Will the peak RONI for the 2026–27 El Niño be at least 2.25 but below 2.5?

- [2026-10-08] The CPC ENSO Diagnostic Discussion keeps the El Niño Advisory in place. It says El Niño continues to strengthen and will keep strengthening through the end of the year. Chances of a historic event (≥+2.5°C on a 3-month RONI) are 54% for SON, 83% for OND and 70% for NDJ. The text gives no figure for ASO or DJF. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-08] The same discussion reports monthly Niño indices of +2.1°C (Niño-3.4), +3.0°C (Niño-3) and +3.9°C (Niño-1+2). Subsurface temperatures are above +10°C at depth. The next discussion is due 2026-11-12. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-18] The CPC RONI table (fetched) shows 2026 values through JAS (Jul–Sep) at about 1.7: JJA 1.4 and JAS 1.7. No ASO, SON, OND, NDJ or DJF value has been published yet. The page says it is updated by the 5th of each month. It also warns that real-time RONI values can change for up to two months. The question uses the first published value. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-18] The NOAA PSL dashboard (rendered) lists the latest RONI as 1.67 for Aug 2026 (JJA) and the ONI as 2.16. Monthly ERSSTv6 Niño-3.4 for September is +2.09°C. Niño-3 is +3.25°C and Niño-1+2 is +3.93°C. https://psl.noaa.gov/enso/dashboard.html - [2026-10-15] CPC's weekly update shows Niño-3.4 at +2.4°C, Niño-3 at +3.4°C and Niño-1+2 at +4.4°C. These are relative-adjusted values, not the conventional ones. https://watchers.news/2026/10/16/very-strong-el-nino-favors-wetter-south-and-warmer-north-in-noaas-2026-27-winter-outlook-u-s - [2026-10-14] Weather West says the weekly RONI analogue is +2.4°C. That is a statistical tie with the record of +2.3°C. The conventional weekly Niño-3.4 is +3.3°C, against a record of +2.1°C. https://weatherwest.com/archives/44119 - [2026-10-15] The Australian Bureau of Meteorology's relative Niño-3.4 for September 2026 was +1.96°C. The Bureau projects a peak near +3.7°C in Nov–Dec, which would be well above the 1902 record of +2.56°C. That is a different index and baseline from RONI. https://weatherzone.com/au/news/el-nino-approaching-record-strength-as-pacific-subsurface-water-reaches-12c-above-average/1891603 - [2026-10-08] WMO says El Niño will keep intensifying ahead of a forecast peak in December. https://wmo.int/news/media-centre/sea-surface-temperatures-reflect-strengthening-el-nino - [2026-09-21] The IRI plume has all 22 models showing a "very strong" event (Niño-3.4 ≥ +2.0°C, not RONI) from SON through DJF. https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/ - [2026-10-12] A secondary tracker reports an "official" peak forecast of +2.7°C RONI, with an AI model at +2.3°C RONI. I could not confirm this on a primary source. A different aggregator cites a CPC middle forecast for SON RONI of about +2.47°C. https://4billionyearson.org/climate/enso - [2026-09-10] The September CPC discussion gave a 75% chance of OND RONI ≥+2.5°C. The October discussion raised this to 83%. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf - [Upcoming] The ASO RONI should appear around 2026-11-05. The SON value should come around early December, OND in early January, NDJ in early February and DJF in early March. The 2026-11-12 CPC discussion falls between these. BOTTOM LINE: Observed RONI is only JAS ≈1.7 so far, and no resolution-window season has been published. CPC puts 83% on OND ≥2.5 and 70% on NDJ ≥2.5, so the 2.25–2.5 band is a minority outcome, roughly the residual below the 2.5 threshold. The weekly relative Niño-3.4 of about +2.4°C and still rising is the most direct near-term indicator. NOT FOUND: Any published ASO–DJF RONI value, CPC's explicit forecast for the peak RONI or DJF, and a CPC-confirmed RONI figure for ASO.

