El Niño 2026-27: Markets Price a Record Peak as NOAA Gives 83% to a Historic October-December

The Pacific is running so hot this autumn that the argument is no longer whether this El Niño is strong, but whether it will be the strongest ever measured. On October 8, NOAA's Climate Prediction Center (CPC) put an 83% chance on the October-December three-month Relative Oceanic Niño Index (RONI) reaching +2.5°C or more, a level the agency calls historic because it would exceed every El Niño in its record back to 1950. Traders on Polymarket agree: on October 31, the market pricing the peak RONI for the 2026-27 event put 74.5% on +2.5°C or above, with 18.5% on the next bracket down, +2.25°C to just under +2.5°C. The two numbers are not the same bet — the CPC figure is for one season, the market figure is for the peak across five overlapping seasons — but they point the same way, and that convergence is what caught my eye.

It helps to be precise about what RONI is, because the headlines calling this a "super El Niño" use a term NOAA does not formally recognise. RONI, NOAA's official ENSO index since February 2026, takes the sea-surface temperature anomaly in the Niño 3.4 box in the central equatorial Pacific and subtracts the average anomaly across the whole tropical ocean, then rescales the result. The subtraction is the point: if the entire tropical ocean is warmer than it used to be, that background warming should not count as a stronger El Niño. On this relative scale, the highest value since 1950 is +2.4°C, reached in late 1982 and early 1983; the famous events of 1997 and 2015 peaked at +2.3°C. So +2.5°C is not just a round number — it sits one tenth of a degree above the existing record.

The observations behind the forecast are already extreme. The latest three-month RONI value, for July-September 2026, stood at +1.7°C. In September, the CPC's monthly regional anomalies reached +2.1°C in Niño 3.4, +3.0°C in Niño 3 and +3.9°C in Niño 1+2, and the weekly Niño 3.4 anomaly was running at +3.4°C in the week centred on October 21. The CPC kept a strong-to-very-strong event likely through January-March 2027, at above 83%. The World Meteorological Organization's separate ensemble, which uses the traditional Niño 3.4 anomaly rather than RONI, forecast about +3.7°C for October-December and a peak around December. Those two indices must not be mixed up, but both tell the same story of an event still strengthening into the end of the year.

One technical detail matters before anyone treats the market price as a forecast in itself. The Polymarket question resolves on the single highest RONI value reported by the CPC across the overlapping seasons from August-October 2026 through December-February 2026-27, using the first published value for each season, even though the CPC notes that recent RONI values can be revised for up to two months. That first-published rule makes the bet cleanly verifiable, and it means the market is betting on the numbers as they first appear, with the December-February value — due in early 2027 — as the last chance for the peak to cross the line.

What would a record peak mean on land? Less than the headlines imply, and more than nothing. A stronger El Niño raises the odds of the pattern's typical impacts — NOAA's October 15 winter outlook already pointed to a much wetter pattern across the southern United States — but it guarantees no single flood, drought or storm anywhere. That is also why I am not simply copying the market price as my own confidence. My prediction, attached to this article, is that the peak RONI will reach +2.5°C or above, at 70% confidence — close to the market's 74.5%, differing by less than 15 points, with the small gap reflecting one specific doubt. RONI subtracts the tropical mean, so a Pacific that is hot in absolute terms can still fall short on the relative scale if the rest of the tropics is hot too. The most likely way this bet loses is a peak landing just under the line, in the +2.25°C to +2.5°C bracket the market prices at 18.5%.

The next test arrives soon. The CPC's next ENSO Diagnostic Discussion is scheduled for November 12, and it will either confirm the October trajectory or start to walk it back. If October-December tracks as hot as forecast, the record will stop being a probability and start being a measurement.