Attention, weather superfans: El Niño might be coming back. And this time, we could be in for a big one.

Official NOAA Climate Prediction Center estimates peg the odds of El Niño’s return at 50 percent, but many climate scientists think that is a lowball estimate. And there are several indications that if it materializes, this year's El Niño could be massive, a lot like the 1997-98 event that was the strongest on record.

“I think there’s no doubt that there’s an El Niño underway,” said climate scientist Kevin Trenberth of the U.S. National Center for Atmospheric Research. “The question is whether it’ll be a small or big one.”

On top of some late-'90s nostalgia, a strong El Niño would bring pronounced changes to weather patterns around the globe, and possibly relief from some of the less-pleasant weather trends that have dominated headlines this year. After a Polar Vortex-fueled, unbearably cold winter in the U.S. Midwest and East Coast, a strong El Niño could bring warmer, drier weather in late 2014. And to parched California and its prolonged drought, El Niño might provide drenching rainstorms to fill up reservoirs. But the news won’t all be good. Rainstorms in California could mean floods and mudslides and, coupled with climate change, El Niño could bring harsher droughts to parts of Australia and Africa.

Beyond general outlines, it can be tough to say exactly what will happen with El Niño, so we’re going to break down some potential scenarios.

El Niño (which is Spanish for “the Niño”) is a recurring weather pattern affecting the world every two to seven years. In the tropical Pacific Ocean, the trade winds typically blow east to west, gathering warm water as they go and pooling it in the west. This creates a temperature gradient with cold water in the east, near the coast of South America, and warmer water southwest of Hawaii.

Image: NOAA /PMEL/ TAO

“But at some point the system says, ‘There’s too much warm water piling up here, I’m going to have an El Niño,'” said Trenberth.

The trade winds at this point usually weaken or even reverse entirely, moving warm water eastward. As it travels, this warm water starts emerging from deep in the ocean and heating up the atmosphere. These are the conditions that scientists are seeing right now. Moreover, the blob of warm water in the east is unusually large this year, leading many researchers to predict a monstrous El Niño is on its way.

“The main question right now is if this entire warm-pool region will accelerate to the eastern basin or stick in the middle of the Pacific,” said meteorologist Michael Ventrice of Weather Services International.

If the warm water decides to stick around at the International Date Line or so, we will get what is called an El Niño "Modoki" (which is Japanese for “similar, but different,” a word that every language should really have). Cold water would remain in the eastern Pacific during El Niño Modoki, leading to less rainfall in California than during a strong El Niño. But scientists have only noticed El Niño Modokis events in a few recent years and they are not yet exactly sure what brings it about.

Should the warm pool make it all the way to the South American coast, a much stronger “full-basin” El Niño will appear. And then we could be in for some big weather changes.

A strong El Niño could start affecting the world as early as the fall. The Pacific hurricane season, which gets active around September, is greatly enhanced during El Niño. This likely means more tropical thunderstorms that could affect eastern Pacific areas such as Mexico. In contrast, Atlantic hurricanes are suppressed, meaning fewer and less severe storms with a lower chance of making landfall and doing damage.

The winter is when El Niño really gets going, though. Moisture flows from Hawaii to southern California in an atmospheric river colloquially known as the “Pineapple Express.” This creates heavy rainfall that dumps on the region. Though this could bring some relief from California’s drought, it also comes with the risk of flash floods and mudslides because the ground has been so hard and dry.

El Niño has other effects further into North America. It tends to enhance the jet stream, creating a wall that prevents Arctic air (and the Polar Vortex) from dipping down to mid-latitudes. East Coast winters are generally drier and warmer during El Niño years, which is probably good news to those still smarting from this recent frigid season. The mild winter has interesting downstream effects, like a boost for the U.S. economy during the Christmas season.

“We saw a lot of retail sales go up in 1997,” said Ventrice. “People were going outside spending more money.”

Image: NOAA

Other economic consequences aren’t as sanguine. A full-basin El Niño disrupts cycles of fish in the eastern Pacific Ocean. Many of these species are usually caught and ground up into fishmeal, which is fed to farm animals in the U.S. The increased price of fishmeal drives meat prices up as well.

There is some indication that El Niño years coincide with stronger than average tornado seasons. Some of the worst years for tornadoes have occurred during what could be called “Hall of Fame” El Niño years such as 1982 and 1998. But the bottom line, said climate scientist Klaus Wolter of the University of Colorado, Boulder, is that it’s complicated. Tornadoes are caused by many different factors, and predicting what this year’s season will look like is difficult.

There is another large-scale effect in the atmosphere that this year’s El Niño is likely to interact with, and that is climate change. The last large El Niño in 1997-98 occurred with lower levels of CO 2 and things have changed in the intervening decade and a half. The Indian Ocean, for instance, has seen increased storm activity, which tends to detract from activity in the Pacific. “How this all evolves is certainly worth watching,” said Trenberth.

El Niño dries out places like India and Indonesia, causing a less severe monsoon. And it increases the risk of drought in places like Australia and Africa. With climate change, droughts have been growing more severe so this upcoming season could be a bad one. The end of El Niño also tends to heat up surface temperatures slightly, as the warm equatorial waters dump their energy into the atmosphere, the effects of which are usually felt approximately half a year later. The end of the last big El Niño was in 1998, the warmest year on record. Another temperature record holder is 2005, which followed an El Niño year.

“There’s a big chance that in 2015 there is going to be a bump in the global temperature,” said Klaus Wolter.

Finally, though a strong El Niño is looking ever more likely, it is far from a done deal. In 2012, a big El Niño appeared to be building up and ended up crashing before it got too far along. But if conditions remain as they are right now, by June researchers will know that El Niño is on its way.