What is El Niño? A defined temperature threshold in one box of the Pacific

El Niño is not a storm, a season or a forecast. It is a measured state of the Pacific Ocean, and it has a definition precise enough to be declared or not declared.

Here is the definition. In a box of the equatorial Pacific called the Niño-3.4 region — between 5°N and 5°S, and between 120°W and 170°W — NOAA tracks how far sea-surface temperature departs from average. **When that departure reaches at least 0.5 °C above average, and holds for five consecutive overlapping three-month periods, it is El Niño.** At least 0.5 °C below is La Niña. In between is ENSO-neutral.

The whole thing sits inside a larger pattern called the El Niño-Southern Oscillation, or ENSO, which shifts irregularly every two to seven years. It is not annual and it is not scheduled.

What produces it is the trade winds. Normally easterly trades push warm surface water westward across the Pacific, letting cooler water rise in the east. When those winds weaken, the warm pool spreads back east — and the band of heaviest tropical rainfall moves with it, because rainfall follows the warmest water. Since that rainfall drives large-scale atmospheric circulation, moving it shifts the jet stream thousands of miles away. That chain is why a sea-surface reading near the equator changes precipitation odds in Georgia.

As of the Climate Prediction Center's diagnostic discussion dated 9 July 2026, an **El Niño Advisory is in effect**, the most recent weekly Niño-3.4 value was **+1.2 °C**, and forecasters give an 81 per cent chance of the event reaching very strong intensity during October to December, with 97 per cent odds of persisting into early spring 2027.

For the United States, a strong El Niño typically pushes the storm track south — tilting the odds wetter across the southern tier from Southern California to the Southeast, and drier across the Pacific Northwest and northern Rockies.

The word doing the work in that sentence is *typically*, and the second word is *odds*.

**El Niño shifts seasonal probabilities. It does not forecast weather.** A region favoured for above-normal precipitation is not being promised a wet season; it is being told the distribution has tilted. A season can land in the unfavoured third without the outlook having been wrong. And it says nothing whatsoever about snowfall, because seasonal precipitation maps combine rain and melted snow and cannot separate the two.

One practical note if you check this yourself. The status changes, and pages that describe it do not always keep up — NOAA's own general explainer still carried text describing the Pacific as neutral while the Climate Prediction Center's advisory recorded El Niño. The dated ENSO Diagnostic Discussion is the document that settles it. If a page does not tell you when it was written, it is not telling you what the Pacific is doing today.