Will the peak RONI for the 2026–27 El Niño be 2.5 or above?

- [2026-10-08] CPC's ENSO Diagnostic Discussion keeps the El Niño Advisory in place. It says El Niño continues to strengthen, with a strong-to-very strong event likely through January–March 2027 (greater than 83% chance). September Niño indices were +2.1°C (Niño-3.4), +3.0°C (Niño-3) and +3.9°C (Niño-1+2). https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-08] CPC's chance of a "historic" event (3-month RONI ≥ +2.5°C), as quoted by secondary sources: 54% for SON, 83% for OND and 70% for NDJ. The Climate.us blog (a NOAA-linked blog) gives the same figures. DJF is not quoted in these sources. https://www.climate.us/news-features/blogs/enso/highlights-noaas-october-2026-el-nino-forecast - [2026-09-10] Previous CPC outlook: 75% chance for OND of a 3-month RONI of +2.5°C or more. The OND figure therefore rose from 75% to 83% in the October update. An elninolive.com page cites 69% for the August outlook. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf - [2026-10-12] The CPC RONI table (fetched today) shows 2026 values from DJF to JAS of -0.9, -0.8, -0.4, 0.0, 0.5, 1.0, 1.4, 1.7. JAS 2026 = +1.7 is the latest value. ASO, SON, OND, NDJ and DJF are not yet published. The page says it is updated by the 5th of each month and that recent values may change for up to two months. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-07] The relative Niño-3.4 anomaly rose from 1.7°C in August to 2.1°C in September. ASO must average above 2.0 before the event can officially be called "very strong". https://www.climate.us/news-features/blogs/enso/highlights-noaas-october-2026-el-nino-forecast - [2026-10-10] Weekly conventional Niño-3.4 reached +3.2°C for the week of 30 Sep, the high of this event, according to elninolive.com (a tracker, not primary). The same site gives the BOM relative Niño-3.4 as +2.8°C for the week ending 4 Oct. The RONI-equivalent weekly values are lower than the conventional ones. https://www.elninolive.com/ - [2026-10-08] WMO's seasonal update forecasts OND-average conventional Niño-3.4 of about +3.7°C, peaking around December. This is a conventional (not relative) index, so it overstates RONI. https://wmo.int/news/media-centre/sea-surface-temperatures-reflect-strengthening-el-nino - [2026-10-05] CPC's "ENSO: Recent Evolution" slide deck (dated 5 Oct) repeats the 10 Sep text: >90% chance of a very strong event. The October 8 text instead says the strong-to-very-strong event is likely through JFM 2027 (>83%). I did not verify the PDF's RONI chart. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-10-11] A tracker site (4billionyearson.org) says the official NOAA RONI outlook projects a peak of about +2.7°C RONI. It also says the SNU CNN model forecasts +2.3°C RONI. This is secondary and unverified against CPC. https://4billionyearson.org/climate/enso - [2026-02] CPC replaced ONI with RONI in February 2026. RONI usually dampens El Niño strength relative to ONI. https://www.drought.gov/news/new-noaa-el-nino-southern-oscillation-index-supports-drought-early-warning-2026-03-11 - [2026-10-08] Scheduled: next CPC ENSO discussion is 2026-11-12. It is scheduled for 12 November 2026. The monthly RONI page update is due by about 5 Nov (ASO), 5 Dec (SON), 5 Jan (OND), 5 Feb (NDJ) and 5 Mar 2027 (DJF). These dates are inferred from the stated "by the 5th" schedule. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-12] No seasonal RONI has reached +2.5 yet. The record RONI for previous events from the CPC table is about 2.4 (NDJ 1982-83 and 2015-16 at about 2.3). The OND and NDJ values for 1982 and 2015 therefore sit just under the threshold. The 2.5 line is a new all-time high. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ BOTTOM LINE: JAS RONI is +1.7, and no season has reached +2.5. CPC's own Oct 8 odds for a 3-month RONI of ≥2.5 are 54% (SON), 83% (OND) and 70% (NDJ), so the peak most likely falls in OND. Because the market resolves Yes on the first published season at or above +2.5, the key dates are the SON release (about 5 Dec) and the OND release (about 5 Jan). The first published value is final, and early real-time values can be revised. NOT FOUND: CPC's DJF 2026-27 probability. The exact numeric RONI forecast chart values. Any published ASO 2026 RONI value, which is due about 5 Nov. A verified primary source for the +2.7°C RONI peak figure.

