CBS San Francisco
The chances that California will begin clawing its way out of the drought with a wet winter got a bump Thursday with a federal report showing an El Niño weather pattern continuing to strengthen in the Pacific.

The U.S. Climate Prediction Center reported that telltale signs of El Niño — which include warming sea surface temperatures and emerging equatorial winds — bore close resemblance to conditions preceding some of the strongest El Niños in recent history.

El Niños carry no assurances for forecasters, but big ones have correlated with increased rainfall across much of California, such as in the winters of 1982-83 and 1997-98, which were among the state’s wettest years.

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“If the event continues to strengthen (this year), then that’s what we’ll see,” said Mike Halpert, deputy director of the Climate Prediction Center, a division of the National Oceanic and Atmospheric Administration. “It’s been a while since we’ve seen one like this.”

The forecast is good news for California, which has seen four consecutive winters of below-average precipitation. Cities and farms statewide are being forced to cut their water use because of shortages.

The next several weeks will be crucial for whether this year’s El Niño really delivers, Halpert said.

Some parts of the Pacific tropics did not warm as quickly as others during June, which could weaken the phenomenon, according to the Climate Prediction Center. However, a giant underwater swell known as a Kelvin wave appears to be beginning to push colder water downward, which could boost surface temperatures.

“I think we’ll see this Kelvin wave,” Halpert said. “The next month or two are going to tell us a lot.”

El Niños are one of the most closely watched weather patterns because of their duration and ability to influence worldwide conditions. The phenomenon is characterized by warm equatorial waters that push moisture into the atmosphere far beyond its origin.

Strong El Niños have been associated with wetter-than-average conditions in the Americas and drier conditions in Asia and Australia. Weak and moderate events have had less association with global weather.

Thursday’s report cites a 90 percent chance that El Niño conditions will persist through California’s rainy season, up from 85 percent last month.

And most of the models now favor a strong event. Things could change, however, Halpert warned. Last year, forecasters said conditions were ripe for an El Niño, but the pattern didn’t take hold.

Kurtis Alexander is a San Francisco Chronicle staff writer. E-mail: kalexander@sfchronicle.com Twitter: @kurtisalexander