Horizontal rivers of rain menace California now, according to "mounting research"

San Francisco Chronicle
‘Horizontal hurricanes’ pose increasing risk for California
By Kurtis Alexander

As increasingly intense hurricanes batter the Southeast and the Caribbean, heightening some of the worst fears about a changing climate, California is facing its own threat of bigger and more destructive storms. Mounting research, much of it done in the wake of the near-record rains that pulled California out of a five-year drought this past winter, shows that seasonal soakers may not come as often as they used to, but could pack more punch when they do arrive.
The potential consequences of the findings are enormous. The massive weather systems that rise out of the Pacific Ocean, now popularly called atmospheric rivers, can drop as much water as Hurricane Irma dumped on Florida this month — billions of gallons that submerged cities and towns.
A few years ago, the U.S. Geological Survey ran a simulation of what a sequence of severe atmospheric rivers might look like in California. It came away with a model of 23 days of rain and wind that caused floods and landslides to the tune of $300 billion in property damage.
A more tangible show of the system’s potential came with the big drenchers earlier this year, which caused mass flooding in San Jose and other cities and triggered a near-catastrophe at Lake Oroville when a pair of dam spillways failed.
Nonetheless, scientists have only begun to understand what drives atmospheric rivers. The term itself was coined just two decades ago. That’s the challenge a growing network of researchers is taking on, including a group in Sonoma County.
At a research station on Lake Mendocino, north of Ukiah, local water officials are working with outside climate experts to measure and monitor atmospheric rivers with the hope of better anticipating the giants. The group recently launched a $19 million radar system that will eventually ring the Bay Area.“Being able to forecast where landfall is going to be, and how long they’re going to linger, is important,” said Jay Jasperse, chief engineer for the Sonoma County Water Agency. “There has been improvement in the forecasting skill and the understanding of what’s causing these storms, but there’s still a lot to learn.”
Sometimes called horizontal hurricanes, atmospheric rivers are exactly what they sound like: airborne channels of water that develop over the Pacific Ocean and are pushed along by strong winds toward the West Coast during the winter.
The systems commonly wring out over California, providing as much as 50 percent of the state’s annual rainfall in a matter of days — dumpings that are critical to water supplies but, at times, bring on disaster.
Jasperse and his colleagues are already looking to apply what they’ve learned about the events to managing drinking-water reservoirs. For example, by better predicting rainfall, they can begin to more accurately free up reservoir space.
Perhaps more important, the Sonoma County research holds promise of providing flood and landslide warnings to residents across California.
“It’s really about getting the science to forecast extreme climate events,” said Sonoma County Supervisor Shirlee Zane, who helped win state and federal assistance for the water agency’s unusually advanced initiative. “The problems (of climate) are all being lived out in the last month, and sadly it’s being lived out in a way that’s so tragic to people.” A group of Southern California scientists, some of whom are partnering with Sonoma County, plan to begin publishing regular 14-day to 21-day outlooks on atmospheric rivers as soon as this winter. Current forecasts typically don’t anticipate the events more than a week out.
“There’s a lot of interest in knowing what the next storm is going to look like: Are we going to get pummeled, or can we handle it?” said Marty Ralph, who as one of the foremost experts on atmospheric rivers is spearheading the forecasting tool.
Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography, is also working to develop a rating system for the storms.
The blasts will be evaluated for intensity on a scale of 1 to 5, similar to hurricanes. A test run of the ratings last winter pegged the strong atmospheric river in February, which contributed to the damage at Oroville Dam, as a category 5 event.

Research published in the journal Geophysical Research Letters in August, which Ralph participated in, indicates that atmospheric rivers are carrying increasing amounts of water.
The authors, who analyzed storms over the past 70 years, speculated that a warming ocean and a warming atmosphere were producing more water vapor and energizing Pacific weather systems, similar to the way Atlantic Ocean hurricanes have been gaining strength.
Another study, published this month in the journal Scientific Reports, suggests that the swelling intensity of atmospheric rivers — while carrying the potential for more damage — will also help California maintain typical rainfall levels amid a drying climate.
In other words, rain might come less frequently but in hardier doses. The combination could mean big swings in rainfall year to year.
 “Whether we get a few or more atmospheric rivers in a year really determines whether we’re in a drought or flood,” Ralph said.
During the height of the drought, California went as long as a year without seeing a single atmospheric river. But the state was hit by more than 30 of the systems last winter, according to Scripps researchers. That explains why the season was one of the wettest on record.
Because of the prospect of significant damage, the U.S. Geological Survey conducted a disaster preparedness exercise in 2011, loosely based on a series of atmospheric rivers that struck during the winter of 1861-62. The string of storms a century and a half ago left much of California covered in water and thousands dead.
Dale Cox, who led the simulation, said it was designed around conditions the state could realistically experience in the near future.
“This wasn’t a way-out-there kind of storm,” he said. “As far as disasters go, earthquakes are the charismatic species. They’re the ones that people tend to fear the most. Floods don’t get the attention, yet that’s what’s going to be California’s big one.”