By Paul Rogers

progers@mercurynews.com

A powerful El Niño that had been emerging in the Pacific Ocean is fizzling out, evaporating hopes it will deliver a knockout punch to California’s three-year drought.

A new report from scientists at the National Oceanic and Atmospheric Administration decreases the probability of an El Niño — the condition that occurs when warm Pacific Ocean water at the equator affects the jet stream — to 65 percent starting in October, down from 82 percent in June.

More significantly, researchers said, the ocean water that had been warming steadily through the spring has cooled off in recent months. Most of the world’s leading meteorological organizations now say that if an El Niño arrives this winter, it is likely to be a weak or moderate one — not the kind historically linked with wetter-than-normal winters in California.

“It’s fair to say that it’s plateaued,” said Michelle L’Heureux, a meteorologist with the NOAA Climate Prediction Center in College Park, Maryland.

Other researchers are more blunt.

“We’re back to square one. It’s finished. I don’t think we even have an El Niño any more,” said Bill Patzert, a research scientist and oceanographer at NASA’s Jet Propulsion Laboratory in Pasadena.

“If I were a betting man, I’d say it’s 75 percent that we’ll have another dry winter,” he said. “The unfortunate fact is that it looks like the last three years all over again.”

To be sure, California could still have a wet winter to help fill depleted reservoirs, replenish streams and raise over-pumped water tables.

If a steady series of low-pressure systems develops off the Pacific Coast later in the year, that could bring tropical storms dumping rain in large amounts. The trend, known as an “atmospheric river” or “Pineapple Express,” has soaked the state in the past. But it has been all but shut down over the past three years as unusually persistent ridges of high pressure off the coast pushed winter storms north to Canada instead.

But the possibility that a strong El Niño won’t be there to help is “not good news, especially if we are using El Niño as an optimism index. It’s not what we want to see,” said meteorologist Jan Null, with Golden Gate Weather Services in Saratoga.

“It’s like in poker,” he added. “If you have one fewer spade out there, the odds of getting that flush are less.”

Generally speaking, the warmer the ocean water during El Niño years, the greater the likelihood of heavy winter rainfall. During mild El Niño years, when the ocean water is only slightly warmer than historic averages, there are just as many drier-than-average winters in California as soaking ones.

Since 1951, there have been six winters with strong El Niño conditions. In four of them, rainfall from the Bay Area to Bakersfield was at least 140 percent of the historic average, Null found.

But in the 16 winters since 1951 when there was a weak or moderate El Niño, California experienced below-normal rainfall in six of them. There was average rainfall in five and above-normal precipitation in the other five.

Thursday’s NOAA report was based on ocean temperature readings from dozens of buoys, wind measurements, satellite images and more than a dozen computer models from scientific agencies around the world.

In April, the report noted, Pacific Ocean waters were nearly 4 degrees Fahrenheit warmer than normal along the equator from the surface down to about 1,000 feet deep. But by last month, they had cooled — and are now half a degree cooler than normal. Wind bursts from Indonesia that had pushed warm water toward South America and the United States diminished. And huge amounts of heat dissipated and failed to trigger weather changes in the atmosphere.

“We’ve seen very lackluster atmospheric response,” said NOAA’s L’Heureux. “What typically happens with warm water in the eastern Pacific is that you see rainfall and winds shifting around. But it didn’t happen. It didn’t coalesce.”

As a result, none of the world’s major meteorological agencies is forecasting strong El Niño conditions this year. Most expect that Pacific waters will range from 1 to 2 degrees Fahrenheit warmer than the historic average this fall, which would signal a weak El Niño.

The last strong El Niño event, in the winter of 1997-98, saw Pacific surface temperatures 5 degrees warmer than normal at some times. That led to drenching rainfall across California, landslides that closed Highway 1 in Big Sur and 35 counties being declared disaster areas.

After three drier-than-normal years, major California reservoirs — from Shasta to Oroville to San Luis — are only 20 to 35 percent full. Farmers in the Central Valley are furiously pumping groundwater wells to keep crops alive. The danger of fire is extreme. And last month the State Water Resources Control Board passed mandatory rules that prohibit all Californians from washing down pavement, irrigating lawns so much that water runs into the street and other excessive practices. Violators face fines of up to $500.

If the drought drags into a fourth year, dozens of cities across California will see strict water cutbacks, including rationing, said Jay Lund, a professor of civil and environmental engineering at UC Davis. The state, he said, also is more likely to put in place rules regulating groundwater pumping and other long-delayed water efficiency reforms.

“It takes big droughts to make big changes in water policy in California,” Lund said. “It would cinch the deal if we have another dry year.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN.