WASHINGTON -- For the third time in 10 years, a huge expanse of unusually warm water has developed in the equatorial Pacific, setting in motion a so-called El Nino system that could disrupt normal weather patterns worldwide, according to National Weather Service scientists.
Evidence of another of the periodic upsets had been accumulating for several months. After analyzing satellite data and other information collected in December, the Weather Service's Climate Analysis Center concluded yesterday that the system has formed and probably will reach its peak in the next six months.
Typically, the phenomenon produces abnormally warm winters in southeastern Alaska, the western- and eastern-most regions of Canada and the northern tier of the contiguous United States. At the same time, areas from Arizona to Florida and along the south Atlantic coast experience cooler-than-normal temperatures and greater-than-normal rainfall.
The Northeast likely will be warmer than normal. The effect on the western United States was uncertain.
El Ninos have produced both very wet and very dry weather.
For example, a severe El Nino in the winter of 1982-83 brought memorable storms to the California, prompting floods and mudslides and causing 1,500 deaths worldwide, but another in 1986-87 went nearly unnoticed.
The effects usually fade with the coming of spring in March, but David Rodenhuis, director of the Climate Analysis Center, warned yesterday: "We should say this is still January, so hold on to your hat."
The warming of an expanse of the tropical Pacific sometimes reaching a fourth of the way around the earth leads to massive changes in cloud patterns and shifts in the jet stream, the high-speed, high-altitude current of air that creates weather patterns.
The warm Pacific pool now being watched is about twice the size of the one in 1986 and 1987, and the water temperature, as high as 86 degrees, averages about one degree more than in January 1987.
For now, weather service scientists said, the new El Nino could be described as "moderate" in intensity.
"We could continue on with a weather pattern like what we have been experiencing for the last six weeks," said Edward O'Lenic, head of forecasting operations at the Climate Analysis Center.