremes 10 years ago just in time to see California experience its driest year on record followed by one of its wettest years, and its driest snowpack followed by record-setting snowfall.
“We are seeing extremes that are breaking previous records — that is what mother nature is delivering us,” Ralph said.
The AR scale is designed to help communities better understand what they are in for, and hopefully give warning of the potential for the kind of catastrophic flooding that took San Diego and Ventura counties by surprise.
In San Diego, nearly three times as much rain fell as forecasters expected.
“The water rose in an hour up to our necks,” San Diego resident Anna Ramirez told the Associated Press. “I had to pull a lady out of the water and she didn’t know how to swim. She was crying for her life. It was very scary, very traumatizing.”
Gov. Gavin Newsom declared a state of emergency in both counties.
“I find that local authority is inadequate to cope with the magnitude of the damage caused by these winter storms,” Newsom announced.
Atmospheric rivers can make or break California’s rainy season, pulling the state out of drought, filling lakes and aquifers and layering a healthy snowpack. However, too much rain too fast or too many atmospheric rivers in a row can be catastrophic. The Golden State experienced both the benefits and the hazards last year when atmospheric river-fueled storms ended the historic drought, dropped 10 feet of snow in a rare Southern California blizzard and caused flooding in Alameda, Merced and Monterey counties.
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The AR scale goes from one to five. An AR 1 is likely to be primarily beneficial while an AR 2 is mostly beneficial but has the potential to be hazardous. Things start to get a little more dangerous with an AR 3, which brings a balance of beneficial and hazardous conditions. Conversely, an AR 4 is likely to be mostly hazardous with some benefit while an AR 5 would be primarily hazardous, according to Ralph.
Even back-to-back AR 1 storms, which California can expect in the next two weeks, can be hazardous.
Today, an atmospheric river is barreling down on the Pacific northwest and is expected to head into the Bay Area Tuesday, according to the National Weather Service. It’s the first of several storms expected to slam the state in the coming two weeks.
The Center for Western Weather and Water Extremes issued an AR scale showing what to expect up and down the state.
“In this particular sequence of storms coming, the atmospheric rivers are primarily going to be hitting the northwest…where it’s batten down the hatches,” Ralph said.
The northern part of the state is facing an AR 3 while the Bay Area can expect an AR 2. The atmospheric river will dissipate by the time the storm reaches Southern California, according to the center’s forecast.
“We are closely watching what could be a notable pattern shift in late January and early February,” the weather service warned. “If this pans out, a period of very wet weather could impact California.”
The chance for abnormally wet weather is high for most of California, and the Sierras are in for days of snowstorms.
Though a subsequent next storm remains far out, there are signs that the following week Southern California may be in for another wallop.
“The heavy precipitation can lead to localized flooding and landslides, particularly in regions that have recently received heavy rains,” the Climate Prediction Center warned. “At higher elevations, such as the Sierra Nevada, Klamath Mountains, and the Mogollon Rim, heavy snow is expected.”
Ralph has his own warning for residents and the era of thousand-year storms.
“If it is an atmospheric river, I think it warrants a special sort of heads up, he said. “Pay more attention when you see an atmospheric river in the forecast.”
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