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

date	predicted	market price	confidence
2026-09-10	66%	62%	57%

1. Planner Fable 5.1 · Fermi decomposition + tool plan

Tools chosen: polymarket_direct polymarket_related kalshi_related resolution_source article_search claude_news wikipedia code_execution

Sub-questions (Fermi decomposition)

What is the current Polymarket YES price for the ≥2.5 bucket, and how are the sibling peak-RONI buckets (e.g., <0.5, 0.5-1.0, 1.0-1.5, 1.5-2.0, 2.0-2.5) priced, so the implied distribution can be checked for coherence?
In the full historical RONI record published by NOAA CPC (1950-present), how many El Niño events reached a peak RONI of +2.5°C or above, and what were the peak RONI values of the 1982-83, 1997-98, and 2015-16 super El Niños?
What is the current ENSO state and what do the latest CPC/IRI ENSO probability outlooks and dynamical model plumes (NMME, ECMWF, etc.) project for Niño-3.4 / RONI during ASO 2026 through DJF 2026-27, and do any credible forecasts suggest a super El Niño (>2.0)?
What is the current equatorial Pacific subsurface ocean heat content and Kelvin wave activity, and does it support rapid El Niño development in 2026?
What is the most recent RONI value published on the CPC RONI page, has the ASO 2026 value or any listed season already been published, and has CPC changed the RONI methodology or publication schedule?
Given the typical gap between ONI and RONI in the current warmer climate (RONI tends to run below ONI), how large an ONI peak would be needed to yield a RONI ≥2.5, and how does that compare with the historical ONI maximum of 2.6?

Planner reasoning

A RONI peak of +2.5°C or higher would be historically unprecedented or near-unprecedented (even the 2015-16 and 1997-98 super El Niños peaked around ~2.3-2.5 in RONI terms), so the question hinges on (a) the historical base rate of extreme RONI values, (b) the current state of ENSO forecasts for late 2026 from CPC/IRI and dynamical models, and (c) the market's own pricing of this tail bucket relative to sibling buckets. The resolution page must be checked for the latest published RONI values and any methodology changes.

2. Research Parallel tool calls

tool	status	time	items	summary
polymarket_direct	OK	0.4s	1	## This Market's Polymarket Data Will the peak RONI for the 2026–27 El Niño be 2.5 or above? - Current price (probability): 73.00% - 7-day price change: +9.50% - 30-day price change: +11.50% - Total volume: $21,062 (USD notional) - Price range: 40.00% - 75.00% - Data points: 34 days
polymarket_related	OK	0.7s	7	Polymarket search over 4 keyword(s), kept 7 matches. keyword 'RONI': 5 markets \| keyword 'El Niño': 2 markets \| keyword 'El Nino 2026': 0 markets \| keyword 'ENSO': 0 markets
kalshi_related	OK	3.9s	0	0 related markets / summaries. keyword 'El Nino': no matches \| keyword 'ENSO': no matches \| keyword 'La Nina': no matches
resolution_source	OK	7.3s	1	already_resolved=no; matches_criteria=True; last_updated=10 September 2026 (ENSO Diagnostic Discussion); RONI table shown appears stale/truncated (visible data ends 2013). The RONI table (fetched, truncated) only shows historical data through 2013 and does not yet include ASO/SON/OND 2026, NDJ 2026-
article_search	OK	47.4s	19	Fetched 19 unique articles across 4 queries (lookback=60d). 'El Niño forecast 2026 super El Niño': 10 articles \| 'NOAA CPC ENSO outlook El Niño watch 2026': 10 articles \| 'Niño 3.4 sea surface temperature forecast late 2026': 10 articles \| 'RONI relative oceanic Niño index': 10 articles
claude_news	OK	24.5s	10	Based on current NOAA CPC/IRI data (as of mid-September 2026): - Current status: NOAA declared El Niño in June 2026, following a rapid transition from La Niña earlier in the year; CPC expects El Niño to continue strengthening through the end of 2026, with a greater than 90% chance of a very st
wikipedia	OK	0.5s	4	Fetched 4 Wikipedia entries (1 missing pages).
code_execution	OK	55.3s	0	## Key Findings - Historical base rate: Among ~26 El Niño events since 1950 (peak ONI values), only 1 event (2015‑16, ONI≈2.6) reached ONI ≥ 2.5 — a raw event‑conditional rate of ~3.8% (Wilson 95% CI: 0.7%–18.9% given the small sample). - RONI adjustment: Applying the assumed 0.1–0

