NOAA’s Climate Prediction Center puts the odds at 83 percent that the October-December 2026 season will register a historic El Niño, one with a three-month Relative Oceanic Niño Index of +2.5 degrees Celsius or higher. Such a reading would exceed every El Niño since 1950.
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The figure first appeared in the center’s Oct. 8 diagnostic discussion and was repeated in its Oct. 26 weekly update, which kept the El Niño Advisory in place.
The 83 percent figure and what +2.5 degrees means
The center’s ENSO diagnostic discussion, issued by NOAA’s National Weather Service, sorts the odds of a historic event by season: 54 percent for September-November, 83 percent for October-December and 70 percent for November-January. The probability peaks in the middle window, which is the three-month stretch the center expects to carry the strongest index value. The yardstick is the Relative Oceanic Niño Index, which measures Niño 3.4 sea surface temperatures against the tropical average, a method NOAA adopted for monitoring on Feb. 1, 2026.
The same discussion says El Niño continues to strengthen, with a strong-to-very-strong event likely through January-March 2027 and a chance above 83 percent. The CFS.v2 model ensemble mean favors El Niño continuing through the northern winter of 2026-27, according to the weekly update.Readings from the equatorial Pacific in October
The center’s Oct. 26 evolution update lists the latest weekly Niño region departures from average: +4.4 degrees in Niño 1+2, +3.6 in Niño 3, +2.5 in Niño 3.4 and 0.0 in Niño 4. The Oct. 8 discussion had them at +3.9, +3.0 and +2.1 for the first three regions, so every eastern and central measure rose in under three weeks. Sea surface temperature anomalies exceeded +4.0 degrees in parts of the eastern equatorial Pacific by early October, and subsurface temperatures topped +10.0 degrees at depth.
The Relative Oceanic Niño Index, which feeds the +2.5 degree threshold, stood at +1.7 for July-September, the most recent value, after running at -0.9 for December-February. That climb from clearly negative to nearly +2 took about seven months and is the arc behind the forecast. Upper-ocean heat content is above average, and the thermocline slope index is below average, both consistent with a mature event. Subsurface anomalies are positive east of the Date Line and negative to the west, and the western negative anomalies are strengthening.
Atmospheric patterns have lined up as well. The Oct. 26 update recorded below-average heights and temperatures over the eastern North Pacific and the U.S. West Coast in late August through early September and again in mid-October, and above-average heights and temperatures over the central United States throughout the period.
Odds that climbed from August to October
NOAA’s confidence has been rising for months. The Inertia reported in August that the center’s monthly discussion put the chance of a historic event at 69 percent, and the October figure for the peak season now sits at 83 percent. The center’s forecast forum page, dated September and last modified in October, carries the same ENSO language and the same three seasonal probabilities.
The outlook still has an end date. El Niño is expected to persist through March-May 2027, with ENSO-neutral conditions favored in April-June and May-July. The center says the next discussion arrives Nov. 12, and the October odds could change by then if the weekly readings flatten.
Rain already tied to the pattern
The wet weeks across the Southwest and Florida offer a first look at what a strong event can do. Jonathan Erdman, a senior meteorologist at weather.com, wrote in a Nov. 3 recap that the soon-to-be record-strong El Niño favors a wetter South, from California and the Desert Southwest to Florida and the Southeast, and that the pattern could ease drought while also bringing more flooding as the southern jet stream strengthens.
Daniel Swain, a climate scientist at the University of California Agriculture and Natural Resources, linked the October Four Corners storms to moisture from record-warm West Coast waters, a strong El Niño and a marine heat wave, as relayed by NPR coverage on OPB. The opening storm alone left as much as half a foot on the San Juan Mountains, and fifteen river gauges in Arizona, southern Colorado and western New Mexico went on to set all-time flood crest records by Nov. 1, per Erdman. The Associated Press, in its Nov. 2 flooding report, connected the same storms to El Niño and warming Pacific waters.
NOAA is careful on the limits. Its discussion says an event of this size makes El Niño-consistent impacts more likely, not guaranteed, and the center refers readers to its seasonal outlooks for specifics. Whether the 83 percent odds verify will not be settled until the October-December index is computed in early 2027, after the winter that readers are now trying to plan around has already begun.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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