Will the peak RONI value during the NDJ 2026-27 season be at least 2.0°C?

- [2026-09-10] The CPC ENSO Diagnostic Discussion states: "El Niño is strengthening, with a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27." This covers the NDJ 2026-27 season. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf) - [2026-09-10] 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 (+2.5°C or more for a 3-month RONI value). (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml) - [2026-09-28] The official CPC "ENSO: Recent Evolution, Current Status and Predictions" update reports the most recent **final** published RONI value, for June-August (JJA) 2026, is **+1.4°C** (ERSSTv6 basis), below the +2.0°C threshold. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf) - [2026-09-28] The CPC Official ENSO (RONI) Outlook chart shows a forecast mean path peaking at roughly **+2.6 to +2.7°C in OND 2026**, with the NDJ 2026-27 mean sitting close to that peak, comfortably above +2.0°C, and the 90% range for NDJ bounded well above +2.0°C on the low end. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf) - [2026-09-28] The CPC Probabilistic ENSO Strength Outlook (issued 10 Sept 2026) puts the chance of a "Very Strong" El Niño (RONI ≥ +2.0°C) at roughly 93-98% for the SON/OND/NDJ seasons, easing only in DJF/JFM 2027. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf) - [2026-09-28] Current weekly SST departures (as of late Sept 2026) are strongly elevated: Niño 3.4 at +2.2°C, Niño 3 at +3.1°C, Niño 1+2 at +3.9°C — all far above long-term averages, consistent with a rapidly strengthening, east-based El Niño. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf) - [2026-09-28] Sub-surface (0-300m) equatorial heat content anomalies remain strongly positive (+3 to +3.5°C in the eastern-central Pacific), a precursor signal historically associated with continued/strengthening warm events. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf) - [2026-09-10] "El Niño is expected to persist through March-May 2027, with a possible chance (55%) of ENSO-neutral in April-June 2027." (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf) - [2026-09-10] A secondary analysis of the official September RONI outlook table reports a median/mean forecast path of **+2.57°C for NDJ 2026-27** (middle-70% range +2.13 to +3.00°C), above both the +2.0°C "very strong" and +2.5°C "historic" thresholds. (https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27) - [2026-09-30] International model consensus (ASMC/Copernicus C3S) corroborates NOAA, projecting El Niño conditions to continue throughout the second half of 2026 into early 2027, with models predicting conditions to begin weakening in December 2026–January 2027, though to still be above the threshold for strong conditions (1.5°C) in early 2027. (https://asmc.asean.org/asmc-el-nino/) - [2026-10-08] The next official ENSO Diagnostic Discussion/RONI update is scheduled for this date, which will provide the next monthly reassessment ahead of the NDJ season itself. "The next ENSO Diagnostics Discussion is scheduled for 8 October 2026." (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf) NOT FOUND: The actual live, current-month (September/October 2026) numerical RONI Strength Probabilities table and RONI Outlook table could not be directly fetched from the official CPC pages (they returned a stale June 2026 cached version or failed to load); figures for NDJ were instead corroborated via the official 28-September CPC PDF briefing and a secondary analysis site citing the September CPC table. No preliminary or final RONI value for any season beyond JJA 2026 has yet been published. BOTTOM LINE: As of the most recent official CPC assessment (28 September 2026), a very strong, east-based El Niño is actively strengthening, with CPC assigning >90% probability that RONI will reach or exceed +2.0°C during the NDJ 2026-27 season, and forecast/mean paths placing NDJ RONI around +2.3 to +2.6°C — strongly supportive of the "Yes" outcome, though no NDJ value has been observed or published yet (the season itself has not occurred).

