El Nino will likely return this summer after a five-year absence, but experts are not yet sure how strong an effect the cyclical warming of Pacific Ocean waters will have on global weather patterns.
“We slightly favor a moderate strength event, although all possible solutions, from a strong event, to this not even developing, are still on the table,” said Mike Halpert, acting director of the U.S. Climate Prediction Center.
The center -- which is part of the National Oceanic and Atmospheric Administration -- and its partner at Columbia University are the chief data keepers for all things El Nino and La Nina.
In March, the partners issued an “El Nino watch” and set the chances of it developing this summer at 50 percent. On June 5, when the latest forecast was issued, the chances went up to 70 percent.
The partners base their forecast on computer models that factor in Pacific Ocean temperatures and other variables, but so far those models aren’t in agreement when it comes to severity.
“I'm hoping that the model runs we see in early July will begin to converge,” Halpert said.
It appears less likely than it did a few months ago that a “super El Nino” will develop.
“Earlier in the spring we had rapid warming beneath the surface in the central Pacific and it was headed east,” said NBC News meteorologist Bill Karins.
“That is why you heard many headlines saying ‘super El Nino possible this fall,'” Karins said, and that it “might be as strong and as bad as 1997-98. But since then the rapid warming has leveled off and even lessened.”
The chief forecaster at NOAA’s partner, Columbia University’s International Research Institute for Climate and Society, agreed.
“Earlier, people thought it might be a giant event,” said Tony Barnston. “Now it doesn't look that way.”
“A moderate strength event appears somewhat more likely than a weak or strong event, and a weak event slightly more likely than a strong event,” Barnston said.
Karins, for his part, said he was “betting we end up with a low-end moderate El Nino.”
That should still be enough to have an impact on hurricanes this year. NOAA earlier predicted a mild Atlantic hurricane season, citing the fact that El Ninos create conditions that deter hurricane formation in the Atlantic Ocean. In the Pacific Ocean, on the other hand, El Ninos tend to support hurricane formation.
Karins noted that, after hurricanes, “the most noticeable impact” in the United States would be a milder winter for most of the country.
“The biggest unknown,” he said, is whether El Nino rains will help drought areas in the Southwest and West, especially California.
El Ninos have dumped heavy rain across the West before, but usually those El Ninos have been “quite strong,” Barnston said.
They can cause dry spells in parts of Australia, Indonesia, the Philippines, northern Brazil, India and southeastern Africa, while leaving other areas like Peru, southern Brazil and northern Argentina more vulnerable to flooding from heavy rains.
El Ninos have been known to stick around for up to two years, but that seems unlikely this time, the experts said.
“The model and expert consensus is that the event will attain peak strength during the fourth quarter and endure into the first few months of 2015 before dissipating,” Barnston said.
And for all the modeling and forecasting, there’s still a chance an El Nino might not form at all this year.
NOAA researcher Michelle L’Heureux last month put the chances of a no show at 20 percent. In fact, she noted in a blog post, that’s what happened in 2012.
“An El Nino Watch was issued,” she wrote, “chances became as high as 75 percent and El Nino never formed.”