See why this year’s El Niño is expected to be the most powerful yet

Early indicators point to extreme weather, with the Pacific Rim already hit and more likely in store for the U.S.

This year’s El Niño is heating up to be one of the strongest on record.

The global weather disrupter has already stoked wildfires in Asia, hurricanes in the Pacific and fishery closures in South America.

The worst may be yet to come. The World Meteorological Organization, a United Nations agency, recently forecast that a very strong El Niño will persist through winter, warning of flooding, drought and extreme heat.

This global weather phenomenon occurs every few years and lasts about a year. Typically, warm water collects in the western Pacific. But in El Niño years, this mass of warm water shifts east toward South America.

This eastward movement of warmer water is one of the harbingers of El Niño. Scientists measure these ocean temperature changes to predict how strong an event will be.

This summer’s temperatures in the east-central Pacific are among the highest on record, according to data from the National Oceanic and Atmospheric Administration. Projections show them exceeding previously powerful El Niños.

As warm waters shift east in the Pacific, the tropical rains they generate move with them—creating fertile conditions for hurricanes. The number of named storms in the northeastern Pacific is about 40% above average for this time of year, according to data from Colorado State University.

Hawaii has seen unusually strong cyclone activity this summer, with Hurricanes Lala and Lowell bringing destructive winds and flooding.

Strong El Niños in 1997 and 2015 fueled devastating blazes in Indonesia. This year more than a half-million acres have burned there, forcing school closures and harming air quality as far away as Singapore and the Philippines.

El Niño’s effects extend far beyond the Pacific.

As ocean evaporation shifts east, it generates warm air that rises into the atmosphere. The warm air migrates north and east, speeding up as it goes.

These high-pressure winds feed into the subtropical jet stream, giving an extra boost that propels it further east over North America, causing it to blow stronger and steadier.

The souped-up jet stream delivers more rain and storms to the southern part of the U.S. Precipitation peaks in winter, potentially bringing flooding and coastal erosion to California and severe thunderstorms to Gulf states.

El Niño sows chaos even farther afield.

“Southern Africa has a pretty strong tendency for drought during El Niño,” according to Emily Becker, an atmospheric scientist at the University of Miami. “These can have very significant impacts because of food insecurity.”

The U.N. World Food Program recently estimated that El Niño could push almost 50 million more people into acute hunger by next year, with southern Africa and Central America particularly at risk.

El Niño isn’t all bad.

While the climate disturbance promotes cyclones in the Pacific, it can stifle them in the North Atlantic.

In El Niño years, warm air rising off the Pacific generates high-altitude winds that blow east toward the tropical Atlantic. “This results in increased levels of Atlantic vertical wind shear,” according to Phil Klotzbach, a hurricane forecaster at Colorado State University. “Too much vertical wind shear tears apart hurricanes.”

The North Atlantic typically gets more than three hurricanes by this time of year. So far, there have been none.

Another redeeming aspect of El Niño: Scientists know where it is likely to strike.

“Disasters happen all the time. And in El Niño years, we actually know where they will happen,” said Becker. “We can prepare.”

Write to Josh Ulick at josh.ulick@wsj.com and Erik Brynildsen at erik.brynildsen@dowjones.com