- [2026-09-10] CPC's official ENSO Diagnostic Discussion states NOAA CPC ENSO Forecast September 10, 2026 Alert Status: El Niño Advisory, and El Niño is strengthening, with a greater than 90% chance of a very strong event during the Northern Hemisphere fall and winter 2026-27 , directly implying a RONI ≥ 2.0°C ("very strong" threshold) for the NDJ season. Source: cpc.ncep.noaa.gov ENSO Diagnostic Discussion. - [2026-09-10] CPC's probabilistic strength chart for the specific NDJ 2026-27 season shows the "very strong" category (RONI ≥ 2.0°C, per the chart's own legend) carrying effectively the entire probability mass, matching the ">90% very strong" headline statement (NOAA CPC ENSO Diagnostic Discussion PDF, p.24). - [2026-09-10] The official RONI point-forecast outlook (the chart CPC itself uses to verify these contracts) puts the mean/median NDJ 2026-27 value at roughly +2.9°C, with the 90% forecast band running from roughly the mid-2s to upper 3s — comfortably above 2.0°C (NOAA CPC ENSO Diagnostic Discussion PDF, p.25). - [2026-09-28] The latest CFS.v2 dynamical model ensemble mean shows Niño 3.4/relative anomalies peaking near 2.2–2.5°C around OND/NDJ 2026-27 and favors El Niño to continue through Northern Hemisphere winter 2026-27 , broadly consistent with the CPC consensus (NOAA CPC ENSO evolution PDF, p.26). - [2026-09-30] Independent trackers summarizing CPC's monthly outlook report Forecasters give El Niño a 97–99% chance of persisting and strengthening through the 2026–27 Northern Hemisphere winter, with a roughly 60% chance it reaches "strong" territory on the conventional Niño-3.4 index — though RONI methodology nets out background warming and CPC's own RONI-based strength chart shows higher confidence (>90%) in "very strong" for NDJ specifically. - [2026-09-15] A third-party aggregator citing CPC's RONI outlook table reports the probability of a very strong El Niño during October–December rose from 81% in CPC's July assessment to 95% in August, with NOAA's August RONI forecast placing the median October–December value at +2.66°C , showing the forecast has been trending upward (more confident, higher median) over successive monthly issuances since June/July 2026. - [2026-10-01] Independent seasonal-model aggregation (Climate Brink/Climate Dashboard, 14 models) put the event's peak at around 4.1C in the Niño 3.4 region (with the middle 80% of ensemble members between 3.4C and 4.6C) on the conventional (non-relative) index — reinforcing that essentially all major forecast sources agree this will be an extremely strong event, far surpassing the 2.0°C RONI bar. - [Historical base rate] NOAA's own RONI table (ERSST.v6, 1950–2026) shows that reaching ≥2.0°C peak RONI in an NDJ/OND-type season is rare but not unprecedented: only the 1982-83 and 2015-16 El Niños are flagged by current trackers as approaching or exceeding that level historically Strongest events on record ranked by peak RONI (1982-83 holds the +2.5°C record) , and the CPC table itself shows OND 2015 at +2.0°C and NDJ 2015 at +2.3°C, confirming it has happened at least twice in ~75 years of record — i.e., historically a low-frequency (roughly 1-in-25- to 1-in-35-season) outcome absent a forecast signal. - [Forecast skill caveat] CPC explicitly cautions that the CPC and the contributors to the Forecast Forum caution potential users of this predictive information that they can expect only modest skill , a standing disclaimer on ENSO statistical/dynamical model guidance, though skill at 1-2 month lead for an already-mature, strongly-forced event like the current one is historically much higher than for onset/neutral forecasts. BOTTOM LINE: Viewed purely as a base-rate matter, reaching RONI ≥ +2.0°C in an NDJ season is historically rare (roughly 2 of ~75 years in the modern record), but the current situation is not a base-rate draw from history — it is a live, already-realized strong El Niño that CPC's own official outlook, the dynamical model ensembles, and independent forecasters all assign >90% probability of qualifying as "very strong" (RONI ≥2.0°C) specifically for NDJ 2026-27, with central estimates (~2.6–2.9°C median) sitting far above the threshold with wide margin. NOT FOUND: A documented, systematic verification/accuracy study of CPC's own RONI-based strength probability forecasts (the metric is new, introduced in 2026, so no multi-year hit-rate track record exists yet); explicit historical hit-rate of CPC's September-issued "90%+ very strong" calls verifying against actual peak values in past (pre-RONI) El Niño cycles.

