Beyond the Forecast: The science of floods

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by JEFF RENNER / KING 5 Meteorologist

Bio | Email | Follow: @Jeffrennerwx

KING5.com

Posted on November 12, 2009 at 8:30 PM

Updated Wednesday, Nov 11 at 4:40 PM

November is here, and so is storm season. Washington state typically sees its wildest weather from November through February; everything from wind storms and snow to drenching rains and floods.

Last January, Western Washington endured epic floods.  Two days of heavy rain on Jan. 6 and 7, more than three-and-a-half inches recorded at SeaTac Airport, and much more in the mountains.

The state emergency management division reports nearly 4,000 people have registered for federal disaster aid and $73.5 million has been paid for individual and public assistance programs.

The science of forecasting floods is improving. New discoveries and new tools are helping meteorologists better anticipate when and where the heaviest rain will fall.

Researchers have found the downpours that push Washington rivers over their banks come from another type of river, atmospheric rivers, and the Northwest is the focus of that research.

If water is the life blood of the earth, then rivers are the arteries that carry it, over hundreds, sometimes thousands of square miles. It's a web of water and nutrients, nourishing a Web of life.

There is no better way to take the measure of a river than in a canoe.  I joined friend and experienced canoeist Steve Salins for a morning on the Green River to do just that.

The ancient design of the craft, combined with skill, judgment and submission to the rhythm of changing currents, allows paddlers to feel and work with the river's power and pulse.

There are silent stretches that glide peacefully in broad valleys, alternating with narrow slots where the river runs hard, squeezing between the muscular shoulders of mountains. As surely as the land sculpts the river, so the river sculpts the land.

"The river is banging into a ninety degree wall to the current, as it does that, it tends to wash away the ground underneath. As you can see right here, there's been a slide where a whole tree slid away into the water," said Salins.

Understand the story of a river and you understand its' potential for damage.

The way the Green River is flowing today, it would fill up Seattle's Green Lake in about three days, but last January during the floods, the stretch above the Howard Hanson dam would have filled up that same lake in under 60 minutes and the pattern responsible for it, called an atmospheric river, several thousand feet up, could have filled up Green Lake in under 90 seconds.

The area of concern, of course, is the Howard Hanson dam.

Meteorologist Larry Schick's focus on the phenomena called atmospheric rivers is natural. He analyzes weather and river conditions throughout the Northwest for the U.S. Army Corps of Engineers.

The conventional satellite images most commonly used by forecasters for years could only hint at the presence and power of these giant airborne flood makers.

It's been microwave images that have been the breakthrough, allowing meteorologists not only to spot the troublemakers, but to peer inside and see what makes them tick.

"The moisture is mainly in the lower five to 10,000 feet of the atmosphere; it's really concentrated. There's sometimes a low level stream that forces the air into the mountains, and that really squeezes the moisture out," said Schick.

In one image, you can clearly see two such rivers surging toward the Northwest, one from the Hawaiian Islands, plus another from Asia.

Such atmospheric rivers can be massive, carrying as much as three times as much water as the entire Mississippi River. It's no wonder they may be responsible for as much as 90 to 95 percent of all major floods in Washington.

That's not unlike taking a hurricane, uncoiling it, and pointing it like a fire hose - narrow, focused and powerful.

"The narrowness is very important, that's why we see flooding sometimes on the Skagit but not south or in the central Cascades, but not the Skagit," said Schick.

One looks like an outstretched fist, ready to clobber the Northwest.

A new flooding defense system called an Atmospheric River observatory was just installed at Westport.

University of Washington Professor Cliff Mass is already analyzing the data.

"One of the things the observatory has is a vertical pointing radar so it can see precipitation above itself. It can also see how fast the precipitation is falling," he said.

Those measurements improve the computer forecast models that forecast atmospheric rivers..

"We have a lot of surface data, but what we really need is data above the surface because the atmosphere is fully three-dimensional," said Mass.

The observatory at Westport may be matched by two others on the Green River. We may not be able to prevent atmospheric rivers, we will be able to better anticipate and prepare for them.

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