La Nina redrawing weather patterns Phenomenon likely to control climate for another winter

January 5, 2000 - USA Today

In Arkansas on Monday, streets flooded. In New York, no one needed a coat. In Illinois, high winds took the roof off a city building. Just a few days before, in San Diego County, Calif., 5 inches of snow topped Mount Laguna.

Welcome to a La Nina winter, where on any given day (Monday, for instance), Philadelphia can be just as balmy as Los Angeles.

Strange as the weather may seem, it comes as little surprise to meteorologists. Since fall, just as they did last year, they've been talking about milder-than-normal conditions in the mid-Atlantic states, an increased risk of severe weather in the Ohio Valley, and below-normal temperatures on the California coast.

''You get warm spells, cold spells,'' says Jim Laver, deputy director of the National Weather Service's Climate Prediction Center. ''We expect those swings.'' By winter's end, he says, everything should even out. ''If you look at last year's results, we don't expect it to be too much different this year.''

La Nina is created by cooler-than-normal waters in the eastern tropical Pacific Ocean. The cooler waters alter the strength and location of jet streams, which steer weather systems. For the second winter in a row, meteorologists predict, La Nina will run the show. Although it hasn't brought as much snow to regions such as the Pacific Northwest this year, it has delivered warmer weather to areas east of the Appalachians.

''I think it was probably the warmest November on record for the United States, and December has probably continued that pattern,'' says Kevin Trenberth, an atmospheric scientist with the National Center for Atmospheric Research.

In Philadelphia, the high was a record 63 degrees. In New York City, the temperature topped at 64. At Boston's Logan Airport, the 64-degree high broke the record of 62 set in 1913. Records also were broken in Concord, N.H.; Portland, Maine; Providence; and Burlington, Vt.

Temperatures in the region were supposed to remain unseasonably warm today and Wednesday.

However, Trenberth warned, ''It's going to be a lot cooler over most of the country in the next week or so, and some wintry weather looks to be in store for us in at least the next couple of weeks.''

In the upper Midwest, that wintry weather could come today. Although overnight lows in Cleveland were expected to stay about 50 degrees Monday night, the city is in line for snow this afternoon. Meteorologists also predict major changes for Chicago, where a moderate Monday edged toward a windy and snowy night.

Trenberth says La Ni´┐Ża weather patterns could linger through March, ''but it could well fade when it comes to the spring.'' By then, folks might no longer need jackets in New York anyway. The trees downed by heavy thunderstorms in Arkansas will have long been removed. And the roof that blew off a garage at the municipal complex in Cambria, Ill., will have been replaced.

As for the snow in San Diego County? It's probably already gone.