- [2026-10-08] CPC's ENSO discussion put the chance of a 3-month RONI of +2.5°C or more at 54% for SON, 83% for OND and 70% for NDJ. It calls +2.5°C historic, higher than any event since 1950. Niño-3.4 was +2.1°C for September. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml - [2026-10-13] The latest published RONI is JAS 2026 at +1.7°C. CPC's median forecast for SON is about +2.47°C, per a secondary guide. RONI is now computed on ERSSTv6. CPC warns that values can change for up to two months after first posting. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-10-18] CPC's RONI table has 2026 at -0.9, -0.8, -0.4, 0.0, 0.5, 1.0, 1.4, 1.7 for DJF through JAS. ASO is the next value due. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-08, secondary] CPC's median RONI path, as relayed by a ski-industry blog, is OND +2.74 (70% range +2.43 to +3.05), NDJ +2.65 (+2.24 to +3.06) and DJF +2.36 (+1.87 to +2.85). The peak is the maximum across ASO to DJF, so it will probably be OND or NDJ. On these numbers, a peak in the 2.25–2.5 band needs the event to come in well below the median. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - Historical reference class (CPC RONI v6 table, which shows 1 decimal): the 1982–83 peak was 2.4 (NDJ 1982, in the table's 1982 row), 2015–16 was 2.3 (NDJ 2015) and 1997–98 was above 2 (its row was not in the fetched text). No event since 1950 has exceeded 2.4, so the 2.25–2.5 band is where the two historic record events landed. Anything at or above 2.5 would be unprecedented. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-15] The weekly RONI is +2.4°C, a statistical tie with the record, per Weather West. Weekly traditional Niño-3.4 is +3.3°C, against a prior record near 2.1 on the old ONI. https://weatherwest.com/archives/44119 - [2026-10-11] Australia's BoM weekly relative Niño-3.4 was +2.81°C, a record. The BoM index uses a different baseline, so it is not directly comparable to CPC's RONI. https://weatherzone.com/au/news/el-nino-approaching-record-strength-as-pacific-subsurface-water-reaches-12c-above-average/1891603 - [2026-09-21] IRI: all 22 models forecast a "very strong" event (Niño-3.4 of +2.0°C or more) from SON through DJF. Weekly Niño-3.4 was +3.0°C. https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/ - [2026-07] The Yale Climate Connections report on model consensus said the pooled models peak around +3.6°C. WMO expects a peak near December. These are traditional-index figures, which run above RONI. https://yaleclimateconnections.org/2026/07/this-could-be-the-strongest-el-nino-on-record/ - [2026-10-15] Polymarket's own peak-RONI market was reported at 78.5% for 2.5 or above and 19% for 2.25–2.5, with about $97k traded. This is the sibling-strike view. A Polymarket page snapshot of unknown date showed 77% and 21%. https://www.riotimesonline.com/paraguay-el-nino-flood-warning-1983-asuncion-river-2026/ - [2026-09-05] Kalshi runs parallel peak-RONI series (KXRONI-27AUG, KXRONI-27APR and KXRONI-27APR2), but I found no current prices for them. https://github.com/alexander-bain/bainluck/issues/3128 - [2026-10-08] The probabilistic threshold figures are CPC's: 83% for OND at 2.5 or more, 70% for NDJ and 54% for SON. They imply at most roughly 17% for OND below 2.5, and that is before taking the maximum across seasons. NDJ is in the 2.25–2.5 band only if OND and SON also stay below 2.5. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml BOTTOM LINE: The 2.25–2.5 band is where the two prior record events landed (about 2.3–2.4), so history favours it. This event is running at or above record pace, and CPC's 83% chance of OND at 2.5 or more leaves roughly 15–20% for a peak below 2.5. A peak below 2.25 is unlikely, since a drop that large would take a sharp weakening. NOT FOUND: Live Kalshi prices. The ASO 2026 RONI first value (due about November 5). An exact CPC probability for a peak below 2.25. The 1997–98 RONI v6 peak (it was in an omitted part of the table). Any track record for CPC's strength probabilities.

