During September 2024, ENSO-neutral continued with near-average sea surface temperatures (SSTs) observed across most of the central and eastern equatorial Pacific Ocean [Fig. 1]. Similar to this time last month, the latest weekly Niño indices ranged from +0.2°C (Niño-4) to -0.4°C (Niño-1+2; [Fig. 2]). Below-average subsurface temperatures persisted [Fig. 3] across the east-central and eastern equatorial Pacific Ocean [Fig. 4]. Low-level wind anomalies were easterly over the east-central equatorial Pacific, and upper-level wind anomalies were westerly over the eastern Pacific. Convection was near average over Indonesia and was slightly suppressed over the Date Line [Fig. 5]. Collectively, the coupled ocean-atmosphere system reflected ENSO-neutral.

The IRI plume predicts a weak and a short duration La Niña, as indicated by the Niño-3.4 index values less than -0.5°C [Fig. 6]. The latest North American Multi-Model Ensemble (NMME) forecasts were warmer this month, but still predict a weak La Niña. As a result of the warmer predictions and the recent weakening of equatorial trade winds, the team still favors a weak event, but has lowered the chances of La Niña. A weaker La Niña implies that it would be less likely to result in conventional winter impacts, though predictable signals could still influence the forecast guidance (e.g., CPC's seasonal outlooks). In summary, La Niña is favored to emerge in September-November (60% chance) and is expected to persist through January-March 2025 [Fig. 7].