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Flash floods remain a major hazard in America a century after the Johnstown disaster that killed at least 2,500 people, but the advent of new weather radar could help ease that danger.

While the new NEXRAD radar system, scheduled for installation in the next few years, is designed to improve weather and storm forecasts, meteorologists say the machines can also form the basis for a much improved flood warning system."It's more than a pipe dream; the reality of NEXRAD" is here, Rafael Bras of the Massachusetts Institute of Technology told a recent meeting of geophysicists.

"I believe our way of predicting floods in the future will change because our sources of data are changing," he said at a meeting of the American Geophysical Union in Baltimore.

Now being tested in Norman, Okla., NEXRAD could begin to be installed around the country by this fall, said spokesman Donald Witten of the National Weather Service.

The new radar system is expected to incorporate a computerized program to warn local weather offices "when too much rain falls in too small an area in too short a time," said Witten.

Deaths from flash flooding have increased in recent years, a trend attributed to increasing development in low-lying areas near rivers and streams.

"Flash floods are the major cause of weather-related deaths in the United States," reports Elbert W. Friday, director of the weather service. "They claimed over 1,800 lives in the 1970s and another 1,000 so far this decade."

On May 31, 1889, waters of the South Fork Reservoir in western Pennsylvania, swollen by two days of steady rain, broke through an earthen dam and cascaded down the Conemaugh Valley into Johnstown, killing at least 2,500 people.

Today parts of flood-prone Appalachia are protected by an automated flood warning system operated by the National Oceanic and Atmospheric Administration.

But while IFLOWS - Integrated Flood Observation and Warning System - covers 179 counties in seven states, flooding remains a hazard in many other areas, too many for human forecasters to keep track of and issue detailed warnings.

Reorganization of the National Weather Service is expected to reduce the agency to 115 local forecast offices scattered across the nation during the 1990s.

"There might be 1,000 points of flooding for each office," to keep track of, said flood analyst John Schaake of the weather service.

But over the next decade or so NEXRAD - Next Generation Weather Radar - will be installed at those offices, and that can make the difference, said Bras.

While current radar systems can see where storms are, NEXRAD will allow meteorologists to track wind movements and estimate rainfall over much of the area.

Bras, using similar radar at MIT, has combined the readings with detailed analyses of water runoff and stream flow patterns in the Souhegan and Squanhacook river basins in New Hampshire in an experiment to see if flooding can be better predicted.

First, he said, he developed detailed topographical maps of the river basins including estimates of the slope to calculate rates of runoff and the velocity of streamflows in the region.

Weather records for the area allowed him to calculate how saturated the soil was to see how much water would be absorbed, and how much would flow through the soil into lower-level streams and creeks.

With all these calculations in a computer, Bras explained, he was able to estimate the stream flows and flood heights when the advanced radar detected storms over the two river basins. In a series of experiments the projections came very close to actual water levels recorded on the scene, Bras said.

Such projections could speed warnings to areas downstream of heavy rains or sudden snowmelts, two major causes of flash flooding in the United States.