Along the Peruvian coast, they call it La Nina, "the little girl," or El Viejo, "the old man," an upwelling of cold water in the eastern Pacific Ocean whose climatic effects are felt throughout the world.

Over the past nine months, La Nina has brought flooding to Australia, heavy rain to Washington state, tornadoes to the Midwest and, here in the middle Atlantic states, an uncommonly dry spring.

La Nina's effects have diminished in the United States, but persist in the western and central Pacific, and will likely return to North America this fall and winter, said research meteorologist Vernon Kousky of the National Weather Service.

In the meantime, however, the District will remain dry, so "the prospects for deep summer are 'watch out,' " said Kevin Trenberth, head of climate analysis for the National Center for Atmospheric Research in Boulder, Colo. "Any storms that come into the area are not going to be drought-breaking storms," he said, unless they are tropical depressions or hurricanes, a heightened prospect in a La Nina year. If the drought ends this summer, it is likely to depart not with a whimper, but with a loud bang.

Over the past 15 years, scientists have enhanced their ability to predict La Nina and El Nino, its warm-water brother, by developing a buoy system to record ocean temperatures, collecting satellite and ship monitoring information, and using new methods of analysis to feed data into sophisticated computer models. These efforts enabled meteorologists to predict six to nine months in advance the onset of the 1997 El Nino, one of the most dramatic in history, and the ensuing La Nina.

The U.S. public is now familiar enough with El Nino and La Nina to blame them for everything from west Texas wildfires to rainouts at midwestern graduations, but this is something of a bad rap, for El Nino and La Nina "are not the only things having an impact on U.S. climate," said Ants Leetma, director of National Oceanic and Atmospheric Administration's Climate Prediction Center.

Scientists have identified another, long-term, Pacific-based weather cycle, and have known for decades about an Atlantic pressure oscillation that profoundly affects European climate. Mix these up with regional peculiarities and the as-yet not-fully-understood effects of global warming, and the result is what Leetma calls an "active area of research."

The 1997 El Nino began late that year with a surge of warm water--frequently above 80 degrees Fahrenheit--off the Pacific coast of equatorial South America. It was a recurring phenomenon that earned it the name El Nino, because Peruvian coastal fishermen noticed that it began every few years around Christmas.

The effects of El Nino-La Nina are global because of the interaction between the ocean and the atmosphere. "Think of the Pacific as a giant bathtub," Leetma said. "Most of the time, trade winds blow steadily from east to west, driving warm surface water to pile up in the western part."

But the trades periodically hiccup and stall, and when they do, "the warm water sloshes back toward the east," Leetma said, and can eventually move all the way across the Pacific, carrying thunderstorms along with it.

"These areas of rainfall are half the size of the United States," Leetma said. "There's a rising motion, like steam," he added, and the northern jet stream blows the moisture eastward, helping the storms' migration.

The jet stream is key. Moving west to east as Earth rotates in the opposite direction, it blows strongest when there is a pronounced temperature difference between the warm air of the tropics and the cold air of the upper latitudes. During an El Nino, this gradient is steep and stretches across the Pacific from southern Japan to San Diego.

In the winter of 1997-98, El Nino storms drenched normally arid areas of northern Mexico, Southern California, Arizona and New Mexico, and brought tornadoes and heavy rain to the Gulf states and the Southeast.

But as the storms moved east, the warm water began to dissipate, and cold water welled up from below, triggering La Nina.

In mid-1998, as the section of the Pacific south of California began to cool in this way, the effect started to spread westward; and as the atmosphere cooled, the jet stream temperature differential shrank, causing the wind to weaken and move north.

What followed was a typical La Nina winter. The jet stream dumped heavy rainfall on the Pacific Northwest, but did not, as it usually does, flip-flop southward, because there was no longer a source of heat in Southern California.

So Seattle got flooding. Then the jet stream dove down through Colorado, bringing a spate of tornadoes and heavy spring rain to the Midwest before it looped northward again, finding its usual summer resting place along the U.S.-Canadian border in April--at least two months earlier than usual.

The mid-Atlantic, which in normal years gets plenty of spring rain, has been largely dry in 1999. As of last week, rainfall in the District since the start of the year was about half the usual eight inches.

The direct effects of La Nina have weakened considerably in the United States, but Trenberth noted that cool water persists in the central Pacific, and Kousky said La Nina will probably be back for a second winter.

In the meantime, the high-pressure system hovering over the mid-Atlantic states should persist. "The pattern is normal, but more so," Trenberth said, because La Nina helped "lock in" the summer-like weather and dryness several months before they usually occur.

La Nina has, however, left one summer legacy. The jet stream's northward drift means there will no longer be a southern El Nino-style wind hurtling into the Atlantic to cut the tops off the cyclonic storms brewing there.

Tropical depressions, including hurricanes, will have a much easier time developing.

The strength of the 1997-98 El Nino and the persistence of the 1998-2000 La Nina have prompted speculation that these climatic events are coming faster and at shorter intervals, but Leetma thinks "the jury is still out."

He noted that examination of weather history over the past 150 years shows there were "very pronounced El Ninos" at the turn of the century, but none in the 1930s, 1940s and through much of the 1950s, evidence of a longer-term weather cycle known as the Pacific Decadal Oscillation.

"The problem is that global warming affects the oceans, too," Leetma said. "The El Ninos come and go; we know there's another longer-term factor at work; and we think the impacts of man are warming up the planet as well. We don't yet know the details."

CAPTION: LA NINA'S EFFECTS (This graphic was not available)