Total precipitable water content on Feb. 17, 2017.
Atmospheric river storms are drought busters in California, dumping huge amounts of rain in narrow corridors within short periods of time. The ongoing storm slamming Los Angeles, the nation’s second-largest city, as well as San Diego on Friday brings with it potentially historic amounts of rain, high winds and flood risks.
To truly understand the mechanisms behind this storm, though, you have to turn your attention to the tropics, where the deep moisture originates from. Satellite imagery showing the transport of atmospheric water vapor reveals the atmospheric river lurching like a ghostly finger toward California from the central tropical Pacific Ocean.
Now that this moisture is present across California, it is being wrung out of the air by a historically vigorous low pressure area that may set all-time intensity records for the month of February in the Golden State. In general, the lower the pressure, the stronger the storm, and this storm is predicted to bottom out near 985 millibars, which is typical of a Category 1 hurricane.
Image: university of wisconsin
The storm’s circulation will siphon the moisture in a south-to-north orientation, allowing it to butt up against the mountains in Los Angeles and San Diego counties, causing even heavier rains and heavy snow to fall in many spots. Rainfall totals of greater than 10 inches are in the forecast for coastal high elevations, with 1 to 2 feet or more of snow expected in areas above 7,000 feet.
Atmospheric rivers are narrow channels of air moving huge amounts of moisture from the tropics to the midlatitudes. How huge?
Consider this estimate from the National Oceanic and Atmospheric Administration, which found that a strong atmospheric river, such as the one striking California Friday and Saturday, can transport "an amount of water vapor roughly equivalent to 7.5 [to] 15 times the average flow of liquid water at the mouth of the Mississippi River."
The storm is likely to cause major traffic delays across Southern California, and may bring air travel to a standstill for several hours as winds gust to 50 miles per hour or more in San Diego, with slightly weaker winds in L.A. Greater than 4 inches of rain may fall during this storm even in downtown L.A., which would be the city’s largest 2-day rainfall total in at least six years or more.
Computer model projection showing the strong Calif. storm.
The peak of the storm in San Diego is likely to occur between noon and midnight on Friday, with winds that will down trees and power lines. The heavy rains will also cause flash flooding, mudslides and debris flows.
This storm is the first in a coming wave of atmospheric river-type storms that will affect California for the next week, raising concerns of continued flooding risks and stresses on water infrastructure like the Oroville Dam in north central California.
Nearly 200,000 people were ordered to evacuate on Feb. 12 due to fears that a part of Oroville Dam’s infrastructure — the nation’s tallest — could fail. Officials are watching Lake Oroville’s levels warily in case they reach record highs once again from heavy rain and snowmelt.
The Friday through Saturday storm is just the latest in a string of Pacific weather systems that have beaten back the state’s epic five-year drought in many areas, although groundwater supplies that take many years to recharge remain depleted.
Parts of the state have seen record rainfall within what’s known as a "water year," a period of time that includes both a wet and dry season between Oct. 1 through Sept. 30. This is remarkable considering there is no El Niño present, and such climate phases are typically associated with the truly blockbuster winters in the Golden State.
Climate change lessons
Without specific climate change attribution studies for this ongoing storm, it’s hard to say how much global warming spurred on by human-caused greenhouse gas emissions has influenced it.
However, there is a growing body of scientific evidence showing that hydrological extremes — both droughts and floods — are more likely to occur and be more severe in a warming world.
This winter’s storms, for example, are probably giving Californians an acute case of weather whiplash, since the storms are punctuating a half-decade of epic drought and record heat.
"The combination of this winter and the recent hot-drought form a pretty reasonable analog for thinking about the challenges to come," says Michael Dettinger, a research hydrologist with the U.S. Geological Survey in Nevada, via email.
Dettinger and other experts interviewed for this story said sudden swings from drought to flood may be the new norm, which will require a fundamental rethinking of infrastructure design for new dams, levees, bridges and other critical infrastructure across the West and the rest of the country.
"The current situation in California —specifically, the dramatic swing from extreme drought to water overabundance and flooding — is indeed a preview of California’s likely climate future," said Daniel Swain, a climate scientist at UCLA. "There is now quite a bit of evidence that future droughts here will be warmer and more intense, yet will be interrupted by increasingly powerful ‘atmospheric river’ storms capable of causing destructive flooding."
Some research shows that extremely prolific atmospheric rivers could become far more common — perhaps twice as common as they are now — in parts of California by the end of the century, though this is still a matter of scientific debate.
Dettinger’s work supports this conclusion, he said. "… What has really been going on is that we’ve just had so many [atmospheric river events] arrive in such a short time this winter. This is actually in keeping with one of the early findings re: west coast atmospheric rivers and climate change… that the numbers of atmospheric rivers will increase faster than their (average) intensities," he said.