YES means: the highest first-published CPC RONI among ASO 2026, SON 2026, OND 2026, NDJ 2026-27 and DJF 2026-27 is at least +2.25°C and below +2.50°C. - [2026-10-13] The CPC ENSO slides were fetched. They show the weekly relative Niño-3.4 at +2.4°C and Niño-4 at only +0.2°C. Westerly wind anomalies run from the west to the east-central Pacific. The subsurface heat anomalies "have remained steady" since late July rather than growing. The cold west of the Date Line has strengthened. A plateau in subsurface heat and a cool Niño-4 limit how far the relative index can climb. That supports a Yes path of a peak just under 2.5. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf - [2026-10-08] CPC's median RONI path was fetched via Mountain Marker. The SON median is +2.47, with a 15–85% range of +2.29 to +2.65. The SON median is itself in the target band. The peak is the maximum across seasons, though, so SON alone would only produce Yes if OND and NDJ also come in low. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-10-08] The same CPC path has the OND median at +2.74. Its 15th percentile is +2.43. The NDJ 15th percentile is +2.24 and the DJF 15th percentile is +1.87. A Yes needs OND and NDJ both to land in the bottom 15–20% of CPC's spread, with the max between 2.25 and 2.5. The 83% and 70% threshold probabilities are consistent with this. This is a tail path, not the central case. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-08-17] CPC's August median for OND was +2.66, with a 25–75% range of +2.37 to +2.95. The October median rose to +2.74. Forecasts have drifted upward, which weakens Yes. The upside is that the 2.5 line has stayed inside the middle of the distribution at every issuance. https://watchers.news/2026/08/17/el-nino-forecast-to-reach-historic-strength-ahead-of-winter-2026-27/ - [2026-10-12] A 4 Billion Years On tracker lists the AI model (SNU CNN) peak at +2.3°C RONI, which is inside the target band. It had been near +2.4 to +2.6 in July and has been trimmed as observations came in. This is the only model-based source I found that points to the 2.25–2.5 band. The same page reports an "official" +2.7 peak, which I could not verify on CPC. https://4billionyearson.org/climate/enso - [2026-10-16] Historical RONI v6 peaks, per a secondary tracker, were 1982-83 at +2.4 (NDJ), 1997-98 at +2.3 (OND) and 2015-16 at +2.3 (NDJ). Every prior record event peaked in the target band. The 2.5 threshold has never been reached on this index. A Yes rests on the historical ceiling holding, i.e. relative Niño-3.4 saturating as it did in those events. Another tracker puts 1982-83 at +2.5, so the figure is not fully consistent across sources. https://www.elninolive.com/el-nino-2026 - [2026-10-18] The monthly RONI values lag the weekly ones. The weekly relative Niño-3.4 is about +2.4. The monthly September ERSSTv6 Niño-3.4 is +2.09 (traditional index), and JAS RONI is +1.69. Three-month averages smooth the peak. OND needs monthly relative values near +2.7 on average to reach 2.5. Mixing in the low Oct values keeps the average lower. https://psl.noaa.gov/enso/dashboard.html - [2026-10-08] CPC's strength probabilities say the very-strong (≥2.0) chance is 100% for SON and OND, 96% for NDJ and 81% for DJF. That removes the "peak below 2.25" tail almost entirely. The Yes band is therefore effectively "peak below 2.5", given ≥2.0 in OND is certain. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 - [2026-10-06] Kelvin wave activity is strong. Scientific American, on Oct 9, cites "extremely strong westerly wind bursts", with more Kelvin waves expected over the next several months. This is the main risk to Yes. A Yes needs the wave energy to fail to drive further relative warming east of 150W. This would happen if tropical-mean warming rises with it, since RONI subtracts the tropical mean. https://www.scientificamerican.com/article/the-kelvin-wave-hitting-california-is-a-potent-warning-about-sea-level-rise/ - [2026-10-13] The RONI formula subtracts tropical-mean SST and rescales variance. A strong east-Pacific event warms the tropical mean as well. Heat released to the atmosphere could push RONI lower than the raw anomaly implies. This explains why weekly relative values (+2.4) sit far below traditional ones (+3.3). A large share of the raw warmth is Niño-1+2 and Niño-3 (+3.4 to +4.4), outside the 5°N–5°S, 170–120°W Niño-3.4 box. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-11-05 upcoming] CPC says RONI values "may change up to two months" after first posting, but the question uses the first value. ASO, with a likely first value around 2.0–2.3, will not be the peak unless the event stalls. An early-November ASO print near 2.2 or below would add a little weight to a lower peak. The Nov 12 CPC discussion would then be the next update. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ BOTTOM LINE: The best Yes path is that OND and NDJ both come in at the low end of CPC's spread (about +2.3 to +2.49), matching 1982-83, 1997-98 and 2015-16, which all peaked at 2.3–2.4. This is supported by steady subsurface heat, a cool Niño-4, and the SON median of +2.47. It runs against CPC's median of +2.74 and its 83% chance for OND. NOT FOUND: Any published ASO–DJF RONI value. A primary CPC source for the "official +2.7" peak. A CPC-stated probability of a peak between 2.25 and 2.5. A primary-source confirmation of the 1997-98 RONI v6 peak.

