Local

Good Question: How Do They Forecast Flooding?

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(credit: CBS) John Lauritsen
John Lauritsen is a reporter from Montevideo, Minn. He joined WCCO-...
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MINNEAPOLIS (WCCO) – When our state has flooding, experts are able to tell us days out exactly how high a river will rise.

And they’re usually correct within a couple inches.

With millions of gallons of water involved, how do they know?

“It’s math, it’s physics, it’s all that stuff, yeah,” said Craig Schmidt.

He’s a hydrologist with the National Weather Service.

He says the DNR, the U.S. Geological Survey, and cities themselves are responsible for measuring river levels.

They use survey tools, gage houses, and sometimes just a stick with numbers on it to get a reading.

The flood stage is when we begin to see water impacts, such as flooded roads and basements.

The crest is the highest level a river will reach, and Schmidt has made a living out of making those predictions.

“You take into account the models,” he said. “They give us an idea of how much volume of water is expected in a basin and you take that volume of water and move it through the entire river system.”

The National Weather Service looks at current river levels and water flow.

They then compare those numbers with precipitation, soil moisture, water released from dams upstream, and even how much moisture is being consumed by plants in the area.

They do that for hundreds of river basins across the state, and all the data is entered into a computer program.
That gives them a flood forecast when it’s needed most.

“You take the output from one, and it becomes the input for the next one, and you keep on going down so it takes a lot of computer power,” Schmidt said.

One good example of this is the Crow River in Delano.

It’s expected to crest at about 21 feet Monday night.

That means the river level will be 21 feet above the river bed, and about 10 feet above the average river level.

Schmidt said as long as the precipitation forecasts are correct, they can come within about 6 inches of predicting how high a river will crest.

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