Hurricane Floyd, one of the largest storms to threaten the United States this century, may be an unwelcome guest, but it is not an unexpected one, scientists say. La Nin~a, the intense upwelling of cold water off the west coast of South America that began a year ago, has been the key player in weather patterns ranging from floods in the Pacific Northwest to this summer's mid-Atlantic drought. Floyd is simply La Nin~a's latest bit of mischief.

But while experts predicted an active hurricane season this year, they have no way of knowing exactly why or how nature can create a monster like Floyd, one of seven Category 4 hurricanes this century.

"It's the right time of year, the water is warm and La Nin~a is here," said Florida State University meteorologist and oceanographer James O'Brien. "But nobody can say. It's like, `How did that basketball player get to be 7 feet, 4 inches tall?' The conditions were right."

Thunderstorms are a regular late summer feature of the Atlantic. Warm water gives off vapor that is picked up by warm summer air and deposited elsewhere as rain. "This convection occurs regardless of what else happens," said Kevin Trenberth, head of climate analysis for the National Center for Atmospheric Research, in Boulder, Colo. The question is, `Are you going to have individual thunderstorms, or are they going to get together?' "

Or, more specifically, Trenberth continued, "are there disruptive winds that tear hurricanes apart" before they can form? This year the winds are not there, and that phenomenon, scientists agree, is mostly La Nin~a's fault. As a result, storms are likely to congregate at sea in the swirling funnels known as hurricanes in the Atlantic and typhoons in the Pacific, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere in conformity with the rotation of the Earth.

The cycle that produced Floyd began in 1998 after a very intense El Nin~o, La Nin~a's warm water cousin, began to dissipate off the coast of Peru and Ecuador, to be replaced by La Nin~a. With cold water in the Pacific tropics, the chill, west-to-east, high-altitude winds known as the jet stream no longer sensed the kind of temperature differential that attracted them southward during El Nin~o.

Instead of hurtling across the United States and out to sea in the Atlantic, the jet stream retired northward, contributing to the Northwest's uncharacteristically wet spring, and the middle Atlantic's summer drought.

More important for the hurricane season, the jet stream's departure meant that there were no strong winds in the middle Atlantic to blow the tops off any big storms that were potentially forming. "There's a lot shear" when the winds are strong, as they are during El Nin~o, O'Brien said. "The wind knocks over the big storms. When El Nin~o's out there, it destroys the environment for hurricanes."

But during La Nin~a, Trenberth said, storms congregate over the sea "where there's plenty of water and very little friction," and, unmolested by the jet stream, they begin to form the funnel-shaped vortex known as a hurricane.

Then, pushed by low-altitude, east-to-west trade winds, they begin to move slowly toward the Western Hemisphere. As they move across the sea, they begin to nose up into the westerly winds that prevail in the Atlantic's middle latitudes. This is what causes hurricanes to curl northward and often, in the end, move out to sea.

There is some speculation that the scope and strength of storms such as Floyd may be caused in part by global warming. But while warmer air can carry more water vapor, experts are reluctant to tout this as a principal reason for monster storms.

"The water vapor environment is better for these storms than it was 20 years ago," Trenberth said. "The relative humidity has increased by 4 percent, and total moisture [in the air] has increased by 10 percent.

"This is climate change," he continued, but global warming probably makes "a relatively modest" contribution to the overall moisture increase, "maybe half."

O'Brien dismissed the global warming thesis as "nonsense," noting that it was La Nin~a's cold water that was crucial to Floyd's formation, while "global warming should mean more El Nin~os and less hurricanes."