3. Evidence Brief Sonnet 5 · 6637 chars

# Current state NOAA/CPC declared El Niño in June 2026 after a rapid transition from La Niña; as of the Sept 10, 2026 ENSO Diagnostic Discussion, Niño-3.4 is already at +1.8°C, Niño-3 at +2.5°C, and Niño-1+2 at +3.4°C — unusually warm for this point in the season versus 1997 or 2015 analogs. However, the CPC RONI table itself has NOT yet published any value for ASO/SON/OND 2026, NDJ 2026-27, or DJF 2026-27 (the fetched page is stale, showing data only through 2013). Resolution hinges entirely on future published RONI values, expected to peak around OND 2026. # Timeline of key events - 2026-06: NOAA declares El Niño onset (reported). - 2026-08-13: Washington Post frames event as a potential "record El Niño" (reported). - 2026-08-20: CPC states >90% chance of a "very strong" event (RONI≥2.0) this fall/winter and 69% chance OND 2026 is a historic event (RONI≥2.5) (confirmed, CBS LA/CNN). - 2026-08-23/25: Scientists (e.g., NOAA's Amaya) publicly state high confidence in a "Super El Niño trajectory" (reported/opinion, CBC/CNN). - 2026-09-03/04: WMO/UN warns of a "supersized" El Niño persisting into 2027 (confirmed, CBS/WaPo). - 2026-09-10: CPC ENSO Diagnostic Discussion raises OND 2026 RONI≥2.5 probability to 75%; Niño-3.4 +1.8°C, Niño-3 +2.5°C reported (confirmed, resolution_source/watchers.news). - 2026-09-14: CNN reports El Niño has "leveled up to super status," 75% chance of becoming strongest event since 1950 (reported). - 2026-09-19: Independent newsletter reiterates 75% OND figure; no update to actual published RONI values (reported). # Event Will the peak RONI for the 2026–27 El Niño reach +2.5°C or above (across ASO 2026–DJF 2026-27 seasons)? # Outcomes to forecast Yes / No # Kalshi market anchor No kalshi_direct data was returned (kalshi_related found 0 matches). The ticker provided is a Polymarket-style contract ID; treating **Polymarket YES = 73%** as the primary market anchor. Trend: +9.5% (7d), +11.5% (30d); range 40%–75% over 34 days; volume $21,062 — thin but clearly trending upward toward "Yes." # Resolution source CPC RONI page (fetched) is stale/truncated, showing data only through 2013 — no 2026-27 season values published yet. The companion ENSO Diagnostic Discussion (Sept 10, 2026) is the most current primary signal: El Niño strengthening, Niño-3.4 +1.8°C, Niño-3 +2.5°C, Niño-1+2 +3.4°C; CPC states 75% chance OND 2026 RONI reaches +2.5°C+ (a historic event) and >90% chance of a "very strong" event (RONI≥2.0). Market is not yet resolved; matches_criteria=True (question is well-defined and awaiting data). # Sub-question answers 1. **Polymarket bucket pricing**: <1.5: ~0.05% Yes; 1.5–2.0: 2.5%; 2.0–2.25: 4.8%; 2.25–2.5: 23%; ≥2.5: 73%. Sum ≈103%, roughly coherent (slight overpricing, immaterial arbitrage). 2. **Historical peak RONI/ONI**: Only 2015-16 reached ONI≥2.5 (2.6, historical max) per code_execution's 26-event dataset. In RONI terms, Climate Brink reports 1982-83 is actually the record holder at 2.69°C once trend-adjusted; 1997-98 and 2015-16 are comparable but slightly lower. 