Seasonal Forecast for May – July 2026
Updated 30 April 2026
The inter-monsoon conditions typically persist over the ASEAN region during May, characterized by prevailing winds that are light and variable in direction in most areas of the region. While scattered to widespread rainfall is typical during this period, brief dry spells may still occur in various parts of the region. From late May onwards, the Southwest Monsoon usually begins to establish itself, as the predominant winds turn to blow from the southeast or southwest. During this period, the traditional dry season for the southern ASEAN region gradually sets in as the monsoon rainband shifts further northwards.
For May 2026, below- to near-normal rainfall is predicted over much of the ASEAN region, apart from over parts of the western and central equatorial region where near- to above-normal rainfall is predicted. Extending to May – July (MJJ) 2026, below-normal rainfall is predicted for the southern ASEAN region. Above-normal temperature is predicted for most of the ASEAN region for May and MJJ 2026. ENSO-neutral conditions are predicted for May 2026, with either ENSO-neutral continuing or El Niño conditions developing in June-July 2026. El Niño conditions tend to lead to an increased chance of below-normal rainfall for much of the southern ASEAN region during June – July.
Even though below- to near-normal rainfall is expected over most of the northern ASEAN region, the return of shower activities at the start of the forecast period should help to gradually reduce the hotspot and smoke haze activities and the risk of transboundary haze occurrence across the Mekong sub-region. Over the southern ASEAN region, an increase in hotspot activity with smoke plumes can be expected as the traditional dry season sets in around the middle of the forecast period, especially over areas where below-normal rainfall is predicted. There is a risk of transboundary smoke haze occurrence over areas experiencing prolonged dry conditions during June and July as the prevailing winds blow from the southeast or southwest. The situation could intensify if El Niño develops.
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 May 2026, below- to near-normal rainfall is predicted over much of the ASEAN region, apart from over parts of the western and central equatorial region where near- to above-normal rainfall is predicted.
For May 2026, over the Maritime Continent, a mix of below-normal to near-to above-normal rainfall is predicted, based on the multi-model ensemble (Figure 1). The highest probability of below-normal rainfall is over the southern Maritime Continent, where most of the models (more than six models, Figure 2) predict below-normal rainfall as the dominant tercile. Below-to near-normal rainfall is predicted over the eastern Maritime Continent, with four to nine models agreeing on below-normal rainfall as the dominant tercile. Over the western and central equatorial region, near- to above-normal rainfall (with four to six models in agreement) or no dominant tercile is predicted. Model skill is moderate for above-normal rainfall over the equatorial region and for below-normal rainfall over the southern Maritime Continent, and moderate to good for the eastern Maritime Continent.
For Mainland Southeast Asia, below- to near-normal rainfall is predicted for much of the region, apart from the easternmost part, where there is no dominant tercile predicted. Where below-to near-normal rainfall is predicted, there is some agreement between the models (generally four to six models). Model skill is moderate to good for below-normal rainfall at this time of the year for this region.
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| Figure 1: Rainfall tercile summary predictions of the multi model ensemble for May 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for May 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 May based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for May 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 MJJ 2026, below-normal rainfall is predicted for the southern ASEAN region.
For MJJ 2026, an increased chance of below-normal rainfall is predicted for the southern ASEAN region, based on the multi-model ensemble (Figure 1). Most models (more than six models) agree with below-normal as the dominant tercile for much of the region, apart from over parts of the equatorial regions where only some models (4 – 6 models) agree. Model skill is moderate to good over the eastern half of the southern ASEAN region and low to moderate over the western half at this time of the year.
For the northern ASEAN region, no dominant tercile is predicted for much of the region, apart from some models (4 – 6 models) in agreement regarding an increase in chance of above-normal rainfall over the northeastern part. Model skill is low for the northernmost part of the northern ASEAN region, and low to moderate elsewhere at this time of the year.
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| Figure 1: Rainfall tercile summary predictions of the multi model ensemble for MJJ 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for MJJ 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 MJJ based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for MJJ 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 May 2026, above-normal temperature is predicted most of the ASEAN region.
For May 2026, above-normal temperature is predicted over most of the ASEAN region, with the highest likelihood in the equatorial region, 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 some of the southernmost part and a few other areas where only some models (four to six) agree. 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 May 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for May 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 May 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 MJJ 2026, above-normal temperature is predicted over most of the ASEAN region.
For MJJ 2026, above-normal temperature is predicted over most of the ASEAN region, apart from over the southernmost part where near-to above-normal temperature is predicted, based on the multi-model ensemble (Figure 1). Most models (more than six models) predict above-normal temperature as the most likely tercile apart from over the southernmost part where some models (four to six) agree that above-normal temperature is the most likely tercile (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 MJJ 2026 (contains modified Copernicus C3S information). | Figure 2: Number of models in agreement in the multi-model ensemble for the dominant tercile for MJJ 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 MJJ 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.