This year’s El Niño weather pattern could be the most powerful on record, federal forecasters said, while warning that the effects of the weather system are never certain.

“We’re predicting this El Niño could be among the strongest El Niños in the historical record,” said Mike Halpert, deputy director of the Climate Prediction Center for the National Oceanic and Atmospheric Administration, in a teleconference with reporters. This year’s El Niño is already the second strongest for this time of year in more than 60 years of recordkeeping, he said.

El Niño, which begins with warmer-than-usual water temperatures in the Eastern Pacific, can affect weather around the world — most notably for Americans by bringing heavy winter precipitation in California and across the American South. El Niño events have also been linked to droughts in Australia and India, and more numerous hurricanes in the Pacific (but fewer in the Atlantic), and a warmer planet overall.

The current El Niño, along with unusual warming in the Northern Pacific, will produce what is “very likely to be the warmest year on record,” said Daniel Swain, a Ph.D. candidate at Stanford who runs the respected California Weather Blog, in an interview.

The federal forecasters announced a greater than 90 percent chance that El Niño would continue through the winter of 2015-16 for the Northern Hemisphere. The likelihood that the effects will last into early spring 2016 is 85 percent, up from last month’s prediction of 80 percent.

Conditions in the Pacific Ocean suggest that what has formed there is as big as anything seen since 1997-98, a system that brought the term “El Niño” into popular culture, and which is remembered for the catastrophic amounts of water it dumped on California, triggering flooding and mudslides.

California receives most of its precipitation in the winter months, and that is when the effects of a potent El Niño system are felt.

William Patzert, a climatologist with NASA’s Jet Propulsion Laboratory in California, said this week in an interview that many of the factors were in place for a big event, with ocean warming “that rivals what we saw with the great Godzilla El Niño of 1997 and 1998.”

The main missing piece of the current patterns that would ensure an event like 1997, he said, is a relaxation of trade winds in the central and western Pacific, which allows the weather patterns to move eastward. “I’m a little cautious — this could happen, it could not happen,” he said.

In other words, Patzert said, “Bob Dylan says it all — the answer is blowin’ in the wind.” Without the relaxation of the trade winds, “this will turn out to be a modest El Niño, with a huge sigh of disappointment here in the West.”

Even if El Niño could bring enormous amounts of rain to California, it will almost certainly not wipe out the state’s four years of drought, experts said. Central and Northern California, which supply much of the state’s water, do not typically get as much precipitation from an El Niño as Southern California, said Kevin Werner, director of western regional climate services for NOAA.

More important, he noted, meeting the current water deficit for the state would require more than twice the average amount of precipitation for a year — “something in excess of the wettest year on record,” he said.

It has to be the right kind of precipitation, too. While a great deal of rain could recharge many of the state’s reservoirs, much of the state’s water supply depends on the amount of snow on the Sierras. “El Niño does have the potential to bring a whole lot of water to California,” Swain said, “but it doesn’t necessarily bring a lot of snow.”

The federal forecasters also warned that the complexities of the El Niño phenomenon mean that the typical patterns of rain and other effects might not emerge at all.

“There are still probably more unknowns regarding temperature and precipitation in the winter than there are knowns,” Halpert said. Experts in the El Niño phenomenon note that they have had relatively few events to compare over the past 60 years, and that each El Niño has proved to be unique.

“You’re working with a very small sample set,” Michael L. Anderson, California’s state climatologist, said in an interview. Even an El Niño that appears to hold promise of extensive precipitation based on similarities to past events may not come through, he said.

“The one important element is that El Niño events are associated with large variability of outcome,” he said. And while people tend to remember years with powerful El Niño effects, he said, “People don’t associate as strongly the years when an El Niño event didn’t lead to a big outcome.”