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

- [2026-09-10] NOAA CPC's ENSO Diagnostic Discussion kept the ENSO Alert at El Niño Advisory and stated El Niño is strengthening, with a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27, with SST anomalies exceeding +3.0°C in the eastern equatorial Pacific . https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-09-10] The same discussion put the odds of the relevant threshold explicitly: During the October-December 2026 season, there is a 75% chance of a historic event that would exceed the strength of previous El Niño events dating back to 1950 (+2.5C or more for a 3-month RONI value) . https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-09-28] CPC's "ENSO: Recent Evolution" update (most recent observed RONI available) reported RONI value (June-August 2026) is 1.4ºC — i.e., the last officially published seasonal RONI value (JJA, outside the market's listed-season window) stood well below 2.5°C. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-10-03] The CPC's official historical RONI data table (fetched directly) shows observed values only through MAM 2026 (-0.1°C); ASO, SON, OND, NDJ and DJF 2026-27 — the seasons that govern this market — have not yet been published. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-09] CPC's official September RONI Outlook (probabilistic forecast, not yet observed) gives a median path peaking at OND 2026 — peak, +2.67°C, middle 70% +2.32 to +3.02°C; NDJ 2026–27, +2.57°C, middle 70% +2.13 to +3.00°C , both medians above the 2.5°C threshold. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-09] The same forecast gives a very strong event (RONI +2.0°C or more) at 97% in September to November, 98% in October to December and 93% in November to January, then eases to 75% in December to February , confirming the event is forecast to intensify markedly before any decline. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-09-28] CPC's weekly SST update noted that in August, Niño indices other than Niño-4 continued rising, reaching +1.8C in… America and abroad, indicates El Niño will continue to strengthen through the end of the year , consistent with the ongoing warming trend feeding into the coming ASO/SON/OND seasons. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-09-23] An independent tracker citing CPC data reported the conventional weekly Niño‑3.4 anomaly at +3.1°C for the week centered September 23, 2026 , indicating the ocean surface remains very warm heading into the ASO/SON seasons even though RONI (a relative, detrended index) reads lower. https://www.elninolive.com/forecast - [2026-10-08] The next official CPC ENSO Diagnostic Discussion and updated RONI/strength outlook is scheduled, which should be the first update to include guidance closer to the ASO 2026 season and may shift probabilities before the market's next data points. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-09-19] A secondary analysis of CPC's probability graphics confirmed The CPC Official El Niño Southern Oscillation (ENSO) Outlook, which synthesizes multiple models from North America and abroad, indicates El Niño will continue to strengthen through the end of the year , reinforcing the CPC's own September stance. https://econcurrents.substack.com/p/rare-strong-pdo-negative-el-nino - [2026-09-21] Regional monitoring centers (ASEAN Specialised Meteorological Centre) corroborate CPC, stating El Niño conditions are supported by strong El Niño conditions in August 2026, expected to continue throughout the second half of 2026 into early 2027 and predicted to continue strengthening until October – November 2026 . https://asmc.asean.org/asmc-el-nino/ BOTTOM LINE: As of early October 2026, no listed RONI season (ASO, SON, OND, NDJ, DJF) has yet been published by CPC — the last observed value (JJA 2026) was +1.4°C — but CPC's own September 2026 outlook assigns a 75% chance that OND 2026 RONI reaches +2.5°C or above, with a median forecast peak near +2.6–2.7°C in OND/NDJ, so official guidance currently favors a "Yes" resolution once the relevant seasons are published (first ASO around early November, OND around early January). NOT FOUND: No observed (as opposed to forecast) RONI value for ASO 2026 or later seasons was available as of 2026-10-04; the October 8, 2026 CPC discussion (which may contain the first ASO-adjacent update) had not yet been issued at the time of research.

