U.S. government forecasters say odds are increasing that El Niño, an ocean-warming pattern that alters weather across the globe, will develop later this year.
Conditions in the eastern tropical Pacific have shifted enough that the National Oceanic and Atmospheric Administration on Thursday activated its alert system to issue an official El Niño watch.
The alert means that meteorologists now believe El Niño, a natural cycle that occurs every two to seven years, has a more than a 50% chance of forming by the summer or fall.
Its re-emergence could bring wetter weather to California and the southern U.S. next winter, temper the Atlantic hurricane season and push up global temperatures in 2015, experts say.
Mike Halpert, acting director of NOAA's Climate Prediction Center, said that satellites, buoys, floats and computer models have shown increasing signs that the ocean is warming below the surface and moving toward an El Niño.
"It looks pretty good at this point," he said, but cautioned that forecasts before spring are notoriously unreliable and sometimes fail to materialize.
If an El Niño develops, it would be the first since 2009-10, a moderate episode that was followed the next season by the cool water cycle, called La Niña. For the last two years, eastern Pacific waters have lingered in an in-between state that is not especially cool or warm.