Seasonal Forecast for May – July 2024
Updated 7 May 2024
The inter-monsoon period is likely to continue throughout much of May 2024. Light and variable winds with occasional shower activities can be typically expected for many parts of the ASEAN region during this period. The Southwest Monsoon usually establishes itself towards the middle of the year, with the prevailing winds transitioning to blow from the southeast to southwest, thus marking the onset of the traditional dry season for the southern ASEAN region. Concurrently, the Mekong sub-region will also transition to the traditional wet season, accompanied by an increase in shower activities.
For May to July (MJJ) 2024 period, models predict an increased chance of above-normal rainfall over the equatorial period, with below-normal rainfall predicted for much of the rest of the ASEAN region in May and below-to near-normal rainfall for the May-July period overall. Above-normal temperature is predicted for the ASEAN region for MJJ 2024. El Niño conditions are predicted to transition to neutral during May 2024. The Indian Ocean Dipole (IOD) is currently neutral. El Niño tends to bring drier and warmer conditions to much of the region at this time of year. The effect of El Niño conditions may still be present for a while after its decay, particularly for temperature.
During the outlook period of MJJ 2024, below-normal rainfall is predicted over the northern ASEAN region in May which could lead to a more prolonged dry season. Nonetheless, the elevated hotspot and transboundary haze situation is expected to gradually improve over the Mekong sub-region with the return of shower activities towards the middle of the year. Although above-normal rainfall is expected for parts of the southern ASEAN region, hotspot activities are likely to rise with the onset of the traditional dry season in June or July. As the prevailing winds are forecast to turn southeasterly to southwesterly, there may be an increased risk of transboundary haze occurrence during dry periods if persistent hotspot clusters with smoke plumes develop.
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 2024, above-normal rainfall is predicted for the equatorial region, with below-normal predicted for much of the rest of the ASEAN region.
For May 2024, above-normal rainfall is predicted for much of the equatorial region based on the multi-model ensemble (Figure 4). The ECMWF model (Figure 2) shows the highest confidence of above-normal rainfall, followed by the UK Met Office model (Figure 3), and the NCEP model (Figure 1). For above-normal rainfall over the equatorial region, models’ skill is moderate to good for the ECMWF and UK Met Office models, and low to moderate for the NCEP model.
Outside of the equatorial region, below-normal rainfall is predicted based on the multi-model ensemble (Figure 4). The NCEP model (Figure 1) has the highest confidence for below-normal rainfall over the northern ASEAN region, followed by the ECMWF model (Figure 2), while the UKMO model has the highest confidence of below-normal rainfall over southern parts of the southern ASEAN region (Figure 3). The models’ skill for below-normal rainfall is moderate to good for the northern ASEAN region and low to moderate for the southern ASEAN region.
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| Figure 1: Rainfall tercile summary predictions of NCEP model for May 2024 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for May 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Rainfall tercile summary predictions of UK Met Office model for May 2024 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for May 2024 (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 2024, above-normal rainfall is predicted for the equatorial region with below- to near-normal rainfall elsewhere.
For MJJ 2024, above-normal rainfall is predicted for the equatorial region based on the multi-model ensemble (Figure 4). All three models predict above-normal rainfall with high confidence over Borneo, northern parts of Sumatra and the eastern equatorial region (Figures 1 – 3). Only the ECMWF model predicts with high confidence above-normal rainfall over the western Maritime Continent, with the NCEP and UKMO models predicting near- to above-normal rainfall for this region. Models’ skill is moderate to good for above-normal rainfall over the equatorial region.
Elsewhere, a mix of below- to near-normal rainfall is predicted based on the multi-model ensemble (Figure 4). All three models are in agreement with the below-normal rainfall over the northern half of the Philippines (Figures 1 – 3). Over Mainland Southeast Asia, the NCEP model predicts below-normal rainfall with the highest confidence, with both the ECMWF and UK Met Office models predicting predominantly below- to near-normal rainfall. For the southern Maritime Continent, the ECMWF and UKMO models predict a high chance of below-normal rainfall, while the NCEP model predicts below- to near-normal rainfall. The models’ skill for below-normal rainfall is relatively good over Philippines and low to moderate over Mainland Southeast Asia and the southern Maritime Continent.
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| Figure 1: Rainfall tercile summary predictions of NCEP model for MJJ 2024 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for MJJ 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Rainfall tercile summary predictions of UK Met Office model for MJJ 2024 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for MJJ 2024 (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 2024, above-normal temperature is predicted over the ASEAN region.
For May 2024, above-normal temperature is predicted over the ASEAN region based on the multi-model ensemble (Figure 4). All three models agree with high confidence on above-normal temperature over most of the region, apart from over the northernmost parts (all three models) and the southernmost parts (ECMWF and UKMO models, Figures 2 – 3). Models’ skill for predicting above-normal temperature is good over the ASEAN region, apart from over the northern most parts.
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| Figure 1: Temperature tercile summary predictions of NCEP model for May 2024 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for May 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Temperature tercile summary predictions of UK Met Office model for May 2024 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for May 2024 (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 2024, above-normal temperature is predicted over the ASEAN region.
For MJJ 2024, above-normal temperature is predicted over the ASEAN region based on the multi-model ensemble (Figure 4). All three models agree with high confidence on above-normal temperature over most of the region, apart from over the southwestern Maritime Continent where near- to above-normal rainfall is predicted by the ECMWF and UKMO models (Figures 2 and 3). Models’ skill for predicting above-normal temperature for the ASEAN region is good over most of the ASEAN region.
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| Figure 1: Temperature tercile summary predictions of NCEP model for MJJ 2024 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for MJJ 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Temperature tercile summary predictions of UK Met Office model for MJJ 2024 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for MJJ 2024 (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.