- [2026-10-08] CPC's official ENSO discussion gives these chances of a 3-month RONI of +2.5°C or more: 54% for September-November, 83% for October-December, and 70% for November-January . These are separate seasons, so the chance that any season reaches 2.5 is at least 83%. CPC does not publish a combined figure. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-08] Current state: Niño indices reached +2.1°C in Niño-3.4, +3.0°C in Niño-3 and +3.9°C in Niño-1+2 . SST anomalies exceeded +4.0°C in portions of the eastern equatorial Pacific . The subsurface was in excess of +10.0°C at depth . The next discussion is 12 Nov 2026. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-12] Outside view, from the CPC RONI table. The record is 2.4 (NDJ 1982-83 and DJF 1982-83). Other peaks include 2.3 (NDJ 2015) and 2.0 (OND 1965 and OND 1972). No season since 1950 has reached 2.5. The 2026 row so far reads: MJJ 1.0, JJA 1.4, JAS 1.7. ASO is not yet posted. The table shows the 2026 values up to the 8th entry, JAS. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-12] The base rate for a YES is therefore 0 of about 75 years. This event is far outside the historical sample, so the reference class is weak. CPC's own forecast (above) is the better anchor. The 2026 trajectory (JAS 1.7) is already above the 1982 and 2015 values for the same season. In 1982, ASO was 1.6 and SON 2.0. In 2015, ASO was 1.8 and SON 2.0. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-12] CPC warns that RONI values may change up to two months after the "real time" value , and the page says the latest values are estimates. The contract counts the first published value for each season. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-09-10] CPC's earlier OND ≥2.5 probability was 75%, up from 69% in August . It reached 83% on 8 Oct, so the forecast has risen each month. September said a greater than 90% chance of a very strong event . https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf - [2026-09-21] IRI/CCSR: all 22 models (13 dynamical, 9 statistical) forecast a "very strong" El Niño (Niño 3.4 ≥ +2.0 °C) from SON 2026 through DJF 2026–27 . The weekly Niño 3.4 was +3.0°C around 16 Sept. The August monthly Niño 3.4 value was +2.52°C. This is the traditional index, not RONI. https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/ - [2026-10-10] WMO forecasts a December peak. The seasonal Niño 3.4 anomaly for OND is forecast at about 3.7°C, against a previous record of 2.6 degrees in November 2015 to January 2016 . The WMO figure is on the traditional index, not RONI. https://indexbox.io/blog/record-breaking-el-nio-forecast-for-late-2026 - [2026-07] Model-based views are far above 2.5. Yale Climate Connections, citing Hausfather, reports that the pooled model consensus is a peak around 3.6 degrees Celsius . A 9 Jul social post cites 3.6°C on RONI. These are secondary sources, and models can overshoot. https://yaleclimateconnections.org/2026/07/this-could-be-the-strongest-el-nino-on-record/ - [2026-10-12] Caveat on the gap between observations and forecasts: the traditional Niño-3.4 monthly value for September was only +2.1°C. RONI subtracts the tropical-mean anomaly, which in recent years is positive. The RONI for 2026 is therefore lower than the traditional index. CPC's 54% for SON and 83% for OND implies that CPC expects a rapid rise in Oct-Nov. The recent monthly Niño-3.4 figures (+1.8 in August, +2.1 in September) are well below the weekly +3.0 reading. I did not find the current weekly RONI. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml BOTTOM LINE: No El Niño since 1950 has reached RONI 2.5, so the historical base rate is zero. CPC's current forecast gives 83% for OND and 70% for NDJ, and any single season at or above 2.5 resolves YES, so the outside view rests mainly on CPC and the model ensembles. The remaining risk is that the monthly Niño-3.4 values (+1.8 in August, +2.1 in September) are well short of what the forecast needs. NOT FOUND: the ASO 2026 RONI value, the current weekly RONI, CPC's combined "any season ≥2.5" probability, the track record of CPC's strength-probability forecasts, and other venues' prices on RONI thresholds.