3. **Current ENSO state/outlook**: El Niño declared June 2026, strengthening rapidly; CPC's Sept 2026 RONI outlook median path peaks at +2.67°C in OND 2026 (mountainmarker.com/CPC), with 75% probability of ≥2.5°C and >90% probability of ≥2.0°C. Independent synthesis (Climate Brink) estimates ~77% chance of a record RONI. 4. **Subsurface heat/Kelvin wave**: CPC reports steady, deep thermocline anomalies with subsurface temperatures >+10°C at depth in August 2026, plus strong low-level westerly/upper-level easterly wind coupling — supportive of continued/rapid strengthening (watchers.news). 5. **Latest published RONI / methodology**: No 2026-27 season RONI values published yet; CPC RONI page data ends 2013 in the fetched snapshot. RONI itself was adopted as NOAA's new standard metric in 2026 (subtracting global tropical SST anomaly from Niño-3.4), no other schedule/methodology change reported. 6. **ONI-RONI gap**: RONI runs below ONI due to global-warming detrending; a 0.1–0.3°C offset implies an ONI-equivalent of 2.6–2.8 needed for RONI≥2.5, near or above the historical ONI record (2.6, 2015-16). Current real-time SST anomalies (Niño-3 +2.5°C by Sept 2026, historically anomalous timing) suggest this event could exceed prior ONI/RONI ceilings. # Key facts (high-confidence, factual) 1. [resolution_source] CPC Sept 10, 2026: Niño-3.4 +1.8°C, Niño-3 +2.5°C, Niño-1+2 +3.4°C; 75% chance OND 2026 RONI≥2.5. 2. [claude_news] CPC RONI Outlook (Sept 2026): median RONI path peaks OND 2026 at +2.67°C. 3. [polymarket_direct] This market's Polymarket YES = 73%, up sharply over 30 days. 4. [code_execution] Historical base rate: only 1 of ~26 past El Niño events (2015-16) reached ONI≥2.5; generic base-rate models (ignoring 2026 real-time data) suggest ~1.5-3.5% conditional probability. # Cross-market signals - Kalshi related: none found. - Polymarket siblings: "Super El Niño this winter" 94.2% Yes; "Strongest El Niño on record" 77.5% Yes — both consistent with high confidence in ≥2.5 RONI. - Sportsbook implied: n/a. # Analyst opinions and speculation - NOAA scientist Amaya (CNN): "We're definitely on a Super El Niño trajectory... I'd bet my house on it" (opinion). - Zeke Hausfather/Climate Brink: independent models give ~77% chance of record RONI, noting unprecedented early-season warmth vs. 1997/2015 analogs (analysis). - UN/WMO: "danger zone of extreme weather," near-100% chance El Niño persists into early 2027 (official statement). # Directional lean per outcome - **Yes (≥2.5)**: Strongly supported — CPC's own 75% probability estimate for OND 2026, record-pace SST anomalies, deep subsurface heat, market consensus (73–77% across Polymarket contracts), analyst confidence. - **No (<2.5)**: Supported only by generic historical base-rate scarcity (only 1 prior ONI≥2.5 event ever) and the fact that no season's RONI has yet actually published a qualifying value — genuine uncertainty remains until October–December data print. # Gaps / unknowns - No actual 2026-27 RONI values yet published (only forecasts/probabilities). - CPC RONI table fetch was stale — cannot confirm most-recent official update beyond Sept 10 discussion. - Historical base-rate model (code_execution) is structurally outdated for this specific record-pace event and likely underweights current real-time signal. # Calibration anchors - Polymarket YES (anchor): 73% (trending up). - CPC official probability: 75% for OND 2026 RONI≥2.5; >90% for RONI≥2.0. - Historical base rate (pre-2026 data only): ~1.5-3.5% — largely superseded by current real-time forecast strength.

