August 2024 Climate Forecast Discussion

Aug 2024 Climate Forecast Discussion for the period Sep-Nov 2024 through Dec-Feb 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 July 2024, weakly positive sea surface temperature (SST) anomalies persisted over the central-eastern equatorial Pacific Ocean, leading to a NINO3.4 anomaly index value of +0.21 °C. The weekly-averaged SST anomalies in the NINO3.4 region were 0.0 °C for the week centered on Aug 07, 2024. This indicates an ongoing ENSO-neutral state in the equatorial Pacific Ocean. According to the early-August CPC ENSO forecast, ENSO-neutral is expected to continue for the next several months, with an onset of La Niña during Sep-Nov, 2024 (66% chances). 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 August 2024 indicate higher chances of above-normal rainfall in Central America, the Caribbean, East Africa, South and Southeast Asia, the Maritime Continent, parts of China, and Australia during the forecast period. These probabilities gradually decrease generally through the forecast period, with the notable exception of northern parts of South America which show strongly elevated probabilities of above-normal precipitation during Nov-Jan, and Dec-Feb, 2025. An increased probability of below-normal precipitation is forecasted for the western and central parts of the U.S. and most of Mexico for Sep-Nov 2024, while the rest of the forecast period shows a shift towards below-normal precipitation for most of the southern U.S. and northern Mexico. An elevated probability of below-normal precipitation is also forecasted over various parts of South America, including northeast South America, areas around Uruguay, southern Chile, and the Middle East during the forecast period, although the strength of this probability decreases towards the end of the period. Many, while not all, of these precipitation patterns align with historical La Niña events, in line with the forecasted onset of La Niña in boreal autumn.

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, and Alaska during the forecast period.