Green River gets beefed-up gauges

More rain gauges and an experimental weather sensor will be installed in the Green River basin to help boost the odds that federal officials can predict which storms might lead to flooding.

Next month, the National Weather Service will put in a system now used in California that measures water vapor in the air and wind speed and direction several miles above the Earth's surface. Forecasters believe that system will help weather-watchers better gauge within 24 hours whether a pending storm is carrying enough rain to cause flooding.

That forecasting, along with 14 new rain gauges to be installed later this month, could help the Army Corps of Engineers more accurately predict whether and how quickly it might need to drain the reservoir behind the weakened Howard Hanson Dam.

"It's easy to conceive of a situation where that data could be very valuable, especially this year," said Brad Colman, with the weather service in Seattle. "And in the end we want to be able to say we did everything we could."

The equipment was requested last month by local emergency planners, who said it could help them avoid releasing more water than necessary or give them a leg up in their ability to plan and begin evacuations. The weather service announced it would set up a new, $400,000 station on the Green after the service's director met with U.S. Sen. Maria Cantwell, D-Wash., who chairs a congressional subcommittee that oversees the National Oceanic and Atmospheric Administration.

"She was able to push them a little faster, maybe, than they were going to otherwise move," said Cantwell spokeswoman Ciaran Clayton.

Since the dam's right abutment was damaged in a storm last winter, the Corps of Engineers has said it may have to release excess water this year during heavy rains to avoid stressing the structure. Such a release could cause major flooding in rural communities and throughout much of Auburn, Kent, Renton and Tukwila.

The new sensor, called an atmospheric river observatory, takes readings and, when a "Pineapple Express" storm arrives, can help forecasters better determine how much rain may fall — and where.

Last month, federal scientists began installing an identical sensor at Westport, Grays Harbor County, to improve coastal forecasts and help fine-tune computer forecast models.

The corps ultimately will release water from Howard Hanson based on many factors — how full the reservoir is during a storm, how many other big storms are coming and how soon, for example. Later this week the weather service and the corps hope to complete the calculations needed to gauge exactly how much moisture falling as rain it would take to force the corps to cause a flood.

The new rain sensors are not foolproof, though, Colman said. They are still experimental, and one of the weather service's most difficult tasks, technologically, is gauging how much rain will fall during a given storm.

"But if before we were calling for 2-4 inches of rain and this lets us refine that to say 2-3 inches ... even removing that bit of uncertainty will be helpful," Colman said.

Craig Welch: 206-464-2093 or cwelch@seattletimes.com