Week of 2026-05-18 · Methodology v1.7

How likely is a super
El Niño this winter?

Updated each Monday from the four major ENSO outlooks (NOAA CPC, IRI, BoM, ECMWF SEAS5) and weekly Niño 3.4 observations. Peak season target: DJF 2026-27. Forecast disagreements are surfaced rather than averaged.

>+2.5°C peak

40%probability

1997 / 2015 magnitude

>+2.0°C peak

69%probability

Very strong / super

>+1.5°C peak

89%probability

Strong

>+1.0°C peak

98%probability

At least moderate

Probabilities use the v1.5 smoothed estimator: a CPC-derived anchor (live offset +0.40°C, week of 2026-05-13, skew-normal fit on the nine-bin strength table) plus a bounded SEAS5 deflection (W = 0.2, capped at ±10 pp per bucket per week). Deltas next to each percentage compare to the issue four weeks prior, aligned with CPC's monthly issuance cadence; weeks where no comparable prior exists (early issues, methodology version changes) show no delta. Full estimator math on the methodology page.

What's interesting this week

Observations from this week's data, beyond what the headline numbers say.

CPC re-issued the strength table. The super (>+2.0°C) anchor moved from 51% to 61% (+10pp). First CPC issuance since the previous brief; the smoothed headline updates accordingly.
CPC and SEAS5 converged. Last week the smoothed super bucket was being pulled up by a SEAS5 deflection of +9.8pp (near the +10pp weekly cap). This week the deflection is +7.8pp — CPC's anchor moved up from 51% to 61%, closing most of the gap with what SEAS5 had been pointing at.
First direct DJF 2026-27 read from CPC. CPC's strength table now includes the peak season directly. Super (≥+2.0 RONI) on the direct DJF read: 31%. Brief still anchors on NDJ for continuity; switching to DJF is queued as a separate methodology decision.
Subsurface heat ahead of both Godzilla analogs. 0–300 m heat content anomaly is now +2.24°C, vs +0.7°C in 1997 and +1.6°C in 2015 at the same calendar week — running ahead of either super-event analog at this stage of development.
Wind forcing has not kept pace. CWWA this week (152) is well below both super-event analogs (1997: 336, 2015: 322), even as SST and subsurface heat run hot. Recent warming is being driven more by accumulated subsurface heat (residual Kelvin-wave propagation) than by ongoing wind events.
Strongest WWB already in super-event territory. 2026's strongest westerly wind burst peaks at 23.9 m/s on 2026-04-12 — vs full-season peaks of 27.7 (1997) and 29.5 (2015). Peak amplitude is super-event-aligned even though cumulative wind energy is lagging.

Analog tracker

2026-27 trajectory vs reference El Niño events, plus the SEAS5 ensemble forecast (median + uncertainty bands) forward.

Read this week: at the JFM tick (month -1 since Mar 1), 2026 sits at −0.15°C. Both 1997 (−0.4°C) and 2023 (−0.3°C) were similarly cool at the same calendar point and went on to become super events; 2015 was already running ahead at +0.6°C in JFM. The takeaway: JFM position is a weak discriminator, ramp speed through MAM–AMJ matters more, and we won't see that until the next 1–2 ONI updates. The dashed line marks the ECMWF SEAS5 ensemble median forward to 2026-11 (peak +2.7°C); the shaded bands show the 25–75 and 5–95 percentile spreads across the 51-member ensemble.

Physical state

Current observations vs the same calendar week in past super-event develop years.

Indicator	Current week of 2026-05-18	1997 same week	2015 same week
Niño 3.4 weekly (traditional)	+0.9°C	−0.1°C	+0.6°C
Niño 3.4 weekly (RONI)	+0.5°C	n/a (pre-RONI)	n/a (pre-RONI)
0–300 m heat content anomaly	+2.24°C	+0.7°C	+1.6°C
Cumulative westerly wind anomaly since Mar 1 CWWA, ERA5 5°N–5°S, 130°E–150°W, m/s·days	152	336	322

Heat content: Above-average and rising. Qualitatively the warmest since Jun 2023; comparable to spring of 2015, well short of spring 1997. New downwelling Kelvin wave initiated in March 2026.

CWWA: Live ERA5 daily 850 hPa zonal wind through 2026-05-15, area-meaned over 5°N–5°S, 130°E–150°W and integrated for positive (westerly) anomalies vs the 1991-2020 same-calendar-day climatology. Higher = more cumulative westerly forcing on the equatorial Pacific, the mechanism that excites downwelling Kelvin waves and drives moderate-to-super event escalation. At the same calendar date, 2026 CWWA (152) tracks closest to 2023 (45); other reference years: 1997 (336), 2015 (322), 2025 (0).

Impact outlook

Aggregation of institutional impact ranges for the developing event. Probabilities below are from named external sources, conditional on the headline strong-to-super case in section 1 materializing.

