Sea surface temperatures in the tropical Pacific have climbed to record levels for this time of year, and forecasting agencies expect the El Niño pattern to keep strengthening through the winter.
In the region used to measure the pattern, water is 6.1 degrees Fahrenheit above normal. The National Oceanic and Atmospheric Administration puts the chance of a very strong event at 90% for autumn and winter, and a 75% chance that it becomes the largest El Niño since 1950.
Second-largest anomaly on record
By NOAA's preferred index, which strips out some of the influence of global warming, this season ranks as the second-largest July-to-September anomaly on record. Climatologists have stopped arguing about the label.
"It feels like super El Niño is appropriate," said Abby Frazier, a climatologist at Clark University.
The physical signs are already visible. The Atlantic hurricane season has been historically calm while powerful storms crossed the Pacific, several of them affecting Hawaii. Earlier this year NASA satellites measured higher sea levels near the equator, a sign that warm water was expanding in volume.
Unusual marine animals, including whale sharks, sea snakes and nesting turtles, have been seen off the California coast farther north than cool water normally allows.
Who carries the risk
UNICEF said its analysis finds more than 347 million children in nearly 100 countries living in areas expected to become drier or wetter because of the pattern. Executive director Catherine Russell said El Niño is expected to increase water shortages, malnutrition and outbreaks of disease.
Researchers at the University of Chicago's Climate Impacts Lab estimate the added global heat could cause 451,000 excess deaths worldwide.
The effects run in both directions. Some regions face intense rainfall and flooding, others face drought and failed harvests, and both strain food prices and electricity systems.
What to watch
Forecasters will track whether trade winds weaken further in the coming weeks, which would let more warm water spread east. Aid agencies are pre-positioning supplies in vulnerable regions while the pattern is still building, on the assumption that the worst impacts arrive in the first half of next year.