Seasonal Forecast for May – July 2025
Updated 2 May 2025
The Northeast Monsoon has transitioned into the inter-monsoon period during the first half of April 2025. Climatologically, the inter-monsoon conditions persist over the ASEAN region during May, marked by light winds that vary in direction across most areas in the region. While scattered to widespread rainfall is typical over the ASEAN region, brief dry spells can still occur in various parts of the region. By late May, the Southwest Monsoon usually begins to establish itself, as the predominant winds shift to blow from the southeast or southwest. During this period, the southern ASEAN region gradually experiences drier conditions while wet weather continues over the northern ASEAN region as the monsoon rainband shifts further northwards.
For May 2025, above-normal rainfall is predicted for much of the eastern Maritime Continent and near- to above-normal rainfall for parts of Mainland Southeast Asia. The above-normal rainfall is predicted to persist over May to July (MJJ), with below-normal rainfall predicted over the western Maritime Continent. ENSO-neutral conditions are predicted during May to June 2025. The Indian Ocean Dipole (IOD) is currently neutral. Above-normal temperature is predicted for the Maritime Continent and near- to above-normal temperature for Mainland Southeast Asia for May, and above-normal temperature for most of Southeast Asia for MJJ 2025.
The return of shower activities over most of the northern ASEAN region at the beginning of the forecast period should help reduce both the hotspot activity and transboundary haze occurrence across the Mekong sub-region. As the southern ASEAN region enters its typical dry season around the middle of the forecast period, an increase in hotspot activity with localised smoke plumes may be expected, particularly in areas with below-normal rainfall. There is also be a risk of transboundary smoke haze occurrence during June and July under the prevailing southeasterly or southwesterly winds, especially over areas experiencing extended dry 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 May 2025, above-normal rainfall is predicted for much of the eastern Maritime Continent and near- to above-normal rainfall for parts of Mainland Southeast Asia.
For May 2025, above-normal rainfall is predicted for much of the eastern Maritime Continent based on the multi-model ensemble (Figure 4). The ECMWF and UK Met Office models (Figures 2 and 3, respectively) predicted an increased chance of above-normal rainfall over much of the region, with the highest probability over the southeastern Maritime Continent. The NCEP model (Figure 1), predicts a mix of below- to above-normal rainfall. Elsewhere in the Maritime Continent, the NCEP model predicts predominantly an increase chance of below-normal rainfall, while the ECMWF and UK Met Office models predict near-normal rainfall or no dominant tercile. The model skill for above-and below-normal rainfall is good over the Philippines and moderate to good elsewhere in the region, while the skill for near-normal rainfall is low to moderate for the Maritime Continent.
For Mainland Southeast Asia, a mix of near-normal and above-normal rainfall is predicted for much of the region, based on the multi-model ensemble (Figure 4). The UK Met Office model (Figure 3) predicts an increased chance of above-normal rainfall for most of Mainland Southeast Asia, while the NCEP and ECMWF models (Figures 1 and 2, respectively), predict a mix of near- and above-normal rainfall or no domain tercile. The models’ skill is moderate to good for above-normal rainfall and low to moderate for near-normal rainfall for Mainland Southeast Asia at this time of the year.
|
|
| Figure 1: Rainfall tercile summary predictions of NCEP model for May 2025 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for May 2025 (contains modified Copernicus C3S information). |
|
|
| Figure 3: Rainfall tercile summary predictions of UK Met Office model for May 2025 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for May 2025 (contains modified Copernicus C3S information). |
|
|
| 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 2025, above-normal rainfall is predicted for much of the eastern Maritime Continent, with a slight increase in chance of below-normal rainfall predicted for the western Maritime Continent.
For MJJ 2025, above-normal rainfall is predicted for much of the eastern Maritime Continent, with a slight increase in chance of below-normal rainfall over parts of the western Maritime Continent, based on the multi-model ensemble (Figure 4). The UK Met Office and ECMWF models (Figures 3 and 2, respectively) predict above-normal rainfall over most of the eastern Maritime Continent, the NCEP model (Figure 1) predicts a mix of below- to above-normal rainfall for this region. For below-normal rainfall, the NCEP model predicts the largest extent covering the western and equatorial parts of the Maritime Continent, while the ECMWF model predicts below-normal rainfall only over the western Maritime Continent, and the UK Met Office model predicts no increase in chance of below-normal rainfall. Models’ skill is generally good for above-normal rainfall and moderate to good for below-normal rainfall over the Maritime Continent.
For Mainland Southeast Asia, there is disagreement between the models, with no dominant tercile for much of the region based on the multi-model ensemble (Figure 4). Model skill is low to moderate for Mainland Southeast Asia at this time of the year.
|
|
| Figure 1: Rainfall tercile summary predictions of NCEP model for MJJ 2025 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for MJJ 2025 (contains modified Copernicus C3S information). |
|
|
| Figure 3: Rainfall tercile summary predictions of UK Met Office model for MJJ 2025 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for MJJ 2025 (contains modified Copernicus C3S information). |
|
|
| 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 2025, above-normal temperature is predicted over the Maritime Continent, with near- to above-normal temperature for Mainland Southeast Asia.
For May 2025, above-normal temperature is predicted over the Maritime Continent and near- to above-normal temperature for Mainland Southeast Asia, based on the multi-model ensemble (Figure 4). All three models (Figures 1 – 3) predict a high likelihood of above-normal temperature over the Maritime Continent. For Mainland Southeast Asia, the ECMWF and UK Met Office models predict a higher chance of near-normal temperature, while the NCEP model predicts a higher chance of above-normal temperature (although still with a lower likelihood than for the Maritime Continent). Models’ skill for predicting above-normal temperature is good for most of Southeast Asia, while the skill for near-normal temperature is moderate for most of Mainland Southeast Asia.
|
|
| Figure 1: Temperature tercile summary predictions of NCEP model for May 2025 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for May 2025 (contains modified Copernicus C3S information). |
|
|
| Figure 3: Temperature tercile summary predictions of UK Met Office model for May 2025 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for May 2025 (contains modified Copernicus C3S information). |
|
|
| 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 2025, the models predict an increased chance of above-normal temperature over Southeast Asia.
For MJJ 2025, an increase in chance of above-normal temperature is predicted over all of Southeast Asia based on the multi-model ensemble (Figure 4). All three models agree with high confidence on above-normal temperature over the Maritime Continent. For Mainland Southeast Asia, the ECMWF model (Figure 2) predicts the highest chance of above-normal temperature, followed by the UK Met Office model (Figure 3), with the NCEP model (Figure 1) predicting a mix of near- and above-normal temperature. Models’ skill for predicting above-normal temperature is good over most of Southeast Asia, and low to moderate for near-normal temperature over Mainland Southeast Asia.
|
|
| Figure 1: Temperature tercile summary predictions of NCEP model for MJJ 2025 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for MJJ 2025 (contains modified Copernicus C3S information). |
|
|
| Figure 3: Temperature tercile summary predictions of UK Met Office model for MJJ 2025 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for MJJ 2025 (contains modified Copernicus C3S information). |
|
|
| 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.