El Niño vs La Niña: the difference, the thresholds, and what neither one predicts

El Niño and La Niña are not two separate phenomena. They are the warm and cool phases of a single pattern in the tropical Pacific called the El Niño-Southern Oscillation — ENSO — and the line between them is a defined number rather than a matter of description.

That number is measured in the Niño-3.4 region, a box in the equatorial Pacific between 5°N and 5°S, and between 120°W and 170°W. NOAA tracks how far sea-surface temperature there sits from average. **At least 0.5 °C above average is El Niño. At least 0.5 °C below is La Niña.** In between is ENSO-neutral. To be formally declared rather than merely observed, the departure has to hold for five consecutive overlapping three-month periods.

The cycle shifts back and forth irregularly, roughly every two to seven years. It is not seasonal and it is not on a schedule.

What separates the two phases physically is the trade winds. During La Niña the easterly trades strengthen, pushing warm surface water further west and drawing cool water up in the east. During El Niño they weaken, the warm pool spreads east, and the band of heaviest tropical rainfall shifts with it. Because that rainfall drives atmospheric circulation, moving it moves the jet stream — which is how a temperature anomaly in the mid-Pacific ends up changing the odds of rain in Georgia.

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

For the United States, a strong El Niño typically pushes the storm track south: wetter odds across the southern tier from Southern California through the Gulf Coast and Southeast, drier odds across the Pacific Northwest and northern Rockies. La Niña generally reverses it.

Now the part that matters more than any of the above.

**ENSO shifts probabilities across a season. It does not forecast weather.** A region shaded for above-normal precipitation is not being promised a wet winter; it is being told the odds have tilted. A season can land in the unfavoured third of the distribution without the forecast having been wrong. And the pattern says nothing at all about snowfall specifically, because seasonal precipitation maps combine rain and melted snow and cannot separate them.

One practical warning about checking this yourself. While preparing this article, two widely available sources disagreed with NOAA's own advisory. One widely syndicated tracker gave the current Niño-3.4 anomaly as +2.3 °C, nearly double the Climate Prediction Center's +1.2. And NOAA's own general explainer page still carried text stating that La Niña had ended and the Pacific was neutral — accurate when it was written, wrong now.

The dated ENSO Diagnostic Discussion from the Climate Prediction Center is the document that settles it. It is issued monthly, it carries its own date, and it states the alert status outright. Anything that does not tell you when it was written is not telling you what the Pacific is doing today.