A La Nina weather phenomenon, which contributed to extreme weather around the globe in the first six months of this year, has reemerged in the tropical Pacific Ocean and is forecast to gradually strengthen and continue into winter, the National Oceanic Atmospheric Administration (NOAA) said Thursday.

According to NOAA, the latest La Nina is a back-to-back event; the previous weather event ended in May. La Nina occurs over the tropical Pacific Ocean from interactions between the ocean surface and the atmosphere. During an event, cooler-than-average Pacific Ocean temperatures influence global weather patterns.

“This means drought is likely to continue in the drought-stricken states of Texas, Oklahoma and New Mexico,” said NOAA’s Climate Prediction Center (CPC) Deputy Director Mike Halpert. “La Nina also often brings colder winters to the Pacific Northwest and the northern Plains and warmer temperatures to the southern states.”

La Nina typically occurs every three to five years, and back-to-back episodes occur about half of the time, the agency said. The current conditions reflect a “redevelopment of the June 2010-May 2011 La Nina episode.”

Forecasters with CPC upgraded their “watch” advisory to a La Nina advisory. Although an official winter outlook isn’t scheduled to be issued until mid-October, “La Nina 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,” they said.

Seasonal hurricane forecasters factored the potential return of La Nina into NOAA’s updated 2011 Atlantic hurricane season outlook, issued in August, which called for an active hurricane season (see Daily GPI, Aug. 5). “With the development of Tropical Storm Nate this week, the number of tropical cyclones entered the predicted range of 14-19 named storms,” the agency noted.

The previous La Nina event — last year — “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,” NOAA forecasters said (see Daily GPI, Feb. 23; Dec. 1, 2010).

In related news the NOAA’s National Climatic Data Center (NCDC) said the heat experienced June through August marked the second warmest summer on record. The average U.S. temperature in August was 75.7 degrees Fahrenheit, which was 3 degrees above the long-term (1901-2000) average, while the summertime temperature was 74.5 degrees, which is 2.4 degrees above average. The warmest August on record for the contiguous United States was 75.8 degrees in 1983, while the warmest summer on record was 74.6 degrees in 1936. Precipitation during August averaged 2.31 inches, 0.29 inches below the long-term average. The nationwide summer precipitation was one inch below average.

The excessive heat in six states — Arizona, Colorado, New Mexico, Texas, Oklahoma and Louisiana — resulted in their warmest August on record, according to the NCDC. This year also ranked in the top 10 for the warmest August for five other states: Florida (3), Georgia (4), Utah (5), Wyoming (8), and South Carolina (9). The Southwest and the South also recorded their warmest August on record. Only nine of the Lower 48 states experienced August temperatures near average and no state had August average temperatures below average.

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