- [2026-09-10] NOAA CPC's ENSO Diagnostic Discussion states: "During the October-December 2026 season, there is a 75% chance of a historic event that would exceed the strength of previous El Niño events dating back to 1950 (+2.5C or more for a 3-month RONI value)." This is CPC's own probabilistic estimate for this specific event, not a historical base rate. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf - [2026-10-04] Reviewing CPC's own published RONI table (1950–2026, ERSSTv5 base period), only one three-month season has ever recorded a RONI of +2.5°C or higher: the 1982‑83 event (OND 1982 = 2.4, NDJ 1982‑83 = 2.5, DJF 1982‑83 = 2.5). All other historically "super" El Niños fell short on this relative index — 1997‑98 peaked at 2.4, 2015‑16 at 2.4, 1972‑73 at 2.3, 1991‑92 at 2.3. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-04] Out of roughly 23 distinct El Niño episodes recorded in CPC's RONI series since 1950, exactly 1 reached the +2.5°C threshold — an unconditional historical base rate of roughly 4-5% per El Niño event, far below the 75% CPC is currently assigning to 2026‑27. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-09-10] A secondary tracker site summarized CPC/related data showing that on the RONI scale, "a 69% chance that October–December 2026 reaches +2.5°C or higher on the RONI... and the claim attached to them is that such an event 'would exceed the strength of previous El Niño events dating back to'" 1950, underscoring that this would be an unprecedented (not just rare) reading if it occurs. https://www.elninolive.com/el-nino-2026 - [2026-10-04] The same tracker's own comparison table shows prior "Super El Niño" peak RONI values were all below 2.5: 1982‑83 peaked at +2.4°C (Nov–Jan), 1997‑98 at +2.3°C (Oct–Dec), and 2015‑16 at +2.3°C (Nov–Jan), reinforcing that even the three strongest post‑1950 events (on ONI) did not clear +2.5°C on RONI. https://www.elninolive.com/el-nino-2026 - [2026-09-10] CPC's most recent official RONI reading, for JJA 2026, was "The most recent RONI value (June-August 2026) is 1.4ºC." This is well below the 2.5 threshold but reflects only the spring/summer average, before the forecast autumn/winter intensification. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-09-10] CPC separately assigns >90% odds to this event reaching "very strong" status (RONI/ONI analog ≥2.0°C) during NH fall-winter 2026‑27, a threshold cleared by only 3 of ~23 post-1950 El Niño episodes (1982‑83, 1997‑98, 2015‑16) on the conventional ONI index, illustrating how unusual official forecasters consider the current event even before invoking the 2.5°C RONI bar. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf - [2026-10-03] An independent round-up of international agencies (WMO, BoM, ECMWF, Met Office, JMA) reported broad agreement that the event is still strengthening, with "all 22 models expect it to stay 'very strong' well into winter" — a multi-agency convergence that is itself unusual relative to past events, where model spread near peak was typically larger. https://www.snowplaza.co.uk/blog/weather-forecast-winter-2026-2027 - [2026-10-03] A news summary independently corroborated CPC's number: "The U.S. National Oceanic and Atmospheric Administration (NOAA) has forecast a 75% chance of a record-strength El Niño during October–December 2026." This confirms the 75% figure is being widely reported as NOAA's current official estimate, not just a single blog's interpretation. https://nationthailand.com/sustaination/40071812 NOT FOUND: A rigorous academic forecast-verification study on CPC's historical track record at translating season-ahead ENSO-strength probabilities (e.g., 75% chance statements) into realized outcomes; no independent calibration score for CPC's extreme-threshold probability statements was located. BOTTOM LINE: The unconditional historical base rate for an El Niño season reaching +2.5°C RONI is extremely low (~1 in 23 post-1950 events, only 1982‑83), making a "Yes" outcome historically rare; however, NOAA CPC's own current, event-specific forecast (75% for OND 2026) and convergent international-agency assessments argue that 2026‑27 is tracking well outside the historical norm, so the outside-view base rate should be heavily discounted in favor of the model-based estimate for this particular event.

