Warm East, Cool West: CPC's End-of-August Read on a Strengthening El Niño

NOAA's Climate Prediction Center (CPC) kept its ENSO Alert System Status at El Niño Advisory in its end-of-month briefing dated 31 August 2026, reporting that El Niño is present and strengthening and that equatorial sea surface temperatures (SSTs) are above average across the east-central and eastern Pacific Ocean. The agency also repeated its headline probability: a greater than 90% chance of a very strong event during Northern Hemisphere fall and winter 2026-27.

The most striking feature of the update is how uneven the warming is along the equator. CPC's latest weekly SST departures were +0.1 °C in the Niño 4 region, +1.8 °C in Niño 3.4, +2.6 °C in Niño 3 and +3.5 °C in Niño 1+2 — a gradient that steepens sharply toward the South American coast. Over the previous four weeks, the agency said, equatorial SSTs were above average east of the Date Line and below average to the west, and since mid-June below-average equatorial SSTs have expanded as far as the Date Line.

How the event got here

CPC traced the sequence back through the year: through February 2026 below-average SSTs covered most of the equatorial Pacific; above-average SSTs emerged in the far eastern equatorial Pacific in early February 2026; and since mid-April 2026 above-average equatorial SSTs have generally increased across the east-central and eastern Pacific.

Below the surface, CPC reported that upper-ocean heat anomalies are above average and the thermocline slope index is below average — the configuration the agency associates with warm episodes. Negative subsurface temperature anomalies emerged in mid-July 2025 and persisted through mid-December 2025, according to the deck; positive anomalies then developed and increased through late April 2026, eased into late May, increased again to late July 2026, and have since remained steady. CPC also catalogued equatorial oceanic Kelvin wave activity, with upwelling waves initiated in August and October 2025 and downwelling waves in December 2025, January 2026, March 2026 and June 2026.

The atmosphere, CPC said, is behaving consistently with El Niño. Over the last 30 days, below-average outgoing longwave radiation — indicating enhanced convection and rainfall — was evident along the equator from near the Date Line to the eastern Pacific, while above-average OLR, indicating suppressed convection, was observed over the 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.

An index bookkeeping change, and the outlook numbers

Several slides deal with the Relative Oceanic Niño Index (RONI), which CPC describes as helping "to place current events into a historical perspective." RONI subtracts the tropical mean (20°S-20°N) SST departure from the Niño 3.4 departure and applies a three-month running mean. CPC noted a dataset change: as of 10 August 2026, with NCEI discontinuing ERSSTv5, RONI values have switched to ERSST.v6, while weekly SST monitoring continues to use OISSTv2.1. The most recent RONI value, for May–July 2026, is 1.0 °C, up from 0.5 °C in April–June and 0.0 °C in March–May. The deck restates the historical episode requirement that thresholds be exceeded for at least five consecutive overlapping three-month seasons.

On forecasts, CPC's probabilistic outlooks updated 13 August 2026 give a greater than 90% chance of a very strong El Niño from September–November through November–January 2026-27, and a 69% chance during October–December 2026 of a historic event exceeding the strength of previous El Niño events dating back to 1950. El Niño is expected to persist through Northern Hemisphere spring 2027. Separately, the NCEP CFS.v2 model forecast issued 31 August 2026 — model output rather than observation — has an ensemble mean that favors El Niño continuing through Northern Hemisphere winter 2026-27.

For the United States, CPC noted that since late July above-average heights and temperatures have been evident over the west-central contiguous states, and that its September–November 2026 seasonal outlooks combine long-term trends, soil moisture and, where appropriate, ENSO.