CPC's September Deck: Niño 1+2 at 3.9 °C, Niño 4 Barely Above Average

NOAA's Climate Prediction Center left its ENSO Alert System at El Niño Advisory in its monthly slide update dated 8 September 2026, reporting that El Niño is present, is strengthening, and carries a greater than 90% chance of becoming a very strong event during the Northern Hemisphere fall and winter of 2026-27.

The most striking feature of the update is the size of the east-to-west contrast across the equatorial Pacific. The CPC lists the latest weekly sea surface temperature departures as 3.9 °C in the Niño 1+2 region off South America, 2.7 °C in Niño 3, 1.8 °C in Niño 3.4 and just 0.1 °C in Niño 4, the westernmost of the four monitoring boxes. Over the previous four weeks, according to the agency, equatorial SSTs were above average east of the Date Line and below average near and west of it.

An east-loaded warming

The CPC's chronology of 2026 shows how quickly the picture changed. Through February 2026, the agency reports, below-average SSTs covered most of the equatorial Pacific; above-average water then appeared in the far eastern basin in early February, and above-average anomalies increased across the eastern Pacific from mid-April onward. During June and July 2026, below-average equatorial SSTs expanded westward to the Date Line — the cool western signal that still shows in the Niño 4 figure.

Below the surface, the CPC says above-average subsurface temperatures have strengthened across the east-central and eastern Pacific over the last two months, with upper-ocean heat content anomalies above average and the thermocline slope index below average. Positive subsurface anomalies increased from late May to late July 2026 and have since held steady. The agency records downwelling oceanic Kelvin waves initiated in December 2025, January 2026, March 2026 and June 2026, following upwelling waves in August and October 2025.

The atmosphere, in the CPC's assessment, is behaving consistently with El Niño. Over the last 30 days the agency observed below-average outgoing longwave radiation — indicating enhanced convection and rainfall — along the equator from near the Date Line eastward, and above-average OLR, meaning suppressed convection, over the central and southern Philippines, Indonesia and Papua New Guinea. Low-level 850-hPa westerly wind anomalies extended from the western to the east-central equatorial Pacific.

Indices and a dataset change

The update also documents a change in the data behind one of the CPC's diagnostics. As of 10 August 2026, the agency notes, NCEI is discontinuing ERSSTv5, so Relative Oceanic Niño Index values have switched to ERSSTv6; the weekly OISSTv2.1 analysis used for the Niño-region monitoring slides is unaffected, and both tables remain available on the CPC website.

RONI is defined in the deck as Niño 3.4 anomalies with the 20°S-20°N tropical mean SST departure removed, variance-adjusted and smoothed with a three-month running mean. The most recent value, for June-August 2026, is 1.4 °C. The CPC's table shows the index climbing through 2026 from -0.9 °C in December-February to 0.0 °C in March-May, 0.5 °C in April-June, 1.0 °C in May-July and 1.4 °C in June-August. The deck's statement of NOAA's operational definitions requires the ±0.5 °C threshold to be met for at least five consecutive overlapping three-month seasons for a full-fledged episode to be classified historically.

What the forecasts say

The CPC's probabilistic products, updated 13 August 2026, put El Niño as expected to persist through Northern Hemisphere spring 2027. For September-November through November-January 2026-27 the agency gives a greater than 90% chance of a very strong El Niño, and for October-December 2026 a 69% chance of a historic event that would exceed the strength of previous El Niño episodes dating back to 1950.

Model guidance points the same way. The NCEP CFS.v2 forecast issued 8 September 2026 — model output rather than observation — has an ensemble mean favouring El Niño continuing through the Northern Hemisphere winter of 2026-27. The CPC says its seasonal U.S. outlooks for September-November 2026 combine long-term trends, soil moisture and, where appropriate, ENSO. The full deck is published by the Climate Prediction Center, and the summary statements are refreshed on the second Thursday of each month alongside the ENSO Diagnostics Discussion.