Forecast system aims to improve flash flood warnings

Scientists hoping to provide notice of 30-120 minutes, reduce uncertainty

By Bill Scanlon, Rocky Mountain News (Contact)

Published July 23, 2008 at 1:41 p.m.
Updated July 23, 2008 at 11:55 p.m.

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Flash floods can kill suddenly, but a new forecast system being tested on the Front Range aims to give a half hour to two-hour notice that waters are rising on streams.

Scientists with the National Center for Atmospheric Research are testing the Front Range Flash Flood Prediction System. It looks at stream flows and detailed atmospheric conditions to make predictions about floods along particular streams and catchments.

"We want to increase the lead time of a forecast . . . and decrease the uncertainty about whether a flood will occur," NCAR scientist David Gochis, one of the developers of the system, said Wednesday.

The Front Range's steep topography and intense summer storms make it unusually vulnerable to killer flash floods. Many of its 3 million people live at the outlet of river canyons.

Thirty-two years ago, the Big Thompson Canyon flood killed 145 people east of Estes Park, the worst flash flood in Colorado history.

On some summer days in Colorado, most people are thirsting for a little rain while people living along one waterway are digging for lost possessions through the mud of a flash flood.

Floods through Front Range canyons can move 10 to 20 feet per second, fueled by heavy rains that might cover a square mile above a watershed but quickly are channeled into a stream just 10 yards wide, Gochis said.

"There's just an enormous, intense collection of water," Gochis said. "It can happen very quickly."

Forecasters can warn that weather conditions might lead to flooding across county-sized areas, but emergency managers may not know that a flash flood on a particular waterway is imminent until the water begins to rise.

That's because so much depends on the soil, topography and hydrology of particular watersheds.

The new model tries to assess how much water is infiltrating the soil relative to how much is moving over the land surface, "how fast and at what location it is accumulating - where things are changing very rapidly," Gochis said.

The model aims to give emergency managers 30 minutes to two hours' warning of flash floods on specific watersheds such as Clear Creek, Cherry Creek or Boulder Creek.

Scientists will monitor the system's performance each day as it gathers information from Colorado Springs to Fort Collins.

"If we can show that our system has some reasonable skill in predicting floods, we think officials may become more interested in using it along with their existing suite of tools," Gochis said. NCAR would like to make the system available to the National Weather Service and Denver's Urban Drainage and Flood Control District.

The project is funded by the National Science Foundation with help from the National Oceanic and Atmospheric Administration.

The system starts with NCAR's weather research and forecasting model, which produces detailed simulations of the local atmosphere.

It then throws in data about hydrology and variables such as terrain slope, soil type and vegetation, which can slow or speed the movement of a flash flood. The system also mixes in data on stream flow on each waterway.

The system can describe "the watershed's likely response to the heavy rain," NCAR scientist David Yates said.

Now, if it would only rain a little bit so the scientists can get a better idea of how their instruments are working.

"We couldn't have asked for a worst year," Gochis said.

Colorado floods

A look at some of the worst floods in state history:

Big Thompson Canyon, July 1976

* 145 people killed

* 418 homes destroyed

* 52 businesses destroyed

* $35.5 million in damage

South Platte River, June 1965

* Six people killed