Seasonal Forecast for September – November 2025
Updated 3 September 2025
In August 2025, Southwest Monsoon conditions persisted over the ASEAN region, characterised by wet weather over the northern ASEAN region and drier conditions over the southern ASEAN region. The low-level winds over the region blew mainly from the southeast or southwest. From the latter half of October, the Southwest Monsoon conditions typically transition into the inter-monsoon period as the monsoon rainband shifts southwards towards the Equator. During this period, the equatorial region usually experiences increased shower activity and the prevailing winds are likely to turn light and variable in direction.
For the September to November (SON) 2025 period, above-normal rainfall is predicted for much of the Maritime Continent and eastern Mainland Southeast Asia. ENSO neutral conditions are predicted for September, with the potential for La Niña conditions to develop in October – November 2025. There are signs of a negative Indian Ocean Dipole (IOD) developing, with models predict negative IOD conditions throughout September – November 2025. Both La Niña and negative IOD events tend to bring wetter conditions to parts of Southeast Asia. Above-normal temperature is predicted for much of Southeast Asia for SON 2025.
The traditional dry season over the southern ASEAN region is likely to last until October 2025. While above-normal rainfall is forecast over most parts of the region, drier conditions can still be expected at times, particularly over Peninsular Malaysia and the northern and central parts of Sumatra, where below-normal rainfall is forecast. There remains a risk of transboundary smoke haze as hotspots with smoke plumes/haze may develop in fire-prone areas, particularly during periods of drier conditions. On the other hand, over the northern ASEAN region, the overall hotspot and smoke haze situation is expected to remain subdued under the wet weather conditions throughout the outlook period.
The qualitative outlook is assessed for the region in general and based on the latest runs from models provided by the SEA RCC-Network LRF node. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.
For September 2025, above-normal rainfall is predicted over most of the Maritime Continent, apart from the northwestern and northeastern regions.
For September 2025, above-normal rainfall is predicted over most of the Maritime Continent based on the multi-model ensemble (Figure 1), with most models (more than six models, Figure 2) predicting above-normal rainfall as the dominant tercile for the southern region. Some models (4 – 6) predict below-normal rainfall over the northwestern Maritime Continent, while there is little to no model agreement over the northeastern Maritime Continent.
For Mainland Southeast Asia, above-normal rainfall is predicted over much of the central region by some models (4 – 6 models in agreement), with most models (more than six models) predicting above-normal rainfall over the northeastern part.
Model skill is good for predicting above-normal rainfall over the southern Maritime Continent, low to moderate for predicting above-normal rainfall elsewhere in Southeast Asia, and low to moderate for predicting below-normal rainfall over the western Maritime Continent.
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| Figure 1: Rainfall tercile summary predictions of the multi model ensemble for September 2025 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for September 2025. Where there no dominant tercile, it refers to the number of models with near-normal tercile (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological rainfall’s lower tercile boundary for September based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for September based on CHIRPS (reference period: 1991-2020). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for rainfall, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
For SON 2025, models predict above-normal rainfall over much of the Maritime Continent and eastern Mainland Southeast Asia.
For SON 2025, above-normal rainfall is predicted for much of the Maritime Continent, based on the multi-model ensemble (Figure 1). The highest probability of above-normal rainfall is over the southern Maritime Continent. Most models agree on above-normal rainfall as the most likely tercile for much of the southern and eastern parts of the region (more than six models, Figure 2). Below-normal rainfall is predicted over the northwestern Maritime Continent by most models. Model skill for predicting above-normal rainfall is good for the southern, central and southeastern Maritime Continent and low to moderate elsewhere. Model skill for predicting below-normal rainfall is moderate over the northwestern Maritime Continent.
For Mainland Southeast Asia, above-normal rainfall is predicted apart from over the northwestern region. Most models agree on above-normal rainfall over the eastern portion (more than 6 models) with only some models agreeing for the central portion (4 – 6 models). No dominant tercile is predicted for northwestern Mainland Southeast Asia. Model skill is moderate to good for southeastern Mainland Southeast Asia and low to moderate elsewhere in the region.
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| Figure 1: Rainfall tercile summary predictions of the multi model ensemble for SON 2025 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for SON 2025. Where there no dominant tercile, it refers to the number of models with near-normal tercile (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological rainfall’s lower tercile boundary for SON based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for SON based on CHIRPS (reference period: 1991-2020). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for rainfall, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
For September 2025, above-normal temperature is predicted over Southeast Asia.
For September 2025, above-normal temperature is predicted over Southeast Asia (Figure 1). The likelihood of above-normal temperature is high, apart from over the northern half of Mainland Southeast Asia and parts of the central Maritime Continent. Most of the models (more than six models, Figure 2) predict above-normal temperature as the most likely tercile, apart from over parts of the central Maritime Continent. Model skill for predicting above-normal temperature is relatively good for most of Southeast Asia.
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| Figure 1: Temperature tercile summary predictions of the multi model ensemble for September 2025 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for September 2025. Where there no dominant tercile, it refers to the number of models with near-normal tercile (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological temperature for September based on ERA5 (reference period: 1991-2020). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for temperature, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
For SON 2025, above-normal temperature is predicted over much of Southeast Asia.
For SON 2025, above-normal temperature is predicted over most of Southeast Asia (Figure 1). Where above-normal temperature is predicted, most models (more than six models, Figure 2) predict above-normal temperature as the most likely tercile. Over parts of central and eastern Mainland Southeast Asia and the central Maritime Continent, either no dominant or near-normal temperature is predicted. There is less agreement between models for these regions, with only some models (4 – 6) predicting near-normal temperature. Model skill for predicting above-normal temperature is good for most of Southeast Asia.
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| Figure 1: Temperature tercile summary predictions of the multi model ensemble for SON 2025 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for SON 2025. Where there no dominant tercile, it refers to the number of models with near-normal tercile (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological temperature for SON based on ERA5 (reference period: 1991-2020). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for temperature, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
Frequently Asked Questions
What is a seasonal forecast?
A seasonal forecast provides forecast information about the general weather conditions at seasonal timescales up to several months ahead. Long range predictions rely on predicting components of the atmosphere and the oceans which show variations at large spatial and temporal scales. Among these key components is the sea-surface temperature – an important indicator in the El Niño Southern Oscillation (ENSO) cycle.
Example of probabilistic long range forecast using the multi-model ensemble. For areas nearby Peninsular Malaysia, the forecast is showing a probability of 50-70% of rainfall being in the below-normal (33%) tercile range. (Source: WMO LC LRF)
To account for the chaotic nature of the atmosphere, seasonal forecast adopts a probabilistic approach and the forecast weather condition is presented as an average over an extended period of time, usually with reference to climatology.
ASMC carries out monthly seasonal forecast predictions based on various seasonal model forecasts from the leading meteorological centres, supplemented by individual assessments from respective countries in South East Asia. The skillfulness of seasonal forecast relies greatly on the performance of seasonal models, which differ from one place to another and for different seasons of the year.
Receiver operating characteristic (ROC) score of UK GloSea5 seasonal hindcast for above-normal precipitation scenario for Dec – Feb (left) and May – Jul (right) over Asia region. Higher ROC score (indicated in red) means better model skill. (Source: UK Met Office)
For example, one way to assess the seasonal model skills is to examine the ROC score. From the figures above, we can see that ROC score map differs greatly from one season to another. During the period from Dec to Feb, a high ROC score region is observed mainly over the Philippines whereas during the period from May to Jul, a high ROC score region is observed over the near equatorial region such as Peninsular Malaysia and Sumatra.