Scientists at NASA say they have identified a rare weather pattern that will help forecasters predict when California will experience periods of intense and potentially prolonged wet weather.
The new findings will help officials assess the possibility of floods, mudslides and levee failures, the scientists say, and will prove critical to regions where rivers are a big part of the landscape – like the Sacramento region, where the American and Sacramento rivers converge. The rivers absorb run off and melting snowpack from the Sierra Nevada, where storms typically deposit much of the state’s precipitation.
“We have found a strong connection between certain phases of two systems and the frequency of winter storms in California,” said Bin Guan, an earth sciences researcher at the University of California, Los Angeles and NASA’s Jet Propulsion Laboratory.
Guan’s study – a collaboration among scientists at UCLA and the National Oceanic and Atmospheric Administration – found a conclusive link between the alignment of two weather patterns in the Northern Hemisphere and the formation of an “atmospheric river” headed for California.
The results were gleaned in part from data provided by NASA’s 11-year-old Aqua weather satellite – one of more than 40 weather-related satellites that circle the globe.
Atmospheric rivers are narrow bands of wind, often a mile high, that can pack the punch of a hurricane. As they move over the ocean, they become laden with water vapor – and can carry with them as much water as the Mississippi River dumps into the Gulf of Mexico in an average week.
An example of the power of such an atmospheric river event was seen in 1999, when a winter storm hit California and caused 15 deaths and $570million in damage.
To arrive at his findings, Guan looked at 50 years of data, including information from the California Department of Water Resources for the winter of 2010-11, when 20 atmospheric river storms made landfall.
The two weather systems studied were the Arctic Oscillation and Pacific/North American teleconnection. These weather patterns rarely align in a certain way, but when they do the result is intense weather for California. Most troubling is that the weather events have the possibility of playing out over an extended period of time – such as the winter storms in 2010-11, said Guan.
The findings will allow scientists to identify the frequency of the wet weather events, said Duane Waliser, chief scientist in Earth Science and Technology at JPL.
“When you get these phases ... you tend to get a lot of them and that’s a pretty significant result,” said Waliser. “Because these have some predictability beyond just four or five days.”
The Holy Grail for such research is long-term forecasting. Waliser said the near-term goal is to be able to forecast such storms three to four weeks out.
“In certain instances, we’ll be able to have a little bit more foreshadowing – as in weeks-ahead notice,” Waliser said. “However, we’re just learning that capability, but this holds promise for that.”
He said the new research adds a next step to weather forecasting, a complex field with a slow evolution that has sped up recently because of satellite data.
“A five-day forecast today is about as accurate as a three-day forecast a decade ago,” Waliser said. “And as accurate as a one-day forecast a decade before that.”
“Basically, if you can load the dice a little bit with forecasting, that’s considered a pretty big improvement,” said Eric Fetzer, a project scientist at JPL, which is charged with interpreting the data from the Aqua satellite.
Some of the research findings might help predict serious storms such as those in 2010-11.
“In 2010 the expectation was that we would have a dry winter, and then in December there was a series of intense storms,” Fetzer said. “There was a significant amount of precipitation over a three-week period.” That extended precipitation set records throughout California. Eleven counties were declared disaster areas.
Fetzer believes that future atmospheric river storms will have a major impact statewide.
“I’m waiting for the other shoe to drop, where we get a week of these really severe storms and we get anything from flooding to reservoir overflow to freeway closures to the disruption of water supplies in the Central Valley,” Fetzer said.
That potential for such widespread and drastic affects was also cited last year by the U.S. Geological Survey in a report on a hypothetical winter storm scenario known as ARkStorm. In that report, the USGS sought to duplicate the 45-day storm of 1861-62, which left most of Sacramento an inland sea. That storm may have been an atmospheric river storm, scientists contend.
The USGS report said that such a long-lasting storm could “realistically flood thousands of square miles of urban and agricultural land and result in thousands of landslides, disrupt lifelines throughout the state for days or weeks, and cost on the order of $725billion.”