YES means: NOAA CPC publishes a RONI value of +2.5°C or higher for any of ASO 2026, SON 2026, OND 2026, NDJ 2026‑27, or DJF 2026‑27, triggering immediate resolution. - [2026-09-21] IRI's CCSR/IRI technical update reported the traditional Niño 3.4 index had already reached "an August monthly anomaly of +2.52°C, placing the current event firmly within what IRI describes as the 'very strong' category." This conventional (non-detrended) monthly reading is already past the 2.5°C bar the market uses for RONI, showing the raw ocean signal is strong enough to support a RONI breach once the warming background-SST adjustment is applied to the autumn seasons. - [2026-09-21] IRI's independent model-based ENSO plume stated bluntly that "this sharp decline in probability during boreal spring is consistent with the tendency for strong El Niño events to weaken rapidly after reaching their peak," implying the peak itself (not the spring decline) is expected imminently in the ASO/SON/OND window the market covers, consistent with CPC's own framing. - [2026-09-21] The same IRI technical discussion went further, stating that "all 22 models included in IRI's September forecast maintained a very strong El Niño through late 2026, with many projecting Niño 3.4 anomalies at or above +3.0°C during the autumn and winter period." A multi-model consensus near +3.0°C on conventional Niño 3.4 provides substantial headroom above the +2.5°C RONI threshold even after the detrending adjustment. - [2026-09] WMO's Global Seasonal Climate Update for SON 2026, published independently of CPC, forecast that for the September–November 2026 (SON 2026) season, multi-model ensemble (MME) forecasts indicate a further development toward stronger El Niño conditions, with the seasonal average Niño 3.4 SST ensemble mean anomaly forecast to reach approximately 3.6°C during SON 2026, with the intensification trajectory peaking in November-December . This WMO consensus figure is markedly higher than CPC's own median RONI path (already noted by colleagues at ~+2.6–2.7°C for OND/NDJ), reinforcing that an independent international multi-model ensemble sees even more room above 2.5°C than CPC's own forecast implies. - [2026-09] WMO added that "the spread among individual forecast systems generally remains narrow, reflecting high forecast confidence," indicating unusually low model disagreement about the magnitude of the ongoing warm event — a factor that historically has not accompanied prior "near-record" El Niño seasons and that argues against a last-minute downward surprise before the ASO/SON readings are published. - [2026-09-21] IRI's forecast indicated El Niño probability itself is essentially locked in through the market's entire resolution window, stating this spring decline comes only after persistence remains high and separately that El Niño probabilities stay at 100% from SON 2026 through FMA 2027 before any decline, meaning there is no live risk of the event collapsing or reversing before the ASO–DJF seasons are observed. - [2026-09-27] A secondary summary of IRI's mid-September data noted that the "preliminary daily sea-surface temperature would exceed the previous reported record of 29.815°C set on 17 November 2015, during one of the strongest El Niño events in the modern observational record," indicating the current event is already testing or exceeding peak daily SST records set by the 2015-16 "Godzilla" El Niño, the second-strongest RONI event on record (2.3-2.4°C peak), suggesting 2026-27 could plausibly exceed it. - [2026-09-28/10-04] Independent live trackers cross-referencing CPC and BOM data showed the conventional weekly Niño 3.4 anomaly running near +3.1°C in late September, a reading on the same order as the IRI and WMO model means, confirming via a second independent methodology (BOM relative index at +1.7°C, a different but corroborating relative metric) that ocean warmth remains exceptionally high heading into the first ASO/SON RONI reports. - [2026-10-08] The next CPC ENSO Diagnostic Discussion, due four days after today, is expected to be the first official update bracketing the ASO 2026 season; given the trajectory in observed conventional indices (Niño 3.4 near +3.0–3.1°C through September) and WMO's +3.6°C SON mean forecast, that update is likely to either hold or raise, not cut, the 75% OND probability figure already in CPC's own September discussion. BOTTOM LINE: Independent, primary-source confirmation from IRI (Sept 21 technical update) and WMO (Sept SON outlook) — both citing conventional Niño 3.4 anomalies already at or above +2.5–3.6°C with unusually narrow model spread and near-100% persistence through the market's entire window — corroborates rather than contradicts CPC's own 75% OND probability, strengthening the case that the 2026-27 event is tracking toward a RONI peak at or above +2.5°C in the ASO–DJF seasons the market covers. NOT FOUND: No official CPC RONI value for ASO 2026 has yet been published (expected after the Oct 8, 2026 discussion or early November); no independent academic verification of how closely conventional Niño 3.4 anomalies translate into the detrended RONI metric for this specific event was located.

