December 2024 Climate Forecast Discussion for the period Jan-Mar 2025 through Apr-Jun 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 November 2024, slightly negative sea surface temperature (SST) anomalies persisted over the central-eastern equatorial Pacific Ocean, leading to a NINO3.4 anomaly index value of -0.14 °C. The weekly-averaged SST anomalies in the NINO3.4 region were -0.6 °C for the week centered on Dec 11, 2024. This indicates an ongoing ENSO-neutral state in the equatorial Pacific Ocean. According to the early-December CPC ENSO forecast, La Niña is favored to emerge in Nov-Jan, 2025 with 59% chances, while a transition to ENSO-neutral is most likely by March-May 2025 with 61% chances. The Indian Ocean Dipole (IOD) index is neutral, and the North Atlantic Ocean continues to exhibit persistent warm sea surface temperature (SST) anomalies.
IRI's seasonal precipitation forecasts issued in December 2024 indicate moderately elevated chances of above-normal rainfall over parts of the northwest and Midwest US, Central America, Siberia, northern parts of South America, the Maritime Continent, and parts of Australia during Jan-Mar 2025. These probabilities become quite weak during Feb-Apr, Mar-May, and Apr-Jun 2025 over the above-mentioned regions. An increased probability of above-normal precipitation is also forecasted for South Asia and Southeast Asia during Mar-May, and Apr-Jun 2025. An increased probability of below-normal precipitation is forecasted for most of the Southern U.S. and Mexico for most of the forecast period. The increased probability of below-normal precipitation is also forecasted over various parts of South America, including tropical northeast South America, southern Chile, and the Middle East during the forecast period, although the strength of this probability is quite weak. Many (while not all) of these precipitation patterns align with historical La Niña events and are consistent with the La Niña forecast in boreal winter and early spring of 2025.
Regarding temperature, strongly enhanced probabilities for above-normal temperatures are forecasted across most land areas throughout the forecast period. The forecast shows elevated probabilities (though weak) of below-normal temperatures over parts of the western Canada and Alaska region during Jan-Mar 2025. The temperature forecasts indicate slightly elevated chances of below-normal temperatures during Feb-Apr 2025 over western and central US, northern and central parts of South America, equatorial east Africa, western Europe, South Asia, Southeast Asia, and the Australia region.
