The El Niño Briefing #6: CPC now puts a record event at 75%
Updated: September 11, 2026 · 2 min read · Live dashboard
The official odds moved this week, and they moved up.
In its monthly discussion on Thursday, 10 September, NOAA's Climate Prediction Center put the chance of a historic El Niño at 75%: one that exceeds every event on record back to 1950, defined as +2.5 °C or more on the three-month RONI during October–December 2026. A month ago the same figure was 69%. The headline odds are unchanged: better than 90% that this becomes a very strong event this fall and winter.
Where we actually are. The weekly Niño 3.4 anomaly rose to +2.7 °C for the week centred 2 September, after three weeks at +2.6. Off the coast of Peru and Ecuador, the Niño 1+2 region reached +4.6 °C. The seasonal numbers moved too: ONI for June–August came in at +1.80, and RONI — the version that strips out the tropics' background warming, and the one CPC's record threshold is written in — at +1.36, up from +0.97 a season earlier.
Like for like. At the same point in the year, June–August:
- 1982: ONI +0.78, RONI +0.89
- 1997: ONI +1.48, RONI +1.50
- 2015: ONI +1.44, RONI +1.27
- 2026: ONI +1.80, RONI +1.36
On the raw ONI, 2026 is ahead of all three. On RONI it sits between 2015 and 1997. Which ruler you pick changes the story, which is why we keep saying which one we are using.
What the 75% asks for. To clear +2.5 on RONI by October–December, the index has to rise about 1.1 °C from its June–August value. Over the same stretch, 1997 rose +0.75 and 2015 rose +0.91. 1982 rose +1.35. So the forecast is asking for a 1982-style climb rather than a 1997 one — it has happened before, but it is the fast end of the record, not the middle. That is the number to watch each month.
Next. NOAA's weekly file updates on Monday, with the week centred 9 September. The next monthly discussion is due on 8 October.
— The El Niño Briefing