Real-time weather system will help Seattle forecast rain

When the wet weather returns in fall, Seattle will be ready with a new system to measure and forecast rainfall with greater precision and accuracy than ever before.

Seattle RainWatch will pinpoint the parts of town where the heaviest rain is falling. The city will be able to dispatch crews and issue alerts for specific neighborhoods where flooding is likely, said James Rufo Hill, of Seattle Public Utilities (SPU).

"It's going to allow us to forecast at the neighborhood scale," Rufo Hill said Saturday at the Northwest Weather Workshop.

The project was inspired by recent storms that caused heavy floods in some parts of town, including the December 2006 downpour that swamped the Madison Valley neighborhood. A woman drowned when she was trapped in her basement by a surge of water.

The unexpected deluge overwhelmed and clogged storm drains. More than 300 damage claims were filed against the city, and SPU paid out $3.2 million.

"The question we asked was: How can SPU know what's happening during these intense precipitation events and take action to prevent property damage and save lives?" said University of Washington meteorologist Cliff Mass, who proposed the new system.

RainWatch combines radar images and real-time data from an expanded network of rain gauges to generate a more detailed picture of how much rain is falling and which neighborhoods are getting hammered the hardest.

"We call it 'now-casting,' " Mass said.

As a weather junkie who regularly surfs the radar, Mass uses "now-casting" to dodge rain showers on his daily bike commute. But governments have been slow to appreciate how valuable it can be to know exactly what's happening now and what's coming in the immediate future, he said.

"There's all this information that people haven't been using."

Radar provides a big-picture view of the way storm cells are moving across the region and reveals overall precipitation patterns. Meteorologists also use radar to estimate rainfall amounts, but those estimates can be off by as much as 40 percent.

Rain gauges provide the ground truth, said UW research meteorologist Phil Regulski. For RainWatch, SPU added more than a dozen new gauges to an existing regional network of 11. Regulski developed the computer model that draws on the nearly instantaneous data from the rain gauges to finetune the radar's citywide rainfall estimates.

The system tallies neighborhood rainfall amounts over several hours to two days, so officials can spot problems as they develop.

"If you're getting an inch an hour in areas where you know you've had problems in the past, maybe you want to get out there and start knocking on doors," Mass said.

RainWatch also peers into the near future, extrapolating storm tracks from the radar and creating highly localized forecasts of the amount of rain likely to fall in the next 30 to 90 minutes.

The scientists are working to extend their forecasting power to several hours, Regulski said.

Those efforts will get a boost when a new radar is built on the Washington coast, with a better view of storms as they approach over the Pacific.

SPU spent about $65,000 on the RainWatch program. The utility is still trying to figure out how best to make use of the information it will provide, Rufo Hill said. During major storms, the utility now relies heavily on calls from residents to alert them to areas where drains are backing up or basements are flooding.

With real-time data on rainfall, SPU may be able to dispatch crews before problems develop or redirect water to prevent sewer overflows, Rufo Hill said.

Testing on the system will continue through the summer.

A preview is online at: www.atmos.washington.edu/SPU/?section=info

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