The alert means that conditions in the eastern tropical Pacific are favorable enough that El Niño has a more than a 50% chance of forming by the summer or fall.
Though it's too early to predict with much confidence, if El Niño re-emerges it could produce wetter weather in the southern United States next winter, bring more rain to California, temper the Atlantic hurricane season and push up global temperatures in 2015, experts say.
The El Niño cycle begins every two to seven years when weak trade winds in the Pacific allow warmer water to build up along the west coast of South America.
El Niño conditions often result in higher rainfall in California, but not always.
If El Niño returns this year it would be the first since 2009-10, a moderate episode that was followed by the next season by La Niña.
Thursday's El Niño watch follows several months of forecasts based on observations from satellites, buoys and floats and computer models that have shown increasing signs El Niño could return later this year.