YES means: NOAA CPC publishes a RONI value of +2.5°C or above for at least one of the five listed seasons (ASO 2026, SON 2026, OND 2026, NDJ 2026-27, DJF 2026-27); NO means every one of those five seasons comes in below +2.5°C once published. - [2026-09] CPC's own official RONI Outlook percentile table (issued September 2026) gives ASO 2026 a 5/15/25/50/75/85/95th-percentile spread of just 1.8–1.9–1.96–2.07–2.18–2.24–2.34°C — meaning even CPC's own 95th-percentile tail for the *first* listed season falls short of +2.5°C. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/outlook/ - [2026-09] The same official table puts SON 2026 at 2.01/2.16/2.26/2.43/2.6/2.69/2.85°C: the median (2.43°C) is still below 2.5°C, and only the 75th-percentile-and-above tail clears it — so CPC itself assigns less than ~30% odds to SON alone crossing the bar. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/outlook/ - [2026-09] Even at the forecast peak, CPC's percentile table shows OND 2026 at 2.12/2.32/2.44/2.67/2.9/3.02/3.23°C and NDJ 2026-27 at 1.87/2.13/2.28/2.57/2.85/3.0/3.26°C — in both cases the 25th percentile sits below 2.5°C, so CPC's own distribution leaves a non-trivial (roughly quarter-to-third) chance that the single best-positioned season still falls short. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/outlook/ - [2026-09] CPC's table further shows DJF 2026-27 at 1.48/1.77/1.94/2.27/2.59/2.76/3.06°C — a median of only 2.27°C, below threshold — confirming that the back end of the window (the season that forces a "No" resolution if nothing has crossed 2.5 by then) is itself more likely than not to miss. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/outlook/ - [2026-09-10] A ski-industry summary of this same CPC table explicitly labels only OND and NDJ as "yes" on crossing +2.5°C in their median path, with DJF 2026-27 marked "no" (median +2.27°C) — i.e., out of five listed seasons, CPC's own median path only clears the bar in two, and even those medians (2.67°C, 2.57°C) sit inside a 70%-range whose lower bound (2.32°C, 2.13°C) is below 2.5°C. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-04-10] ECMWF's own science blog on 2026 ENSO forecasting recalls that the "worst March ENSO forecast on record" for ECMWF's SEAS5 occurred in 2017, when model consensus favored a substantial warm event but the Niño3.4 region instead cooled into La Niña by winter, and states forecast busts "are not common but are possible," directly undercutting any claim that a ~75%-confidence official forecast guarantees the outcome. https://www.ecmwf.int/en/about/media-centre/science-blog/2026/el-nino-2026 - [2026-09-10] CPC's own 75% figure for OND 2026 (raised from 69% in August) still implicitly carries a 25% chance that even the best-positioned single season misses +2.5°C, and that number applies to only one of five listed seasons — the cumulative "all seasons stay below 2.5°C" scenario is not negligible once ASO's and SON's much lower sub-threshold odds are folded in. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf - [2026-09-30] A detailed recap of the September CPC release notes that the historic-strength (+2.5°C) threshold is distinct from and higher than the "very strong" (+2.0°C) threshold, and stresses that the 75% figure "is a forecast, not certainty," explicitly warning against treating model medians as settled outcomes. https://stormbag.co/blogs/stormbag-flood-protection-blog/super-el-nino-2026-27-what-noaas-off-the-charts-forecast-means-for-the-year-ahead - [2026-03-11] NOAA's drought.gov explainer on the RONI methodology confirms that, structurally, RONI accounts for the long-term ocean temperature trends in a way that the traditional ONI does not, and as a result, in most cases, RONI will dampen the strength of El Niños and amplify La Niñas when compared to the traditional ONI — meaning headline-grabbing raw Niño-3.4 readings (e.g., weekly +3.1°C in late September) systematically overstate what RONI itself is likely to show. - [2026-10-04] CPC's historical RONI table shows the single precedent for crossing +2.5°C (1982-83) only barely cleared it in two seasons (NDJ and DJF at 2.5 exactly), while the 1997-98 and 2015-16 "super" events — despite comparable or higher raw SST anomalies than currently reported — peaked at 2.3-2.4°C on RONI, underscoring how narrow the margin for a "Yes" has historically been even in record-strength events. https://nsco.unl.edu/news/what-does-2026-el-nino-mean-nebraska/ - [2026-10-08] CPC's next ENSO Diagnostic Discussion and updated RONI outlook — the first to incorporate observations closer to the ASO 2026 season — had not yet been issued at time of writing; until then, no listed season's actual (as opposed to forecast) RONI value is on the books, so the market remains unresolved and status-quo inertia (no observed season yet at or above 2.5°C) still technically favors "No" pending new data. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf BOTTOM LINE: Even NOAA CPC's own official percentile forecast table — not a skeptic's assumption — shows the first two listed seasons (ASO, SON) very unlikely to clear +2.5°C, the strongest seasons (OND, NDJ) carrying roughly a quarter-to-a-third chance of falling short individually, and DJF's median itself below threshold; combined with a documented history of "overconfident" ENSO forecasts busting (ECMWF's 2017 case) and the fact that only one El Niño since 1950 has ever reached 2.5°C on RONI, there remains a material, evidence-based path to "No." NOT FOUND: No October 2026 (post-October 8 discussion) observed RONI value for ASO or later seasons was available at time of writing; no independent statistical verification/calibration study of CPC's specific 75%-type extreme-threshold probability statements was located.