Mid-September ENSO Deck: Niño 3.4 at 2.0 °C, 55% Neutral Odds by Mid-2027

The NOAA Climate Prediction Center has kept its ENSO Alert System Status at El Niño Advisory in a monitoring update prepared on 14 September 2026, reporting that El Niño is present and strengthening, with a greater than 90% chance of a very strong event during Northern Hemisphere fall and winter 2026-27.

In the update, CPC states that equatorial sea surface temperatures are above average across the east-central and eastern Pacific Ocean, and that atmospheric circulation anomalies over the equatorial Pacific are consistent with El Niño. The agency notes that its summary statements are refreshed once a month in association with its ENSO Diagnostics Discussion.

A sharp east-west split in the index values

The latest weekly sea surface temperature departures reported by CPC are -0.1 °C in the Niño 4 region, 2.0 °C in Niño 3.4, 2.8 °C in Niño 3 and 3.7 °C in Niño 1+2. That gradient matches the agency's description of the past four weeks, when equatorial temperatures ran above average east of the Date Line and below average near and west of it.

CPC's four-week change analysis shows positive equatorial anomaly changes across the eastern Pacific, in most of the Atlantic and in parts of the Indian Ocean, with negative changes near Indonesia and the west-central equatorial Pacific. Over the same window, the agency reports above-average sea surface temperatures across most of the global oceans, with near-average values around Indonesia.

The deck also carries the Relative Oceanic Niño Index, a relative diagnostic that CPC constructs by subtracting tropical-mean (20°S–20°N) sea surface temperature departures from those in the Niño 3.4 region, rescaling the variance to match the original Niño 3.4 index and applying a three-month running mean. CPC says the index helps place current events in historical perspective. Its most recent value, for June–August 2026, is 1.4 °C, following 0.0 °C in March–May, 0.5 °C in April–June and 1.0 °C in May–July. CPC's stated standard for classifying a full-fledged El Niño or La Niña episode is that the ±0.5 °C threshold be exceeded for at least five consecutive overlapping three-month seasons. The agency also flags a dataset change: on 10 August 2026 RONI values switched from ERSSTv5 to ERSSTv6, as NCEI is discontinuing ERSSTv5, while the weekly OISSTv2.1 data used for weekly monitoring is unaffected.

Ocean heat and atmospheric signals

Below the surface, CPC reports that upper-ocean heat anomalies are above average and the thermocline slope index is below average — the configuration the agency associates with Pacific warm episodes. Above-average subsurface temperatures have persisted across the east-central and eastern Pacific over the past two months. CPC traces the longer sequence: negative subsurface anomalies emerged in mid-July 2025 and persisted through mid-December 2025; positive anomalies then developed and increased into late April 2026, eased from late April to late May, increased again from late May to late July 2026, and have since remained steady. Downwelling equatorial Kelvin waves were initiated in December 2025, January 2026, March 2026 and June 2026, following upwelling waves in August and October 2025.

Over the last 30 days, CPC observed below-average outgoing longwave radiation — indicating enhanced convection and rainfall — along the equator from near the Date Line to the eastern Pacific, and above-average OLR, indicating suppressed convection, over the southern Philippines, Indonesia and Papua New Guinea. Low-level 850-hPa westerly wind anomalies extended from the western to the east-central equatorial Pacific, with easterly anomalies over the eastern Pacific; at 200 hPa, anomalies were easterly from the western to east-central Pacific and westerly over the far-eastern Pacific.

Outlook into 2027

CPC's probability panels, which carry an update date of 10 September 2026 — four days before the deck itself was prepared — show El Niño expected to persist through March–May 2027, with a 55% chance of ENSO-neutral conditions in April–June 2027. The agency's strength outlook gives a greater than 90% chance of a very strong El Niño from September–November through November–January 2026-27, and for October–December 2026 a 75% chance of a historic event that would exceed the strength of previous El Niño events dating back to 1950.

Model guidance issued on 14 September points the same way. CPC presents the NCEP CFS.v2 ensemble mean as PDF- and spread-corrected forecast output, not observation, and says it favours El Niño continuing through Northern Hemisphere winter 2026-27. For the United States, the deck notes that since late July above-average heights and temperatures have been evident over the central contiguous states, while the North Pacific height pattern has been variable since mid-July. CPC says its seasonal outlooks combine long-term trends, soil moisture and, where appropriate, ENSO.