The weather phenomenon known as La Nina, which contributed to extreme weather around the globe in the first half of 2011, returned in August and is expected to gradually strengthen and continue through winter in the Northern Hemisphere, U.S. government forecasters said Thursday.

"It is not yet clear what the ultimate strength of this La Nina will be," according to the National Oceanic and Atmospheric Administration.

La Nina is the name given to the unusual cooling of the temperatures of the equatorial Pacific Ocean, which has wide-reaching impact. The opposite occurrence is El Nino, in which unusual warming takes place.

In the U.S., temperature and precipitation impacts linked to La Nina are expected to remain weak through early autumn, and to generally strengthen during the late autumn and winter, NOAA said.

"During September-November 2011, there is evidence that La Nina favors an increased chance of above-average temperatures across the mid-section of the country, and an increased chance of above-average precipitation across the Pacific Northwest," NOAA said.

The active 2010 Atlantic Hurricane season was aided by La Nina conditions the current season, which continues through November and is also expected to show above-average activity.

NOAA said 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.

"This means drought is likely to continue in the drought-stricken states of Texas, Oklahoma and New Mexico," Mike Halpert, deputy director of the Climate Prediction Center, said in a press release. "La Nina also often brings colder winters to the Pacific Northwest and the northern Plains, and warmer temperatures to the southern states."

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. With the development of tropical storm Nate this week, the number of tropical cyclones entered the predicted range of 14-19 named storms.

The strong 2010-11 La Nina 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 said.

La Nina 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 Nina, cooler-than-average Pacific Ocean temperatures influence global weather patterns. La Nina typically occurs every three-to-five years, and back-to-back episodes occur about 50% of the time. Current conditions reflect a re-development of the June 2010-May 2011 La Nina episode, NOAA said.