(Adds dropped word Kolli in third paragraph)

GENEVA, July 20 (Reuters) - La Nina, the cooling of sea surfaces in the Pacific Ocean which can wreak havoc with weather patterns, is likely to develop by the end of the year, the World Meteorological Organisation (WMO) said on Friday.

In its latest update on ocean temperatures, the United Nations weather agency also said it was "very unlikely" that an El Nino event, where Pacific water become abnormally warm, would occur in the rest of 2007.

WMO expert Rupa Kumar Kolli said a typical La Nina lasts nine to 12 months, and that Pacific cooling has been unsteady to date.

"We are now in the development phase and it is likely to peak around the (northern) winter season," Kolli told a news briefing. "But it is likely (to be) a weak event rather than a strong event."

La Nina is a "planetary-scale climate anomaly" known to affect precipitation and temperature and also the likelihood of severe weather across the globe, he said.

"Historical data indicates La Nina conditions are frequently associated with strong monsoon rainfall and flooding over the Asian monsoon region," Kolli said.

"It is also associated with drier than normal conditions over Africa and also with high frequency of hurricanes in the Atlantic," he added.

But it was too soon to predict specific effects in regions.

Currently, sea-surface temperatures are neutral across the tropical Pacific basin as a whole, though cooler-than-normal surface waters have been found off the western coast of South America, according to the Geneva-based WMO.

Its next WMO statement on La Nina, based on a consensus forged from weather forecasting centres, is due in three months.

La Nina last occurred from 1998 to 2001 -- a prolonged event -- and resulted in drought in much of the western United States.

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