YES means: the highest first-published CPC RONI among ASO 2026, SON 2026, OND 2026, NDJ 2026-27 and DJF 2026-27 is at least +2.25°C and below +2.50°C. This report argues the NO side. NO happens if the peak is +2.5°C or higher (the main path) or below +2.25°C. - [2026-10-08] CPC's median path, relayed by a ski blog, puts the peak season at OND at +2.74°C, with a 70% range of +2.43 to +3.05. The whole 70% range sits at or above about 2.4, and the median is 0.24°C above the band's ceiling. The August CPC median for OND was similar, at +2.66°C (middle 50% from +2.37 to +2.95). The forecast has been stable to slightly higher since August. https://www.mountainmarker.com/blog/el-nino-ski-season-2026-27 ; https://watchers.news/2026/08/17/el-nino-forecast-to-reach-historic-strength-ahead-of-winter-2026-27/ - [2026-10-08] CPC probability of a 3-month RONI of +2.5°C or more rose from 69% (August) to 75% (September) to 83% (October) for OND. The direction of every monthly revision has been toward NO. https://watchers.news/2026/08/17/el-nino-forecast-to-reach-historic-strength-ahead-of-winter-2026-27/ ; https://watchers.news/2026/10/08/noaa-gives-el-nino-an-83-chance-of-reaching-historic-strength-in-late-2026/ - [2026-10-08] NOAA's L'Heureux said the event is forecast to peak in OND, but that some uncertainty remains. There is no sign of an early peak that would cap the event below 2.5. https://www.newsweek.com/el-nino-forecast-reveals-winter-outlook-as-event-strengthens-cpc-12540301 - [2026-10-14] A westerly wind burst under way is described as the strongest in months. Weather West expects it to persist for 2+ weeks and calls it the "last shove" toward record territory. The ASO and SON values will incorporate October and November warming still to come. The weekly CPC relative Niño-3.4 of +2.4 on Oct 15 is already above the 3-month values needed to reach 2.5 by OND. https://www.edhat.com/news/california-el-nino-winter-forecast-2026-storm-flood-threats/ - [2026-10-16] Weekly traditional Niño-3.4 is up +0.5°C over 4 weeks (+3.3°C on the week of Oct 7). BoM's relative index is up +0.4°C over 4 weeks, and the record value has been held two weeks in a row. The trend is upward, not plateauing. https://www.elninolive.com/ - [2026-10-07] GFDL SPEAR is the one dynamical counterweight. It says the El Niño is "nearing its peak" with weakening by early winter, and calls its forecast the strongest in 30+ years. Even this model implies the peak is high, and its weakening starts after OND and NDJ. That timing matters because the peak is the maximum across ASO–DJF. A faded DJF cannot pull the peak into the band if OND or NDJ is already at 2.5 or more. https://www.gfdl.noaa.gov/prediction/october-2026-el-nino-predictions/ - [2026-10-18] Arithmetic of the 3-month averages: JAS is +1.7 (CPC table). Reaching OND ≥2.5 needs monthly relative values of roughly +2.5 to +3 in Oct–Dec. September is only +2.09 (PSL). Weekly values of +2.4 and above are already running above the monthly numbers. For a Yes, both OND and NDJ must stay below 2.5, so CPC's 17% for OND and 30% for NDJ are not independent. If OND misses, NDJ is likely to miss too, and these two misses are positively correlated. This is my inference, not a published figure. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-18] Rounding and boundary risk. The CPC table publishes one decimal. A true value of 2.45–2.49 would likely be posted as "2.5" and resolve NO. The PSL dashboard uses two decimals, so the resolving source for a boundary case is ambiguous. The band's effective upper edge is therefore about 2.45 if the table is used. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-18] The record events are weak precedent for a Yes. The 1982-83 peak was 2.4, and 2015-16 was 2.3 (CPC table). But this event is far ahead of them at the same point: JAS 1982 was 1.1 and JAS 2015 was 1.5, against 1.7 now. The sub-2.5 landing of past events is a base rate from a smaller sample and a slower start. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-18] Revision and first-value risk. CPC says real-time RONI can change for up to two months. Only the first published value counts. A modest downward revision cannot rescue a Yes, because a first print at 2.5 or more governs. Each first print also takes the latest data, with warm anomalies still to come. https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso/roni/ - [2026-10-15] Prediction-market sibling strikes, per a Rio Times snapshot: 78.5% for 2.5 or more, 19% for 2.25–2.5. An earlier snapshot (about 138 days old) showed 77% and 21%. The market has hardly moved while CPC and observations strengthened. I take the 2.25–2.5 band as possibly slightly overpriced if the strengthening trend continues. A Polymarket "Super El Niño" market was at 97% in late Sept. https://www.riotimesonline.com/paraguay-el-nino-flood-warning-1983-asuncion-river-2026/ ; https://polymarket.com/weather BOTTOM LINE: CPC's median peak is about +2.7°C in OND, with 83% for OND at 2.5 or more, and observations and the westerly wind burst are strengthening. NO therefore means the peak lands at 2.5 or more, with rounding to 2.5 as a further boundary leak, and a sub-2.25 collapse is a negligible alternative. A Yes requires CPC's central forecast to miss on the high side, with both OND and NDJ landing in the 2.25–2.5 band. NOT FOUND: Any published ASO–DJF RONI value. CPC's explicit peak-RONI probability for the 2.25–2.5 band. Any official statement on rounding for the resolution value. Live Kalshi prices. A track record of CPC's ≥2.5 probabilities.