YES means: NOAA CPC publishes a first-release RONI of +2.5°C or higher for at least one of ASO, SON, OND, NDJ or DJF 2026-27. The first such value resolves the market. - [2026-10-08] CPC's official RONI outlook table, which I fetched, puts the median at +2.74 for OND and +2.65 for NDJ. SON is +2.47 and DJF is +2.36. This gives the numbers behind the 83% and 70% headlines. - OND percentiles are 5% = 2.25, 25% = 2.54, 75% = 2.94 and 95% = 3.23. - The 25th percentile of OND is already above 2.5, so most of the forecast distribution clears the threshold. - NDJ is also a median above 2.5, with a 25th percentile of 2.39. - SON's median sits right at 2.47. That is a coin-flip path to an early YES in early December. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/outlook/ - [2026-10-08] The YES case does not need one particular season to work. Any of SON, OND or NDJ at or above +2.5 resolves YES, and the three medians are 2.47, 2.74 and 2.65. Their errors are strongly correlated, so they are not independent chances. Still, the median path overshoots the threshold by about 0.15–0.25 for two consecutive seasons. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/outlook/ - [2026-10-04] BoM's relative Niño-3.4 is a close analogue of RONI. It was +2.72°C for the week ending 27 Sep and +2.83°C for the week ending 4 Oct. BoM says most models peak above +3.0°C and that values are forecast to exceed the warmest on record since 1950. Weekly relative values of about 2.8 already sit above 2.5. Consecutive months averaging above 2.5 would make the 3-month RONI follow. https://www.bom.gov.au/climate/enso/ ; https://www.dpird.nsw.gov.au/climate_applications/state-seasonal-update/climate-outlook-and-drivers/global-climate-drivers/el-ninosouthern-oscillation-enso - [2026-10-07] The arithmetic for SON ≥ 2.5. The monthly relative Niño-3.4 values were 1.7 in August and 2.1 in September. SON ≥ 2.5 would need October and November to average roughly 2.7–2.8, since SON is the mean of Sep, Oct and Nov. Weekly BoM readings are already about 2.7–2.8. OND ≥ 2.5 needs only a continued climb: Oct, Nov and Dec averaging 2.5. Observed weekly relative values and the +3.0 to +3.7 forecasts for the Dec peak (a December peak is consistent with WMO) give room for this. https://www.climate.us/news-features/blogs/enso/highlights-noaas-october-2026-el-nino-forecast - [2026-10-08] WMO's multi-model ensemble gives OND Niño-3.4 of about +3.7 on the 1993–2009 baseline, with narrow spread among systems, peaking around December. RONI subtracts the tropical mean and applies a variance adjustment. One tracker estimates the ONI–RONI gap at about 0.4–0.5 (secondary source). On those numbers, a +3.7 raw forecast converts to about 3.2 RONI, well above 2.5. This conversion is rough, and the baseline differs. https://wmo.int/resources/publication-series/global-seasonal-climate-updates/gscu-ond2026 ; https://4billionyearson.org/climate/enso - [2026-10-14, reported as of 2026-10-09] The Washington Post tracker's October update projects the central Pacific peaking at about +4.2°C in December on its own index. That is higher than any event since 1850. The conventional weekly Niño-3.4 was +3.2 for the week of 30 Sep, the event high, and Niño-1+2 was +5.3. These are conventional indices, not RONI. https://www.washingtonpost.com/climate-environment/interactive/2026/track-how-extreme-this-years-el-nino-could-get/ ; https://www.elninolive.com/ - [2026-08-17, AccuWeather analysis] An AccuWeather analysis of NOAA RONI data and the McNoldy dataset put the 2026 forecast mean at 2.66 RONI. That would be the strongest three-month mean on record. The prior record is 2.40. I did not fetch the page, so treat this as a secondary report. https://www.accuweather.com/en/climate/the-2026-el-ni%C3%B1o-could-become-the-strongest-in-a-lifetime/1924444 - [2026-10-08] The forecast has moved up each month, which is the main momentum argument for YES. The OND ≥ 2.5 probability went 69% (Aug) → 75% (Sep) → 83% (Oct). The very-strong probability for SON is now 100%, per the Climate.us post, up from 97% in September. Observations have kept running at or above the previous month's forecast. https://www.climate.us/news-features/blogs/enso/highlights-noaas-october-2026-el-nino-forecast ; https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-10-08] Oceanic and atmospheric coupling supports further intensification. Climate.us notes the rainfall and cloud pattern shifted east, which it describes as the feedback loop essential for continued strengthening. The Sep-Oct IOD is positive, at +0.60 for the week to 4 Oct, a combination described as reinforcing El Niño. Together with the deep subsurface warmth noted in the earlier reports, there are no signs of a stall. https://www.climate.us/news-features/blogs/enso/highlights-noaas-october-2026-el-nino-forecast ; https://4billionyearson.org/climate/enso - [2026-10-08] Timeline to a YES call. SON data arrive about 5 Dec; if SON is at or above 2.5, the market resolves YES then. Otherwise OND arrives about 5 Jan, and NDJ about 5 Feb. Since the first published value is final, a high real-time OND print would settle it despite later revisions. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ BOTTOM LINE: The most likely path to YES is OND 2026 (median 2.74 per CPC's October outlook table, 25th percentile 2.54) published about 5 Jan 2027. It needs October and December to keep climbing from the Sep relative anomaly of 2.1 and the weekly BoM readings of 2.7–2.8. SON at about 2.47 is a coin-flip early path, and NDJ (median 2.65) is the backstop. NOT FOUND: the ASO 2026 RONI value, the DJF probability, the actual monthly RONI for October, the verified accuracy of CPC's RONI outlook at extreme values, and any data suggesting a stall.

YES means: NOAA CPC publishes a RONI of +2.5°C or higher for any of ASO 2026, SON 2026, OND 2026, NDJ 2026-27 or DJF 2026-27 (first published value). Otherwise it resolves NO once the DJF value is published. - [2026-10-08] The CPC discussion fetched today gives the 54% (SON), 83% (OND) and 70% (NDJ) odds only for the "historic" tier. It gives no DJF figure and no combined "any season" figure. The synopsis headline is the softer "strong-to-very-strong… likely through January-March 2027." The record-tier claim sits in a single sentence ("Chances remain high"). The OND figure is also an official-outlook judgment, not a model output. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-08] Observed relative Niño-3.4 was +2.1°C in September and +1.8°C in August (the August figure comes from the Sept 10 discussion, as quoted by abovethenormnews). A 3-month season needs an average of 2.5. SON ≥2.5 therefore needs Oct and Nov to average about 2.7 or more. OND needs the Oct–Dec average to be about 0.4 above the latest monthly reading. The path from 2.1 to a 2.5+ three-month average has to start rising immediately, and no season has ever done this. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-07] The tracker sites report the CPC official RONI outlook median as roughly +2.67°C for OND (Sept issuance). That sits only about 0.17°C above the threshold, and the 4billionyearson site puts the peak at +2.7. Forecast error for a 1–3 month lead is easily 0.3–0.5°C, so a median this close to the line leaves a real chance of a 2.3–2.4 peak. This is secondary reporting, not verified on the CPC chart. https://pogodnik.com/en/news/winter-2026-2027-forecast-europe-usa-canada-super-el-nino - [2026-10-11] The same tracker says the SNU deep-learning model projects a peak of only +2.3°C RONI (+2.8°C raw). That is an independent model below the line. The statistical and dynamical plumes quoted in