Mediterranean

Spain, Portugal, Italy, Greece, southern France. Probability of severe summer 2026 heat and drought: high, >70% Iberia, ~65% Italy/Greece/southern France. The 2024 July Mediterranean heatwave was characterized by World Weather Attribution as "virtually impossible without human-caused climate change". A 2003-magnitude European heat event (~70,000 excess deaths) is rated medium probability (~25-30%) on a strong El Niño compounded with the multi-year Mediterranean drought baseline.

Amazon basin

Probability of major drought 2026: high (>70%); probability of fire season exceeding 2024 hotspot levels: medium-high (~50%). NASA SERVIR characterized the 2023-24 drought magnitude as "roughly double" the 2015-16 event. The 2024 fire season produced a 76% increase in hotspots vs 2023.

Australia and the Great Barrier Reef

Probability of severe bushfire season austral summer 2026-27: high (>65%); GBR mass bleaching: very high (>85%); agricultural drought: high (>70%). The reef has bleached six times since 2016; another super event makes a sixth-in-eight-years bleaching baseline. Australian winter wheat is the cleanest El Niño short on record, with declines of 16% to 46% under prior strong events (1965, 1977, 1982, 1994, 1997, 2023).

Southern Africa

Probability of major drought repeat: ~70% if rains arrive late, per OCHA framing of the 2023-24 baseline ("worst impacts in 40 years"). Probability of a humanitarian appeal exceeding $5 billion: medium-high (~50%). Six SADC countries declared emergency in 2024; back-to-back is the asymmetric humanitarian risk.

India and South Asia

IMD's April 2026 monsoon outlook is 92% of the long-period average, the first below-normal April call since 2015. Of 16 historical El Niño years since 1950, 7 produced below-normal Indian monsoons (IMD MMCFS). Pre-monsoon heat already reached 43.8°C at Akola in mid-April 2026.

United States

California atmospheric river season: above-normal Pacific storm count winter 2026-27 high (~70%), with ~50% probability of a major atmospheric river damage event January-March 2027. Pacific Northwest: warmer-drier winter (~70%) with significant 2026 fire season (~50%). Atlantic hurricane season: high probability (~70%) of below-normal activity from El Niño wind shear, partially offset by warm Atlantic SSTs (2023 produced 20 named storms despite an El Niño base state). Southern Plains drought relief: low-medium (~25-30%); the 2023-24 super event underdelivered there.

Southeast Asia

Significant drought in Indonesia: high (>70%); palm oil production decline: medium-high (~55%). The 2015-16 super event delivered a 13.2% Malaysian palm oil production decline at a 12-month lag. Vietnamese coffee output fell 20% in 2023-24.

Global coral

The 2023-25 fourth global bleaching event already affects ~84% of world reefs (International Coral Reef Initiative, April 2025). Continued mass bleaching across all tropical basins is essentially certain into 2026-27.

Source-by-source check

What each agency said this week, verbatim where useful.

NOAA CPC strength table, NDJ 2026-27 (RONI)issued 2026-05-14
super 37%, strong 30%, moderate 22%, weak El Niño 9%, neutral 2%, La Niña 0%.
IRI plume, DJF 2026-27issued 2026-05-19
El Niño 98%, neutral 2%, La Niña 0%. Strength not broken out in the public Quick Look.
BoM ENSO Outlookissued 2026-05-12
Tropical Pacific continues to warm as models suggest a transition to El Niño during winter. Categorical only.
ECMWF SEAS5run 2026-05-01
51-member SEAS5 ensemble for 2026-11: median Niño 3.4 anomaly +2.68 deg C; 51/51 members above +1.5 (~100% above +2.0, ~75% above +2.5).

Caveats this issue

The CPC anchor for the +2.5°C bucket carries a 30–33% range. It comes from a bootstrap that perturbs CPC's published bin probabilities by Gaussian noise (sigma 1 percentage point, matching CPC's whole-percent reporting precision) and refits the skew-normal each time. The range reflects reporting-quantization uncertainty in CPC's table, not underlying forecast uncertainty. The smoothed headline (40%) sits above this anchor range because the bounded SEAS5 deflection (+8.7pp this issue) lifts it.
ECMWF SEAS5 vs CPC anchor on the upper tail above +2.5°C trad ONI: SEAS5 has 38/51 members (75%) at 2026-11 (max available lead); the CPC anchor lands at 30–33%. The v1.5 smoothing absorbs 20% of this gap (capped at ±10pp/week), moving the smoothed headline to 40%. We subtract SEAS5's own model climatology, which removes its known ENSO warm bias; an observational-climatology subtraction would put SEAS5 higher still. For broader context, multi-model pools (e.g., the Climate Brink dashboard's 13-model 637-member view) currently report a meaningfully higher probability for the same threshold; the gap reflects CPC's analyst-correction vs raw multi-model breadth, documented as methodology limitation #7.
Spring predictability barrier: April–May forecasts at any of these centers carry materially wider error bars than what we'll see in July–August. Treat all numbers as preliminary.