4. Forecasters 5 draws · Fable 5.1 + GPT-5.6 Sol

claude-fable-5-1

58% conf

Yes	74%
No	26%

The Polymarket anchor sits at 73% Yes and is trending upward, closely matching CPC's own Sept 10, 2026 estimate of a 75% chance that OND 2026 RONI reaches ≥2.5 with a median forecast path peaking at +2.67°C. Real-time observations (Niño-3.4 +1.8°C, Niño-3 +2.5°C, deep subsurface heat >+10°C, strong wind coupling) are running ahead of 1997 and 2015 analogs, and independent analysis (Climate Brink ~77%) corroborates the official outlook. The main risk is that 2.5 RONI has essentially never been reached (only 1982-83 at ~2.69 in trend-adjusted terms), so a modest underperformance or late-season stall would land the peak in the 2.25–2.5 bucket, which the market prices at ~23%. No RONI values for the qualifying seasons have published yet, so this remains a forecast rather than a documented outcome; I stay close to the market/CPC consensus.

claude-fable-5-1

58% conf

Yes	76%
No	24%

CPC's Sept 10, 2026 discussion gives 75% for OND 2026 RONI ≥2.5 with a median outlook path peaking at +2.67°C, and Polymarket prices YES at 73% (trending up). Real-time anomalies (Niño-3.4 +1.8°C, Niño-3 +2.5°C in September, deep >+10°C subsurface warmth) are running ahead of 1997 and 2015 analogs, and independent analysis (Climate Brink ~77%) corroborates. Since the market resolves on the peak across ASO–DJF rather than OND alone, the probability should sit slightly above CPC's single-season 75%. The main downside risk is the historical rarity of RONI ≥2.5 (only 1982-83 in trend-adjusted terms) and the fact that no qualifying value has printed yet, so I stay near the consensus rather than pushing higher.

claude-fable-5-1

62% conf

Yes	75%
No	25%

CPC's Sept 10, 2026 discussion gives a 75% probability that OND 2026 RONI reaches ≥2.5 with a median forecast path peaking at +2.67°C, and Polymarket prices YES at 73% (trending up), so the official forecast and market consensus are tightly aligned. The question resolves on the peak across ASO–DJF, which is marginally more permissive than CPC's single-season OND figure, and real-time signals (Niño-3 +2.5°C, Niño-1+2 +3.4°C, deep subsurface heat >+10°C, strong wind coupling) support continued strengthening. The main downside risk is that RONI ≥2.5 would be near or above the historical record (1982-83 ~2.69 trend-adjusted), so a modest shortfall into the 2.25–2.5 bucket (Polymarket ~23%) is a realistic outcome given forecast spread around the threshold. No actual 2026-27 RONI values have printed yet, so genuine uncertainty remains until the OND value publishes.

gpt-5.6-sol

74% conf

Yes	75%
No	25%

The 73% Polymarket YES price provides the market anchor, while CPC's September outlook independently assigns a 75% chance that OND 2026 RONI reaches at least 2.5°C and projects a median peak of 2.67°C. Rapid strengthening, Niño-3.4 already at +1.8°C, exceptional warmth farther east, deep subsurface heat, and favorable atmospheric coupling support the qualifying bucket. Independent estimates near 77% and related market pricing reinforce this outlook. I assign 25% to No because no qualifying RONI has yet been published, RONI typically trails ONI by 0.1–0.3°C, and reaching 2.5 would remain historically exceptional.

gpt-5.6-sol

78% conf

Yes	76%
No	24%

Polymarket’s 73% YES price provides the market anchor, while CPC’s September outlook independently assigns a 75% chance that OND 2026 RONI reaches at least 2.5°C. CPC’s median peak forecast of 2.67°C, unusually advanced regional warming, deep subsurface heat, and favorable atmospheric coupling support a small upward adjustment from the market price. The historical rarity of RONI/ONI near 2.5°C and the absence of any published qualifying 2026–27 RONI value leave a meaningful 24% chance of falling short. Resolution-source uncertainty is limited but nonzero because the fetched CPC RONI table is stale and RONI is a newly adopted metric.

6. Final plain mean of 5 draws

75% (66% confidence)

Yes: 75% No: 25%

Mean of 5 draws (0.74, 0.76, 0.75, 0.75, 0.76). The Polymarket anchor sits at 73% Yes and is trending upward, closely matching CPC's own Sept 10, 2026 estimate of a 75% chance that OND 2026 RONI reaches ≥2.5 with a median forecast path peaking at +2.67°C. Real-time observations (Niño-3.4 +1.8°C, Niño-3 +2.5°C, deep subsurface heat >+10°C, strong wind coupling) are running ahead of 1997 and 2015 analogs, and independent analysis (Climate Brink ~77%) corroborates the official outlook. The main risk is that 2.5 RONI has essentially never been reached (only 1982-83 at ~2.69 in trend-adjusted terms), so a modest underperformance or late-season stall would land the peak in the 2.25–2.5 bucket, which the market prices at ~23%. No RONI values for the qualifying seasons have published yet, so this remains a forecast rather than a documented outcome; I stay close to the market/CPC consensus.

Pipeline Timing

Total pipeline time: 156.0s

Per-tool research timings shown in the Research section above.