Weather researchers have found that thunderstorms are not always the random local downpours of a summer afternoon, but can sometimes grow and merge into one huge storm holding enough rain to cause flash floods.

The discovery of the way local storms can mass and produce larger ones is considered a significant step toward improved prediction not only of when rain will fall, but of how much, something meterologists have been unable to predict well.

The ability to make such predictions about heavy rain would be of great value to agriculture and public safety, according to a spokesman at the National Oceanic and Atmospheric Administration, where the discoveries were made.

Dr. Robert Maddox and J. Michael Fritsch of NOAA discovered the weather patterns by studying the satellite photographs of local storms. Pictures of American weather systems, taken every half hour, 24 hours a day, have been available to researchers for only about four years, Maddox said.

"When you build up a record of these pictures over several summers, then some things just jump out at you. We've never had the all-encompassing view from space like we have now," he said.

Some of the world's most violent and destructive weather comes from the intense thunderstorms generated east of the Rocky Mountains during spring and summer, according to NOAA, and it is the formation of these powerful systems that Maddox and Fritsch say they have worked out.

The standard thunderstorm that builds and breaks over a small area, dumping its burst of heavy rain quickly, is unconnected to the national weather systems, such as the large swirling systems displayed on television weather broadcasts. They are not part of the cold and warm fronts that sweep across the nation ahead of high and low pressure areas.

Instead, the hundreds that occur, mostly in the East and Midwest in the spring and summer, are caused by rising currents of hot air that blossom into tall, billowy clouds when they hit the colder air of the upper atmosphere.

These can sometimes remain small and local when they dump their moisture, buw the NOAA researchers have found that several of these storms can be drawn together into a system large enough to cover one or two states, and can dump huge amounts of rain in a single night.

The scientists are beginning work that would track these storms closely. They hope to learn enough about when and why they merge to predict both the merging of the storms and the amount of rainfall they might produce.