Supercomputer’s Forecasts Are a Breeze : Technology: Weather Service’s Cray C90 does 15 billion calculations per second. It predicts conditions throughout the country.

It analyzes upper air wind two weeks ahead. It can calculate and compare 42 different weather forecasts at once. It can do 15 billion math operations per second.

But to someone standing in front of the National Weather Service’s new Cray C90 computer, the machine seems disarmingly simple. A screen that says it’s running, and a few buttons, the main ones labeled “start” and “stop.”

Shouldn’t it be a little more complicated than that, Cray President John Carlson was asked.

“When you push start, it knows what to do,” he replied.

It lets National Weather Service meteorologists flex their numerical muscles with blinding speed. The C90 can slice the country into bits, analyze the weather and calculate how it will change. It’s the fastest such computer in service anywhere, Carlson says.

“We have, in effect, stepped into a new era,” said Commerce Secretary Ron Brown after snipping a ribbon in front of the computer.

“Each day in this country there are hundreds of thousands, perhaps millions, of decisions made based on the weather forecast,” explained Ronald D. McPherson, director of the National Meteorological Center.

Those decisions range from whether to send a child to school with a raincoat to planting rot-resistant seed to permitting barge traffic on the Mississippi River, he said.

“The information on which those decisions are based comes from the Cray C90,” McPherson said.

The $45-million machine joins a Cray Y-MP8 computer, purchased in 1990, which could only do 2.5 billion calculations a second. The older model will concentrate on research now, with the new one handling the daily forecasts for the National Weather Service.

The national weather forecasts, which local offices use as the basis of city and area forecasts, are based on complex sets of calculations that use mathematical formulas to mimic the behavior of the atmosphere. These are called numerical models.

Some models look at the atmosphere as a series of waves--waves of wind, waves of heat, waves of pressure. Others use grids to divide the world into squares. They measure the weather and try to predict what it will do next.

The faster computer lets the work be done more quickly and also permits the computer to look at the nation more closely.

For example, the old computer was able to look at the country in squares about 50 miles across. The new one reduces the squares to less than 25 miles, and officials hope to reduce them even more.

The new computer is already calculating changes in upper air winds--at about 30,000 feet where airliners fly--out to 16 days in the future. The old one looked 10 days ahead. Besides hurrying or slowing planes, those winds help steer weather movements.

Another forecasting technique is to run the models several times, with slightly different conditions at each start, to see how the results differ and how they are similar. It’s a method that helps meteorologists decide which models are more accurate for a given situation.

The old computer provided 14 model runs; the new one will do 42, said Anthony J. Mostek of the Weather Service. The more that results cluster together, the more likely the outcome, he said.

National Weather Service Director Elbert W. Friday noted that the new computer is but one part of the agency’s $4-billion modernization. The program also includes new satellites, measuring equipment, radars and other changes.

“I’ve been in operational meteorology for 34 years and this is the most exciting time I can remember,” Friday said.