An El Nino that has developed in the tropical Pacific could become one of the largest events of its type on record, US forecasters say.

The National Oceanic and Atmospheric Administration says El Nino conditions developed over the past month and are expected to strengthen into the northern hemisphere winter.

“There is a 63% chance of a very strong El Nino during November-January that would rank among the largest El Nino events in the historical record going back to 1950,” an advisory notice this week said.

During a typical El Nino northern winter, the jet stream over the north Pacific Ocean tends to shift southward, bringing the storm track over the southern part of the US, the NOAA’s National Weather Service says.

The southward shift in the storm track also leads to drier conditions over the northern Rockies, and the Ohio and Tennessee valleys. El Nino often leads to a warmer than usual winter over the northern US.

“Every El Nino is not the same; each one is unique with its own imprint on our weather,” National Weather Service director Ken Graham said.

“Advanced monitoring and an improved understanding of El Nino patterns allow the [weather service] to better predict and better prepare the public and our core partners for what is to come.”

A US El Nino Southern Oscillation update is expected on July 9.

Australia’s Bureau of Meteorology, which has different declaration thresholds, says an El Nino is likely to form this southern winter.

A June 9 update notes sea surface temperatures in the central tropical Pacific now exceed El Nino thresholds, while atmospheric indicators are starting to align.

“This suggests some coupling between the ocean and atmosphere may be occurring. Should this be sustained, an El Nino event is likely to become established,” it said.

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