How much rain will cause Green River to flood?

It seems like a simple question: How much rain has to fall, and for how long, to cause the Green River to flood?

Actually, like most things concerning the weather, the answer is far from easy. But a group of meteorologists is taking a stab at it.

The National Weather Service, with help from other agencies, will crunch numbers from rainfall and river records to try to determine how much rain under what conditions could increase the flood risk for the Green River Valley south of Seattle. With problems at the Howard Hanson Dam limiting the U.S. Army Corps of Engineer's ability to prevent the river from flooding this winter, it has become an important, if difficult, question.

"That's a tough one because storms, you know, aren't one point - they're a variety of conditions," says Larry Schick, a meteorologist with the corps' Seattle District. "But they're going to give it a try."

Land on one side of the flood-control dam in the Cascade foothills was found to be severely weakened after a record 15 inches of rain fell in 12 hours on the Green River's upper watershed in January. To avoid further damage to that abutment, the Corps of Engineers has greatly reduced the amount of water that can be stored in the dam's reservoir. That has increased the chances the corps might have to release enough water from the reservoir to cause flooding in the heavily developed valley downstream.

Ted Buehner, a weather service meteorologist in Seattle, said the Federal Emergency Management Agency recently asked the service and other federal agencies to come up with scenarios for when heavy rain might overwhelm the reservoir's limited capacity. FEMA officials want to get as much warning as possible so they can get people and resources in place well in advance.

"What they're really looking for is trying to get a sense of the probability or likelihood of a heavy rain event as far out as five days in advance. And there's a lot of uncertainty when you get that far in advance," he said.

Long-range forecasts are dicey, Buehner said. A Pacific weather system five days out "is probably on the east coast of Asia." A lot can happen as the system moves across the ocean, including strengthening, dissipating, or missing the Pacific Northwest altogether.

Buehner said such information, of course, would also be highly useful to the corps in managing the dam and to state and local emergency agencies faced with the possibility of evacuating thousands of people and defending property from billions of dollars in damage. And the sooner they can have an idea of what's coming, the better.

The corps especially wants to know how much water is forecast to pour into the reservoir and how much rainfall will swell the river below the dam. "That combination along with the duration of the event will play a key role in deciding how they are going to operate the dam with its limited capacity," Buehner said.

The corps provided information on the water flows coming into the reservoir that would cause concern, while the U.S. Geological Service provided data from river gauges it operates on the river.

The weather service will plug that data into stream flow simulations for the Green River, then try to determine how much rain could be a problem, said Brent Bower, the service's hydrologic program manager in Seattle.

But both he and Buehner said that in this particular "What if?" exercise, "if" will be a large part of the answer.

"There are so many variables and that's why there hasn't been a number" before, Bower said.

First and fundamentally, he said, no two storms are the same, and there is an endless list of scenarios: Where rain falls, when it falls, how much comes down, how long it lasts, how warm it is, how much snow is on the ground, how much water is in the soil, the time of year, the amount of wind, the direction of the storm.

Then there are the separate issues of what's happening at the dam, including how much water is in the reservoir, how much it can hold, how much can be safely released, whether more storms are on the way. Plus, Buehner says, there's the question of what's going on in the lower Green River, where a drenching storm would add to the flood risk.

The best the service can do, Bower said, is to take a lot of assumptions - for instance, it's a single storm and that soil and river conditions are at seasonal norms - then provide a range of possible rainfalls.

No deadline has been set for the answers, he said, but they're expected well before winter storms start in earnest.

"I know they want it soon," he said. "Everybody's asking. That's the million-dollar question right now."