La Nina will whip up U.S. winter weather 

 A typical La Nina reorients winter storms northward, drying the southern states. (NOAA) Like a rude guest who refuses to leave, La Nina will hang around the tropical Pacific for several more months, upsetting U.S. weather for the second winter in a row, government forecasters predicted this week.

 The climatologic opposite of El Nino, La Nina chills the waters of the equatorial Pacific Ocean and redirects the air currents carrying weather across North America. The current incarnation of La Nina developed in July 1998, following the strongest El Nino warming of this century.

"We have learned in recent years that local weather is determined by global climate," says D. James Baker, head of the National Oceanic and Atmospheric Administration (NOAA). "By looking at the global things that are happening, we can do a better job of forecasting what will happen locally."

 The Pacific cooling has dwindled in recent months, but computer models forecast that it should strengthen over the winter and stick around at least through March, says Ed O'Lenic of NOAA's Climate Prediction Center in Camp Springs, Md. Some other researchers, however, question whether this will be a La Nina-style winter.

 A typical La Nina winter blows cold air over the northwestern states and northern Great Plains while warming much of the rest of the country. O'Lenic and his colleagues modified this prediction to take account of a global heating trend that has been boosting U.S. temperatures over the past 25 years.

 The combination of both influences will keep most of the country warmer than usual this winter, with normal temperatures confined to the Northwest and northern Great Plains, predicts NOAA.

 Whenever a La Nina appears, it tends to shove atmospheric winds into a looping route north of a high-pressure ridge over the northeast Pacific. This polar jet stream will swoop down from Canada carrying storms into the Great Lakes region and the Ohio and Tennessee Valleys, predicts NOAA. Other storms will drench the Pacific Northwest. In contrast, the Southwest, Texas, and the Southeast will have a dry winter, says O'Lenic.

 La Nina typically has a fickle effect on U.S. winter weather, with conditions swinging from unseasonably cold to mild and back. Last year, though, the United States remained warm throughout winter. O'Lenic explains that the North Pacific had so much leftover warmth from the previous El Nino that La Nina's true personality could not shine through. This winter, all of that relatively tepid water has disappeared, so the weather should follow the expected erratic pattern.

 The continuing La Nina contrasts with conditions over the past 23 years, during which the Pacific has hosted an unusually large number of strong and long appearances of El Nino. Some researchers have identified this shift as a potential sign of greenhouse warming, although others see it as a natural fluctuation.

 The current Pacific chill does not wipe away the overall warmth of recent decades, says Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colo. In fact, the tropical Pacific water is only marginally colder than normal. The situation, he says, "doesn't qualify as a La Nina at the moment. It's a lot weaker than it was at this time last year, and for La Nina conditions to dominate this winter, it'll have to get colder again. And I'm not sure that winter will be strongly influenced by La Nina."