La Nina Weather Pattern Expected to Dissipate by May, NOAA Says

La Nina, the Pacific Ocean cooling phenomenon that affects weather around the world, is expected to weaken through the Northern Hemisphere winter and dissipate by May, the U.S. Climate Prediction Center said.

The weather pattern, which has been blamed for record rain in Australia and increases the chance of Atlantic hurricanes, probably will still affect U.S. temperatures and rainfall into April, said the center, a division of the National Oceanic and Atmospheric Administration. La Nina is a counterpart of El Nino, a periodic warming of the Pacific Ocean.

“It’s really too early to know what will happen during hurricane season” as forecast models from now through June become less reliable, Michelle L’Heureux, a center meteorologist, said in a telephone interview from Camp Springs, Maryland. “I wouldn’t want to rule out anything out for the hurricane season.”

Temperatures may be above-average in the southeastern and south-central U.S. into April, while the Northwest will have an increased chance of below-average temperatures, the center said in a report today. There may be more precipitation than normal in the Northwest, Northeast and the Ohio River and Tennessee River valleys, while the South will probably be drier.

La Nina events occur on an average of every three to five years and usually last from nine to 12 months. They sometimes occur in back-to-back years, as is happening now.

Parched Crops

This La Nina has parched crops in Argentina and Brazil and flooded plantations in Thailand and Malaysia. The phenomenon also contributed to Australia’s wettest two-year period on record, as 140.9 centimeters (55.5 inches) of rain fell in 2010 and 2011, the nation’s Bureau of Meteorology said Feb. 7.

La Ninas tend to produce lower vertical wind shear across the Atlantic, giving hurricanes an expanded area to form. The 2011 hurricane season tied 2010, 1995 and 1887 as the third-most active on records going back to 1981, spawning 19 named storms with winds of 39 miles (63 kilometers) an hour or more, NOAA said. The average season produces 11.

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