The El Nino weather pattern, which can bring global weather chaos such as droughts and floods, could develop within weeks, the Climate Prediction Center in the United States said.
El Nino is driven by an abnormal warming of the eastern Pacific and the forecaster said conditions were favourable for a switch to El Nino conditions during June to August 2009.
The forecast is the latest warning of the increased chances of El Nino developing after months of rising ocean temperatures in the tropical Pacific.
Australia's Bureau of Meteorology said on Wednesday that if recent trends in Pacific climate patterns held up, there was an above-50 percent chance an El Nino event would be established by July.
El Nino occurs when the eastern Pacific Ocean heats up, with warmer, moist air moving east, leaving drier weather in the western Pacific and Australia and putting crops at risk of failure.
The most devastating El Nino was in 1997/98, when it caused drought in Australia and Indonesia and floods in Peru and Ecuador.
El Nino can also bring wetter weather to parts of the United States and can affect the monsoon in India.
The prediction center, part of the National Oceanic and Atmospheric Administration, said sea surface temperatures had risen for the fifth consecutive month in the equatorial Pacific.
Sub-surface temperatures also continued to rise.
"These surface and subsurface oceanic anomalies typically precede the development of El Nino," the center said in a statement.
"Current observations, recent trends, and the dynamical model forecasts indicate that conditions are favourable for a transition ... to El Nino conditions during June-August 2009."