"Although a weak El Nino is likely this fall and winter, impacts should be minor going into late autumn," it said.

El Nino, Spanish for "boy child," is an abnormal warming of water in the Pacific Ocean every four to five years which can last up to 18 months.

The last one, from May 2002 through March 2003, caused record rains in Europe and Australia's worst drought in a century. In the United States, it aggravated drought in the Plains states and unleashed heavy storms in the south and east.

If an El Nino does develop and persists through the end of the year, NOAA said it may ease drought in the U.S. Southwest.

"There should be a slight increase in the chances for above-normal precipitation and increased streamflows over the Southwest this winter and early spring," the agency said.

In its drought outlook, NOAA said abnormally dry weather that hit major crop-growing regions in the Midwest this month would only be short term.

In its weekly drought report, also published on Thursday, NOAA said showers last week helped ease droughts in parts of Oklahoma, New Mexico, Texas, Nebraska and South Dakota. Experts forecast more heavy rainfall next week for the U.S. Plains.

NOAA said on Aug. 6 that sea surface temperatures had risen sharply in the central Pacific, possibly signaling the start of the anomaly first noticed by Latin American anchovy fishermen in the 19th century and named in honor of the Christ child as it tends to take place around the Christmas season.

Severe El Ninos, as happened in 1997/98, could cause searing drought in Australia, the Philippines and Indonesia while spawning rampant flooding in Chile, Ecuador and Bolivia.

The warming of Pacific waters can cause floods and drought as far as South Africa and trigger severe winter storms in California. El Nino killed hundreds of people in 1997/98 and caused billions of dollars in damage.