Inside the August ENSO Discussion: Two Separate Thresholds, One Strengthening El Niño

The NOAA Climate Prediction Center held its ENSO Alert System Status at El Niño Advisory in the ENSO Diagnostic Discussion issued on 13 August 2026, reporting that El Niño strengthened over the past month and that there is a greater than 90% chance of a very strong event during Northern Hemisphere fall and winter 2026-27.

The centre's monthly assessment describes ocean and atmosphere moving together. Sea surface temperature anomalies exceeded +2.0 °C in the eastern equatorial Pacific, according to the CPC, and July values of its monitoring indices were +1.4 °C in Niño-3.4, +1.7 °C in Niño-3 and +2.9 °C in Niño-1+2 — the easternmost of the three regions again the warmest relative to average.

Below the surface, the CPC said its equatorial subsurface temperature index, averaged from 180° to 100°W, increased during July, reflecting a deeper-than-average thermocline, and it characterised subsurface temperature anomalies as significant. Subsurface heat matters because it represents warm water positioned to reach the surface as an event evolves; the centre treats the deeper thermocline as part of the evidence that this event is intensifying.

On the atmospheric side, the CPC noted low-level westerly wind anomalies and upper-level easterly wind anomalies extending from the western to the east-central equatorial Pacific, with convection and rainfall enhanced from the central to eastern Pacific and suppressed over Indonesia. Both the traditional and the equatorial Southern Oscillation indices were significantly negative, with values near 2 standard deviations during July. Taken together, the centre concluded, the coupled ocean-atmosphere system reflected a strengthening El Niño.

Two different numbers, two different questions

The discussion contains two headline probabilities, and they answer separate questions.

The first is the greater than 90% chance of a very strong event during Northern Hemisphere fall and winter 2026-27. That is the statement the CPC repeats as its synopsis and its closing summary, and it covers a two-season window.

The second is narrower and more demanding. For the October-December 2026 season specifically, the CPC's Official ENSO outlook — which the centre describes as a synthesis of multiple models from North America and abroad — puts the chance at 69% of a historic event that would exceed the strength of previous El Niño events dating back to 1950. The discussion defines that threshold precisely: a 3-month RONI value of +2.5 °C or more. The 69% figure therefore refers to a record-exceeding event over one three-month season, not to the broader "very strong" category described by the greater than 90% number.

The CPC also attached a caveat to the strength forecast. With an event of this magnitude, the centre said, the chances of experiencing impacts consistent with El Niño are larger, but they are not guaranteed, and it pointed readers to its own outlooks for probabilities of seasonal anomalies.

Reading the index values carefully

The index figures reported above are July monthly values for the individual Niño regions. The document does not publish a running three-month ONI or RONI value for the latest period; the only RONI number it gives is the +2.5 °C threshold used to define a historic event in the October-December 2026 forecast. Monthly regional anomalies and multi-month running means are not interchangeable, and the CPC labels each quantity explicitly for that reason.

Comparisons across agencies also need care. The CPC frames this assessment using Niño-3.4, Niño-3, Niño-1+2 and RONI; other forecast centres monitor their own index regions and apply their own thresholds and averaging periods to declare an event, so numbers quoted elsewhere will not line up one-for-one with these. Our science and glossary pages set out how the regions and indices are constructed.

What comes next

The discussion is described as a consolidated effort of the National Oceanic and Atmospheric Administration, NOAA's National Weather Service and their funded institutions. Oceanic and atmospheric conditions are updated weekly on the Climate Prediction Center web site, and the centre notes that a probabilistic strength forecast is also published. The next ENSO Diagnostic Discussion is scheduled for 10 September 2026, with an e-mail notification list available for the monthly releases. Earlier statements are collected in our news archive.