YES means: the NOAA CPC official RONI table publishes a final (non-preliminary) value of at least +2.0°C for at least one of the three-month periods overlapping NDJ 2026-27 (i.e., the single most-likely qualifying period being Oct-Dec or Nov-Jan 2026-27). - [2026-09-21] Independent modeling aggregation (Carbon Brief, citing NOAA CPC, Copernicus C3S, ECCC and JAMSTEC) shows 14 seasonal forecast models projecting the models project a peak monthly anomaly later this year of around 4.1C, with 80% of the 674 individual model runs falling between 3.4C and 4.6C on the conventional Niño 3.4 index — and critically, every single model run peaks above the 2015-16 record , meaning no model run in this large ensemble falls short of even the 2015-16 level (which itself produced an RONI near +2.0-2.3°C per colleagues' prior research). - [2026-09-21] Carbon Brief's own RONI-specific tracking states RONI stood at around 2.5C in mid-September, some 0.7C below the 1982 record , confirming the RONI metric itself — not just the conventional index — is already running well above the 2.0°C bar before the NDJ season has even begun. - [2026-09-21] Historical analogue evidence from the same source notes that for the three strongest past events, the 1982-83, 1997-98 and 2015-16 events added between 0.5C and 1.9C to their RONI values between mid-September and their peaks — implying this year's ~2.5°C mid-September RONI reading is highly likely to climb further into the NDJ window rather than plateau or retreat. - [2026-09-29] The Australian Bureau of Meteorology's independent weekly relative Niño3.4 index (a comparable, though not identical, relative-index methodology to RONI) registered the latest relative Niño3.4 index value for the week ending 27 September 2026 is +2.72 °C, well above the El Niño threshold (+0.80 °C) , corroborating the US-based relative-index readings from an independent national agency. - [2026-09-29] BOM's own outlook states El Niño is at near-record levels. Both oceanic and atmospheric indices reflect values consistent with a strong El Niño, with further intensification forecast during the rest of spring , and specifically that Relative-Niño3.4 values are forecast to reach levels above the warmest observed since reliable records began in 1950, with the majority of models indicating a peak value above +3.0 °C — i.e. a majority-model peak far above the 2.0°C bar, from a second independent agency using a RONI-like methodology. - [2026-09-21] Columbia's IRI, in its September Quick Look, reported the traditional Niño 3.4 index shows a pronounced and sustained rise, with the seasonal mean reaching +2.03°C during June–August 2026 and the August monthly value increasing further to +2.52°C , and that the latest weekly Niño 3.4 index, centered on September 16, 2026, climbed to +3.0°C, providing further evidence that El Niño is already very strong and continuing to strengthen . - [2026-09-21] IRI explicitly flags that its own forecast categories are inadequate for this event: our current intensity classification is capped at +2.0°C, yet a majority of models in the plume—including both dynamical and statistical models—project Niño3.4 values that could reach or exceed +3.0°C, well beyond our highest defined category. This is more than a high-probability El Niño: the ensemble shows an unusually strong and consistent signal for an event of exceptional intensity . - [2026-10-01] A tracker site summarizing the 21 September IRI plume update reports all 22 models expect it to stay 'very strong' well into winter , with unanimous model agreement rather than a probabilistic split, strengthening the case that no plausible model trajectory undercuts the +2.0°C RONI bar for NDJ. - [2026-09-19] Carbon Brief further corroborates that the current event already broke the all-time daily Niño 3.4 record, noting it As of 21 September, the daily anomaly in the Niño 3.4 region stands at 3.11C, surpassing the previous record of 3.08C set in 2015 — a season that still produced a final RONI comfortably above 2.0°C, reinforcing that this year's trajectory, running well ahead of 2015's pace at the same calendar point, has essentially no realistic path to falling back under 2.0°C by NDJ. - [2026-09-29] BOM also flags a compounding atmospheric driver: the Indian Ocean Dipole (IOD) index value for the week ending 27 September 2026 is +0.76 °C. This is the third consecutive week that the index has exceeded the positive IOD threshold , and a positive IOD tends to reinforce El Niño-related Pacific warming, providing an additional independent physical mechanism supporting continued strengthening into NDJ. BOTTOM LINE: Beyond the >90% official CPC probability already noted, multiple independent sources — Carbon Brief's model ensemble (peak ~4.1°C conventional, all runs above 2015-16), BOM's relative Niño3.4 index (+2.72°C and rising), and IRI's own strained classification (models exceeding its +2.0°C category) — converge on the same conclusion: current observed levels and nearly every model trajectory already sit at or above the 2.0°C RONI threshold well before the NDJ season, with historical analogues suggesting further intensification into winter, making the YES path look near-deterministic rather than merely probable. NOT FOUND: No preliminary or final CPC RONI value has yet been published for SON, OND, or NDJ 2026-27 (data lag means the season's own figures are not yet out); the October 8, 2026 CPC Diagnostic Discussion update (post-dating this report) was not yet available to confirm continued trajectory.