the headlines are on traditional Niño-3.4 (IRI Sept 21: many models ≥3.0, WMO OND about 3.7). RONI is about 0.5°C lower than raw in this event, so the headline model numbers overstate what counts here. https://4billionyearson.org/climate/enso - [2026-10-05] The CPC 5 Oct slide deck lists "latest weekly" SST departures of Niño-3.4 +2.3°C and Niño-3 +3.3°C. That is well below the +3.0 to +3.2°C weekly traditional figures in tracker and IRI reports. Weekly CPC values appear to be running below the headline numbers. The deck also still carries the Sept 10 "greater than 90%" text, so it may be stale. I could not confirm the date of the weekly row. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-10-05] The same deck notes that on 8/10/26 NCEI discontinued ERSSTv5 and RONI switched to ERSSTv6. A dataset change mid-event adds revision and calibration risk. The tropical-mean subtraction in RONI also rises if the tropics warm broadly (record global SST per BoM, July +0.57°C). That mechanically caps RONI relative to the raw index. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-09-27] BoM relative Niño-3.4 (week ending 27 Sep) was +2.72°C on its GAMSSA dataset, not the CPC ERSST dataset. BoM says the "majority" of models peak above +3.0 on its relative index. Weekly values run above monthly ERSST values (CPC's September monthly was 2.1). Three-month ERSST RONI averages smooth out weekly spikes, and the weekly high has to persist for 3 months. https://www.bom.gov.au/climate/enso/ - [2026-10-12] Deadline arithmetic favours the status quo only weakly. NO needs every one of ASO, SON, OND, NDJ and DJF to print ≤2.4 (one decimal). The first YES chance is SON (about Dec 5, odds 54%). OND (about Jan 5) is the main one. NDJ (odds 70%) gives a further chance, and CPC publishes DJF about Mar 5, before the Apr 1 close. For NO, the peak needs to stall or be damped by early winter. In 1982 and 2015 the peak came around NDJ/DJF at 2.3–2.4, and the RONI series never exceeded 2.4. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-12] RONI is published to one decimal, so a true 2.46 shows as 2.5. That cuts against NO. Values at or just under the line (2.3–2.4) are the most plausible NO outcome, and such a miss would be by a margin inside CPC's own forecast error. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-10] The tracker elninolive notes ASO (about Nov 5) is the first print. ASO has a very low chance of ≥2.5, since JAS is 1.7 and September monthly is 2.1. So no early YES is available before SON, and the market stays open at least until about Dec 5. A weak ASO print (about 1.9–2.0) would be the first test of CPC's 54% SON figure. https://www.elninolive.com/el-nino-2026 - [2026-09-05] Other venues listed the same underlying question as separate Kalshi peak-RONI markets and this Polymarket market. Their prices are not given in the snippet I could read, and a Polymarket page snippet 132 days old showed a separate 2.25–2.5 bucket at 20%. That bucket's existence shows that the "just below 2.5" outcome has been a live competing outcome. https://github.com/alexander-bain/bainluck/issues/3128 BOTTOM LINE: The strongest NO case is that 2.5 has never been reached on RONI (record 2.4). It would need Oct–Dec monthly values far above the September 2.1 and the weekly CPC readings in its deck. CPC's official median (about 2.67) is only slightly above the line, an independent SNU model sits at 2.3, and a damped-RONI outcome of 2.3–2.4 is plausible. That is a real miss scenario, but it needs CPC's 83% OND call to be wrong. NOT FOUND: the CPC RONI outlook chart's numeric values (median and spread) from a CPC-hosted page; the DJF 2026-27 probability; any published ASO value; the current weekly RONI from CPC; a CPC verification record for its "historic" tier probabilities; current Kalshi and Polymarket prices.