National Oceanic and Atmospheric Administration meteorologists now believe an El Nino has a 66 percent chance of forming by this winter.
If the prediction materializes and the influential climate pattern sets in, it could bring wetter weather to California and the southern U.S., suppress the Atlantic hurricane season and compound global warming by boosting temperatures in 2015.
Government forecasters say observations and computer models in recent weeks indicate that water below the surface in the eastern tropical Pacific Ocean is warming and showing increasing signs that an El Nino is afoot.
“The odds are increasing but still not one hundred percent,” said Mike Halpert, acting director of NOAA’s Climate Prediction Center.
After inching slowly upward, the chances of an El Nino surpassed 50 percent last month, prompting NOAA activate its alert system and issue an official El Nino watch.
A forecast by Australia’s Bureau of Meteorology on Tuesday predicted a 70 percent chance of an El Nino in the coming months.
The naturally occurring climate cycle occurs every two to seven years when weak trade winds allow warmer surface water to pile up along the west coast of South America, shifting storm activity and altering precipitation patterns around the globe.
Although El Nino is associated with above-average rainfall in the southern U.S. and Southern California, its return would not be a guaranteed drought-buster.
A potent El Nino in the winter of 1997-98 doubled rainfall in Southern California, but other episodes, including strong El Ninos in 1965-66 and 1991-92, resulted in below-normal rainfall.
The pattern has been in a lull for the last two years, with the eastern Pacific never warming enough to form a El Nino or cooling to generate a La Nina. The last El Nino, in 2009-10, was moderate and followed by the cool-water phase, La Nina.
NOAA’s latest forecast gives a 30 percent likelihood of conditions remaining neutral by year’s end and a 4 percent probability of a La Nina.
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