CCSR/IRI Seasonal Forecast Issued October 2026

Note: The CCSR/IRI seasonal forecasts of precipitation and temperature issued this month are based on an objective calibration procedure that combines forecasts from four models using the Extended Logistic Regression (ELR) method. The four NMME models are CanSIPS-IC4, CFSv2, COLA-RSMAS-CESM1, and NASA-GEOSS2S. The climatological base period for normal is 1991-2020. The SST anomalies cited below refer to the OISST.v2.1 SST data set. Starting from June 2024, the observed climatology used to compute the dry mask was updated to 1991-2020, and the criterion modified to mask points where 10% of the training sample (i.e. 3 or more years) received less than or equal to 1 mm over a 3-month season.

During September 2026, sea surface temperature (SST) anomalies strengthened further across the central and eastern equatorial Pacific, with the monthly Niño 3.4 index reaching +2.84°C, placing the ongoing El Niño event firmly in the “very strong” category. The latest weekly Niño 3.4 index, centered on 7 October 2026, rose to +3.3°C, indicating continued strengthening of El Niño conditions across the tropical Pacific. Notably, Niño 3.4 values have remained above +3.0°C over the past four weeks, highlighting the persistence and exceptional strength of the ongoing El Niño. Meanwhile, the Indian Ocean Dipole (IOD) has been in positive territory since September 2026, with the latest weekly value reaching +0.88°C for 5–11 October 2026.

The CCSR/IRI objective multi-model probability precipitation forecast for November–January (NDJ) 2026/2027 indicates increased probabilities of below-normal precipitation (yellow to brown shading) across northern South America, parts of Central America, South Africa, Southeast Asia, and the Maritime Continent. These dry signals persist through the forecast period (DJF to FMA) across several regions, although the intensity and spatial extent vary by region and forecast period. The probability of below-normal precipitation also increases across Alaska, northwestern Australia, and Madagascar during the DJF–JFM period. In contrast, the southern United States—including the Southwest, Southeast, and adjacent Southern Plains—along with Uruguay and surrounding areas, northeastern Africa, the Middle East, and parts of Eurasia show high probabilities of above-normal precipitation during the forecast period. Many of the observed wet and dry precipitation patterns are broadly consistent with the influence of an exceptionally strong El Niño across the central and eastern tropical Pacific. The latest observations already show Niño 3.4 index values exceeding +3.0°C, underscoring the event’s exceptional intensity, while forecasts indicate the potential for further strengthening toward its peak in fall 2026. This evolving ocean–atmosphere state raises the prospect of continued shifts in regional precipitation patterns. However, ENSO alone cannot explain all the observed anomalies. The developing positive Indian Ocean Dipole (IOD), together with regional atmospheric and oceanic processes, may further reinforce or modify rainfall patterns across affected regions.

The temperature forecasts indicate a high likelihood of above-normal temperature (red shading) across much of the globe throughout the forecast period. Of particular interest are regions showing increased probabilities of below-normal temperatures (blue shading), including Uruguay and surrounding areas during NDJ and DJF, and the southern and southeastern United States during JFM and FMA.