Sep 2024 Climate Forecast Discussion for the period Oct-Dec 2024 through Jan-Mar 2025
**Note: ** The IRI seasonal forecasts of precipitation and temperature issued this month are based on an objective calibration procedure that combines the NCEP-CFSv2, CanSIPS-IC4, COLA-RSMAS-CESM1, GFDL-SPEAR, and NASA-GEOSS2S models. 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 August 2024, slightly negative sea surface temperature (SST) anomalies appeared over the central-eastern equatorial Pacific Ocean, leading to a NINO3.4 anomaly index value of -0.07 °C. The weekly-averaged SST anomalies in the NINO3.4 region were -0.2 °C for the week centered on Sep 11, 2024. This indicates an ongoing ENSO-neutral state in the equatorial Pacific Ocean. According to the early-September CPC ENSO forecast, La Niña is favored to emerge in Sep-Nov, 2024 with 71% chance, and is expected to stay through Jan-Mar 2025. The Indian Ocean Dipole (IOD) index is currently neutral. The North Atlantic Ocean continues to exhibit persistent warm SST anomalies that are forecast to continue for the next couple of months, while the Indian Ocean Dipole is forecasted to remain in the neutral state.
IRI’s seasonal precipitation forecasts issued in September 2024 indicate higher chances of above-normal rainfall in Central America, the Caribbean, western side of equatorial East Africa, Southeast Asia, the Maritime Continent, parts of China, and Australia during Oct-Dec, 2024. These elevated probabilities become very weak during the rest of the forecast period over the above-mentioned regions. From Nov-to-Mar, 2025 the forecasts show a shift of odds towards the above-normal precipitation over the northern parts of South America. An increased probability of below-normal precipitation is forecasted for most of the Southern U.S. and Mexico for most of the forecast period. An elevated probability of below-normal precipitation is also forecasted over various parts of South America, including tropical northeast South America, areas around Uruguay, southern Chile, scattered parts in equatorial East Africa and the Middle East during the forecast period, although the strength of this probability decreases towards the end of the forecast period. Many, while not all, of these precipitation patterns align with historical La Niña events and are consistent with the forecasted onset of La Niña in boreal autumn and persistence during the boreal winter.
Regarding temperature, strongly enhanced probabilities for above-normal temperature are forecasted across most land areas throughout the forecast period. The forecast shows elevated probabilities (though weak) of below-normal temperatures over parts of western Canada, the central U.S. and Alaska into the boreal winter of 2024-25.
