Atmospheric 'rivers' drench North Coast with rain



The storms now pummeling Sonoma County are being driven by "rivers in the sky," low-lying streams of moisture which account for half the rainfall that falls on coastal California and has been blamed for most Russian River flooding.

It is a phenomenon scientists have been grappling to understand since 1998 and, with an array of new coastal observation stations and satellite imaging, are using Sonoma County and the storms this week as a test case.

"It basically comes up from the equator, a narrow band of extremely high water-content air that gets focused in one direction, like a fire hose," said Jay Jasperse, Sonoma County Water Agency chief engineer. "It is basically as the term suggests, a river of water in the atmosphere. We are in one of the prime areas to receive it."

Jasperse and scientists from the National Oceanic and Atmospheric Administration on Thursday were tracking the storms at NOAA's Earth Research Laboratory in Boulder, Colo.

The Water Agency has spent $300,000 and NOAA $270,000 for these studies, which are meant to eventually strengthen long-range forecasting, and help flood control and frost protection.

The storms appear in computerized graphics as a stark band of red, orange and yellow that move laterally across the Pacific Ocean, taking dead aim at Santa Rosa.

Sometimes referred to in California as "the Pineapple Express," because they often form near Hawaii, they are properly called atmospheric rivers and are the moisture-heavy conveyor belts for storms that occur worldwide.

"Pineapple Express is a specific version, but the concept is more general and applies across the world," said Marty Ralph, a chief researcher for NOAA. "They are finding that in England, in the U.K., much like California and Washington state, the major flooding is associated with winter storms and atmospheric rivers that come off the Atlantic Ocean."

In California, 42 atmospheric rivers have been identified, six of which hit Sonoma County. Although they only occur 3 percent of the time, those streams drop more than half of all annual rainfall.

Between 2000 and 2006, the atmospheric rivers have accounted for all seven floods that occurred on the Russian River, including the flood of 2006, when 10 inches of rain fell in a two-day period, Ralph said.

Because the atmospheric rivers flow in the atmosphere up to 8,000 feet, the research requires special equipment, the first of which was put in at Bodega Bay in 1998 and was the basis for the initial study, Ralph said.

That unmanned station at the Bodega Marine Lab provides wind speed, moisture, temperature and atmospheric pressure.

It will be replaced next year by a new station with sophisticated radar that can see rain and wind that even weather satellites cannot identify. Similar stations will be built in Eureka, Point Sur and Santa Barbara.

Early in the 14-year study, NOAA even flew its Hurricane Hunter aircraft into the atmospheric rivers to measure moisture and wind.

Ralph said that moisture sensors have also been placed around Sonoma County to measure where and how much rain has fallen.

The storms are also tracked by a special, polar orbiting weather satellite that has a view of the North Coast twice a day. It is equipped with special sensors to detect moisture content.

"That is very helpful. The way we learned about the atmospheric rivers was seeing these long narrow features with high water content," Ralph said.

You can reach Staff Writer Bob Norberg at 521-5206 or bob.