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Originally published Thursday, October 15, 2009 at 12:12 AM

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Storm sensors sought for Green River flood forecasts

Local officials want new weather sensors in the Green River watershed to help forecast and possibly mitigate flooding this winter.

Seattle Times science reporter

Federal scientists will begin installing new weather sensors on the Washington coast today to improve understanding of "atmospheric rivers," like the Pineapple Express that delivers the region's heaviest rain and most punishing floods.

While the sensors will improve coastal forecasts and help fine-tune computer models, state meteorologists and emergency planners say they want help with a more pressing problem.

Threatened by the leaky Howard Hanson Dam, locals are pushing the feds for more money to set up two similar sensor stations near the Green River watershed, to help forecast — and possibly mitigate — floods this winter.

"I believe these instruments could have a major impact on the Corps of Engineers' ability to keep the water in the banks," said Hillman Mitchell, Tukwila's emergency-management coordinator.

Tukwila, Kent and Auburn are among the downstream cities that could be swamped if the corps is forced to release more water than usual to reduce stress on the 50-year-old dam. A leaking abutment, discovered after a January storm, led the corps to declare the earthen structure unsafe.

New sensor stations installed upstream and southwest of the dam could show how storms are evolving as they move into the mountains, provide real-time measurements of how much rain is falling and pinpoint the freezing level. That could help forecasters predict Green River flows and help the corps estimate how much water is headed into the reservoir and how much can be spilled without overtopping levees downstream, Mitchell said.

Nine of the past 10 major rainstorms in the Green River basin were caused by a Pineapple Express.

The new sensors wouldn't be a magic bullet, cautioned Corps of Engineers meteorologist Larry Schick. "The main thing it would give us is more confidence in the forecast," he said. "There's always a curveball Mother Nature can throw at us."

The National Oceanic and Atmospheric Administration (NOAA) scientists who developed the sensor stations agree that additional locations would be valuable, but say they don't have the $400,000 or more it would take to pay the bill.

"We're stretched very thin," said F. Martin Ralph, of NOAA's Earth System Research Laboratory in Boulder, Colo.

Mitchell and others are scrambling to find the money and have appealed to Washington's congressional delegation and former Gov. Gary Locke, now secretary of commerce and NOAA's top boss.

The sensor stations that are so in demand are called "atmospheric river observatories." Ralph and his team have been building and testing them for more than 10 years. The observatories, which can be mounted on flatbed trailers, provide detailed snapshots of the sopping bands of air that account for most global moisture transfer. The Pineapple Express shuttles moisture northward from the subtropics.

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The atmospheric rivers, which Ralph said can carry more water than the Mississippi, are many hundreds of miles long and 50 to 200 miles wide.

"They're long and skinny, like a fire hose," said Brad Colman, Seattle meteorologist-in-charge for the National Weather Service. NOAA scientists estimate atmospheric rivers deliver a third of the Northwest's rainfall. The rest arrives during less intense storms.

Satellite instruments provide a bird's-eye view of the airborne rivers, and give forecasters a good idea where the hose is pointing, Colman said. Atmospheric river observatories complement that information, with a radar that peers upward to profile wind speed and direction. Other instruments locate the freezing level and measure the amount of precipitation in the air. Some observatories have sensors that count raindrops and estimate their size.

In California, where experimental observatories have been deployed for a decade, the data helped flesh out the atmospheric rivers' role in floods and mudslides, and helped forecasters improve their models.

"We've been able to document the frequency and intensity of atmospheric rivers in ways that have never been possible before," Ralph said.

The instruments on Washington's coast, which will be located at Westport, Grays Harbor County, will expand that insight to the Northwest. They also will help forecast rain and floods in the southern Olympic Mountains and around Chehalis and provide an initial look at atmospheric rivers as they hit the land, Colman said.

But the location, which was picked before Howard Hanson's woes were obvious, is too far away to be of much value for the dam and the Green River basin, said University of Washington meteorologist Cliff Mass.

Ralph said he and his colleagues could assemble the two additional observatories and deploy them before the winter is over. But someone would need to provide cleared sites for the instruments.

If the funding comes through, Colman said he's confident he can find places to put the observatories.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

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