Seasonal Forecast for August – October 2026
Updated 4 August 2026
The Southwest Monsoon conditions typically persist for most of the outlook period of August – October (ASO) 2026 with the prevailing low-level winds blowing mainly from the southeast or southwest. With the monsoon rain band positioned north of the Equator, the dry season for the southern ASEAN region is expected to continue while the northern ASEAN region experiences wet weather conditions. Around October, the prevailing winds typically become light and variable in direction as the region gradually transitions into inter-monsoon conditions.
For August 2026, below- to near-normal rainfall is predicted over most of the southern ASEAN region. During August – October (ASO) 2026, below-normal rainfall is predicted to persist for the southern ASEAN region with high likelihood, with below-normal rainfall predicted for much of the northern ASEAN region. El Niño conditions are now present and predicted to continue strengthening over the next few months. El Niño conditions tend to lead to an increased chance of below-normal rainfall for much of the southern ASEAN region during August – October. During August and August – October (ASO) 2026, above-normal temperature is expected over most of the ASEAN region.
Under El Niño conditions and below-normal rainfall expected over most of the southern ASEAN region during the outlook period, the hotspot and smoke haze situation could escalate, heightening the risk of transboundary haze under the prevailing southeasterly or southwesterly winds. As El Niño conditions are predicted to strengthen during the outlook period, there is a higher likelihood of extended periods of persistent dry conditions, thereby further increasing the risk of transboundary haze. Over the Mekong sub-region, the situation is expected to remain mostly subdued. However, the risk of hotspot activity may increase over areas with expected below-normal rainfall conditions.
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 August 2026, below-normal rainfall is predicted over most of the southern ASEAN region and southern Mainland Southeast Asia.
For August 2026, below-normal rainfall is predicted over the southern ASEAN region, based on the multi-model ensemble (Figure 1). The highest probability of below-normal rainfall is over the central and eastern equatorial region, where most of the models (more than six models, Figure 2) predict below-normal rainfall as the dominant tercile. Model skill for below-normal rainfall is good over the eastern equatorial region and moderate elsewhere over the Maritime Continent.
For the northern ASEAN region, below-normal tercile is predicted for parts of southern Mainland Southeast Asia and southern Philippines, with above-normal tercile or no dominant tercile predicted elsewhere. There is good agreement between models (more than six models) for the coast of western Mainland Southeast Asia and southern Philippines, with some agreement between the models (four to six models) elsewhere in general. Model skill is low over both southern and eastern Mainland Southeast Asia.
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| Figure 1: Rainfall tercile summary predictions of the multi model ensemble for August 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for August 2026. 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 August based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for August 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 ASO 2026, models predict below-normal rainfall for most of the ASEAN region.
For ASO 2026, an increased chance of below-normal rainfall is predicted for most of the southern ASEAN region, in particular over the eastern, central, and southern Maritime Continent region, while and above-normal rainfall is predicted over parts of the northwestern Maritime Continent based on the multi-model ensemble (Figure 1). Most models (more than six models) are in agreement with below-normal as the dominant tercile for much of the region. Model skill is good over the eastern and central region and low to moderate over the western part at this time of the year.
For the northern ASEAN region, below-normal rainfall is predicted for much of the region, apart from the northeastern parts where a mix of below- to above-normal tercile is predicted. Most models (more than 6 models) are in agreement regarding an increase in chance of below-normal rainfall over regions of higher likelihood predicted, with little to some models in agreement elsewhere. Model skill is low for both below- and above-normal rainfall, apart from over southern Mainland Southeast Asia, where skill is low to moderate.
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| Figure 1: Rainfall tercile summary predictions of the multi model ensemble for ASO 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for ASO 2026. 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 ASO based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for ASO 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 August 2026, above-normal temperature is predicted for most of the ASEAN region.
For August 2026, above-normal temperature is predicted over most of the ASEAN region, with the highest likelihood over most of the Maritime Continent, and southern parts of Mainland Southeast Asia, based on the multi-model ensemble (Figure 1). Most of the models (more than six models, Figure 2) predict above-normal temperature as the most likely tercile for almost the entire ASEAN region, apart from the southeastern part of the region with some models in agreement (4 to 6 models). Model skill for predicting above-normal temperature is good for much of the region, apart from the northernmost part where it is low to moderate.
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| Figure 1: Temperature tercile summary predictions of the multi model ensemble for August 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for August 2026. 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 August 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 ASO 2026, models predict above-normal temperature over most of the ASEAN region.
For ASO 2026, above-normal temperature is predicted over most of the ASEAN region, apart from the southeastern part of the region where near- to above-normal temperature is predicted, based on the multi-model ensemble (Figure 1). Most models (more than six models) are in agreement for the region predicting above-normal temperature as the most likely tercile, with little to some models in agreement elsewhere (Figure 2). Model skill for predicting above-normal temperature is good at this time of the year.
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| Figure 1: Temperature tercile summary predictions of the multi model ensemble for ASO 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for ASO 2026. 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 ASO 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.