Factbox: U.S. weather forecaster's El Nino/La Nina watch

(Reuters) - The U.S. weather forecaster issued its first El Nino watch in almost 18 months on Thursday, warning the phenomenon that can wreak havoc on weather and roil global crops could strike as early as the Northern Hemisphere summer.

The latest outlook brings the forecaster in line with other global meteorologists that have raised their outlook for El Nino's potential return.

Here are some details about the Climate Prediction Center's (CPC) recent weather alerts:

*The CPC issues an alert when there is a chance of El Nino or La Nina conditions developing in the coming six months.

*This is the CPC's first El Nino watch since October 2012.

*The anomaly heats up tropical oceans in East Asia, sending warm air into the United States and South America, often causing flooding and heavy rains. It can also trigger drought conditions in Southeast Asia and Australia, regions that produce some of the world's major food staples, such as sugar cane and grains.

*The previous El Nino alert lasted five months from June until October, but the anomaly did not materialize. The CPC dropped the alert in November 2012.

*The global weather system has not been roiled by El Nino or its infamous counterpart La Nina for several years.

*In 2011, La Nina conditions, an abnormal cooling of waters in the equatorial Pacific, caused extreme weather conditions in Asia and the Americas.

*The phenomenon was also blamed for the worst drought in a century in Texas, the country's biggest cotton growing state, and severe dry spells in South America that killed crops.

*The CPC dropped its La Nina watch in May 2012.

(Reporting by Josephine Mason; Editing by Sophie Hares)