Seventy-five percent is the figure NOAA’s Climate Prediction Center attached to the most extreme outcome on its board: an El Niño that outstrips every one on record back to 1950. The center’s September 10 ENSO Diagnostic Discussion puts the odds of that historic event at three in four for the October-December 2026 season, and a separate, even higher probability on a merely “very strong” one.
Both numbers come from the same product, issued by a NOAA office whose official outlook synthesizes multiple models from North America and abroad into a single forecast. The next scheduled update is October 8.
The CPC sentence behind the 75%
The discussion states the claim in one sentence: during October-December 2026, “there is a 75% chance of a historic event that would exceed the strength of previous El Niño events dating back to 1950,” defined as +2.5°C or more for a three-month value of the Relative Oceanic Niño Index. The probability therefore belongs to the record-strength outcome itself and to a specific season, not to El Niño conditions in general.
That distinction matters because the same discussion carries a second, broader figure. Its synopsis says El Niño “is strengthening, with a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27.” A very strong event and a record-setting one are different thresholds, and the Climate Prediction Center assigns them different odds. The 75% applies only to the three-month Oct-Dec window; it is the fall-into-early-winter peak that the center singles out, rather than the entire winter, and a reader comparing the two probabilities should keep that window in mind because the broader 90% figure spans several overlapping seasons through the January 2027 period the center lists, each of which can only be scored once the readings are in.
The center’s late-September status slides repeat both lines unchanged, dated 28 September 2026, and add that El Niño is forecast to persist through March-May 2027 with a 55 percent chance of neutral conditions by April-June.
Relative index and the 2.5-degree threshold
The yardstick matters as much as the percentage. CPC’s monitoring page explains that the Relative Oceanic Niño Index, which subtracts the warming of the tropical oceans as a whole from the Niño-3.4 reading, is now the official measure for monitoring and predicting ENSO. The record the 75% chance is measured against is therefore a record on that index, with 2.5°C as the bar, and not a ranking of rainfall, storms or damage.
The current event is already large. The August Niño-3.4 reading reached +1.8°C, and the CPC discussion notes sea surface anomalies exceeding +3.0°C in parts of the eastern equatorial Pacific. Meteorologist Sam Collentine of OpenSnow, summarizing the center’s outlook, wrote that the June-August Niño-3.4 value was the second-warmest for that season in the record back to 1950, and that the 1982-83 event, at roughly +2.4°C, is the mark in play.
Impacts, snowpack and the limits of an ocean forecast
NOAA is careful about what the label implies. The same paragraph that carries the 75% adds that with an event of this magnitude, “the chances of experiencing impacts consistent with El Niño are larger, though not guaranteed.” Impacts are handled by separate products: the monthly seasonal outlooks, issued mid-month, translate the ocean signal into temperature and precipitation odds for each region.
In OpenSnow’s tally of the four strongest El Niño winters, Western snowpack ran from 139 percent of normal in 1982-83 to 73 percent in 1991-92, with 104 and 98 percent in 1997-98 and 2015-16. The Southwest leaned wetter in most of them, but, as Collentine put it, the pattern does not hold every time.
The gap between the ocean signal and a local forecast is the reason the discussion stops where it does. It quantifies how strong the Pacific warming is likely to get, then sends readers to the separate outlooks for anything about rain, snow or temperature at a specific place. A record-strength El Niño raises the odds of El Niño-type weather without writing the season in advance, and the center says so in its own text.
The October 8 update
The Climate Prediction Center’s next ENSO Diagnostic Discussion is scheduled for October 8, 2026, and it will either hold the 75% chance of a record-breaking October-December or move it. By then another month of Niño-3.4 readings will be in, and the October-December season to which the probability is tied will have barely begun, which leaves the number open to revision in either direction. Until that release replaces it, the September 10 discussion is the version of record, and its 75% figure remains NOAA’s stated chance of an El Niño stronger than any measured since 1950.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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