The U.S. Climate Prediction Center raised the prospect that El Niño conditions could return after the Northern Hemisphere summer, causing adverse weather that could potentially disrupt the harvest of vital crops such as cotton, corn and soybeans.
A return in the fall of La Niña's more infamous counterpart could increase rainfall, although farmers typically prefer it to the La Niña phenomenon. La Niña has been blamed for a bad dry spell in South America.
In its monthly climate forecast, the CPC said there is still considerable uncertainty for the remainder of the year, but it "slightly favors (La Niña) neutral or developing El Niño conditions over a return to La Niña conditions."
To the relief of U.S. farmers, La Niña has been fading since February and its impact is expected to disappear by the summer, CPC said on Thursday reiterating its previous forecast.
With it dissipating between now and June, there is an increased chance of above-average temperatures in the south-central United States and below-average temperatures in the Northwest, the CPC, an office under the National Oceanic Atmospheric Administration, said.
El Niño has in the past caused drought in Southeast Asia and flooding in South America and Australia.
La Niña is an abnormal cooling of waters in the equatorial Pacific, which can last for years and wreak havoc over weather conditions in Asia and the Americas. El Niño is the abnormal warming of those waters, which can also affect weather.
"Any undue moisture can hamper harvest," said Sterling Smith, an analyst at Country Hedging Inc in Minnesota.
But he said it is far too early to cause alarm among farmers, whose planting season has started.
Modern technology means that farmers can harvest in a matter of days rather than weeks and as a result can act quickly if adverse weather, which could wreak havoc, was predicted.
"It's a concern but not a concern of years past," Smith said.
It would have the biggest impact on corn. U.S. farmers are expected to plant the most corn in 75 years to ease extremely tight supplies that have kept prices near historic highs.
While La Niña fades, drier-than-average conditions are more likely across Utah and Colorado and along the western Gulf of Mexico.
Those conditions would not necessarily affect key U.S. farming regions in the Midwest and the south, with the exception of eastern Colorado, which has some corn farming but is well irrigated, Smith said.
But La Niña was blamed for last year's drought in Texas, the biggest cotton growing state in the country, which was the worst drought in a century.
Its dissolution would coincide with the start of the annual Atlantic hurricane season on June 1. The U.S. crude oil industry is particularly worried about storms in the Gulf of Mexico threatening to topple platforms and rigs in the area.
La Niña has been keenly felt in Latin America, where estimates for the 2011/12 corn crop from Argentina, the world's No. 2 supplier, have been cut. Brazil's soy crop has also been hit. (Reporting By Josephine Mason; Editing by Bob Burgdorfer)