La Niña returning to bring more rain in valley and north Pacific

La Niña, which contributed to extreme weather around the globe during the first half of 2011, has re-emerged in the tropical Pacific Ocean and is forecast to gradually strengthen and continue into winter. Forecasters with the U.S. Climate Prediction Center Tuesday upgraded last month’s La Niña watch to a La Niña advisory.

The National Oceanic and Atmospheric Administration said it will issue its official winter outlook in mid-October, but La Niña winters often see drier than normal conditions across the southern tier of the United States and wetter than normal conditions in the Pacific Northwest and Ohio Valley.

La Niña is a naturally occurring climate phenomenon located over the tropical Pacific Ocean and results from interactions between the ocean surface and the atmosphere. During La Niña, cooler-than-average Pacific Ocean temperatures influence global weather patterns. La Niña typically occurs every three to five years, and back-to-back episodes occur about 50 percent of the time. Current conditions reflect a redevelopment of the June 2010 to May 2011 La Niña episode.

Only a week ago weather officials were talking about a neutral situation in the Pacific, although they said there was a probability that la Niña would return.

In Costa Rica the Instituto Meteorológico Nacional said that a growing La Niña means more rain on the Pacific coast and in the Central Valley, which would be similar to last year which
in places set records for rainfall. The weather institute said that the maximum intensity of La Niña would likely be between December and February and would extend to March 2012.

The country is likely to feel the effects during the second half of September.

On the Caribbean coast the drought there is likely to continue through October with a chance of improvement in November.

The weather institute expects normal rainfall from November until February.

The Caribbean has been struggling under below average rain from March to August with the exception of May where rainfall was a bit above average. That is the same pattern as in 2010 when the dry spell ran from April through November.

The weather institute estimated the Caribbean rainfall under the coming La Niña conditions from 15 to 20 percent less than normal.

The north Pacific was expected to be 35 percent above normal with both the Central Valley and the central Pacific getting about 15 percent more rain. The northern zone and the southern Pacific were expected to be close to normal.

The strong 2010-11 La Niña contributed to record winter snowfall, spring flooding and drought across the United States, as well as other extreme weather events throughout the world, such as heavy rain in Australia and an extremely dry equatorial eastern Africa.

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