YES means: the peak RONI value during NDJ 2026-27 reaches at least +2.0°C; this dimension argues for NO, i.e., that the peak NDJ RONI stays below +2.0°C. - [2026-09-30] Even in NOAA's own most-favorable-to-YES tabulation, CPC's own strength-probability table assigns only a 69% chance of exceeding the historic +2.5°C threshold for OND, and the NDJ "very strong" (≥+2.0°C) probability is explicitly below 100%, i.e., CPC itself leaves a residual chance NDJ RONI stays below the 2.0°C bar even as it calls the event historic. (https://stormbag.co/blogs/stormbag-flood-protection-blog/super-el-nino-2026-27-what-noaas-off-the-charts-forecast-means-for-the-year-ahead) - [2026-09-ish] A detailed breakdown of CPC's September strength table states the NDJ-specific "very strong" probability is 93% in November to January -equivalent terms (quoting the source's own NDJ figure), which leaves roughly a 1-in-14 residual chance the season closes below +2.0°C even on CPC's own numbers — not the "effectively certain" framing some secondary sites use. (https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27) - [2026-09-ish] The same CPC-derived table's "middle 70%" forecast band for NDJ 2026-27 runs from +2.13°C to +3.00°C — meaning the 15th-percentile model guidance sits only about 0.13°C above the 2.0°C strike, a thin margin that a modest downward revision or cooling trend could erase. NDJ 2026–27 +2.57°C median, middle 70% of forecasts +2.13 to +3.00°C (https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27) - [2026-09-21] Carbon Brief's independent analysis found actual August 2026 observations already running cooler than model projections: "The models also predicted temperatures slightly warmer than observed this summer, with real-world observations for August coming in around 0.3C below forecasts." A repeat of this modest cool-bias in the NDJ window could be enough to pull the 15th-percentile-adjacent scenarios under 2.0°C. - [2026-09-21] Carbon Brief also flags an unresolved structural concern specific to RONI itself: the index "can also diminish the apparent strength of strong El Niño events, such as the current one, which extend well outside the Niño 3.4 region" — i.e., the very methodology used to adjudicate this market is designed to produce lower readings than raw SST anomalies, and does so by an amount (tropics-wide mean subtraction) that is itself model/observation-dependent and could shift with data revisions. (https://www.carbonbrief.org/analysis-super-el-nino-reaches-remarkable-all-time-record) - [2026-09-21] Carbon Brief's own caveat on forecast verification is explicit: "No seasonal forecast system has ever been verified against an event of this size, because none has ever occurred." This is a genuine first-of-its-kind event for the models now being relied upon to justify >90% confidence, undercutting the historical-skill basis for that confidence. - [2026-09-21] Historical analogues show late-season deceleration risk: Carbon Brief notes "The 1982-83, 1997-98 and 2015-16 events added between 0.5C and 1.9C to their RONI values between mid-September and their peaks," meaning the exact size of the September-to-peak jump is itself variable by nearly 1.4°C across the three best analogues — a swing large enough, if it landed on the low end from a lower September starting point, to leave some uncertainty about where NDJ specifically (as opposed to OND, the forecast peak month) nets out, since NDJ is a three-month mean straddling both the peak and the beginning of decline. - [2026-10-05 scheduled] Resolution requires a NOAA-designated "final" (non-preliminary) RONI table value, not merely a preliminary or forecast one; the next official diagnostic update is due only on 2026-10-08, and the final NDJ figure itself will not be published and finalized until well into Q1 2027, so no "final" NDJ value exists yet, and all current evidence (however strong) remains forecast/preliminary, leaving open the technical possibility of a late downward data revision before finalization. (https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf) - [2026-09-10] Even the headline ">90% very strong" claim is explicitly a seasonal-average statement covering SON/OND/NDJ collectively with decay starting immediately after; CPC's own probability table shows the very-strong probability already easing to 75% in December to February and 39% in January to March , confirming the event is forecast to be past its peak intensity by the back half of winter — meaning NDJ sits on the descending limb of confidence, not at the most-certain point of the distribution. (https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27) BOTTOM LINE: The NO case does not rest on the event failing to be a major El Niño — virtually all evidence shows it will be — but on the residual (~5-10%) tail probability CPC's own strength tables assign to NDJ RONI landing below +2.0°C, the thin ~0.13°C buffer at the 15th percentile of the forecast band, a documented recent tendency for realized values to run slightly below model projections, and the fact that RONI's own tropics-wide adjustment is structurally designed to suppress readings relative to raw SST anomalies in ways with no track record for an event this large. NOT FOUND: No official CPC NDJ-specific numeric forecast table (beyond secondary-source citations) could be directly fetched; no documented case of a CPC RONI "very strong" seasonal probability above 90% subsequently failing to verify (the RONI record is too new, adopted only Feb 2026, for any such track record to exist).