Seasonal Forecast for July – September 2024
Updated 1 July 2024
The existing Southwest Monsoon conditions over the ASEAN region are expected to persist throughout the outlook period. The prevailing winds over the region typically blow from the southeast or southwest during this period. The Southwest Monsoon is also the traditional wet season for the northern ASEAN region, characterised by increased showers over the northern ASEAN region For the southern ASEAN region, there were no clear signs of a start to the traditional dry season as continued shower activities were observed over many areas in June 2024. However, the dry season is likely to commence in the coming months with the passage of the monsoon rainband further north of the Equator.
For July to September (JAS) 2024 period, models predict an increased chance of above-normal rainfall over the equatorial region. Above-normal temperature is predicted for the ASEAN region for JAS 2024. ENSO state is neutral, with the potential for La Niña conditions to develop from August onwards. The Indian Ocean Dipole (IOD) is neutral, with models predicting neutral conditions to persist for the next few months.
Despite the above-normal rainfall forecast over most parts of the southern ASEAN region during the outlook period, there is still an increased likelihood of hotspot activity and smoke haze occurrence during the dry season , especially over areas in the southern parts of the Maritime Continent where no increase in chance of above-normal rainfall is forecast.. While the risk of transboundary smoke haze remains during the outlook period, it is less likely to occur compared to 2023, when there was El Niño conditions and a positive IOD. In the northern ASEAN region, the hotspot and smoke haze situation is expected to remain suppressed under the wet weather 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 July 2024, above-normal rainfall is predicted for much of the equatorial region.
For July 2024, above-normal rainfall is predicted for much of the equatorial region based on the multi-model ensemble (Figure 4). The NCEP model (Figure 1) shows the largest extent of above-normal rainfall across the region, followed by the UK Met Office model (Figure 3), and with then the ECMWF model (Figure 2). For above-normal rainfall over the equatorial region, models’ skill is moderate to good for the central and eastern regions, and moderate for the western region.
Above-normal rainfall is predicted over parts of western and central Mainland Southeast Asia based on the multi-model ensemble. The NCEP and ECMWF models show some increase in chance of above-normal rainfall over this region, while the UK Met Office model shows a mix of near- to above-normal rainfall. Models’ skill for the region is generally low.
Elsewhere, there is either no dominant terciles (southern Maritime Continent, eastern parts of Mainland Southeast Asia) or mix of below- to above-normal rainfall (the Philippines). Model skill is generally good for the southern Maritime Continent and poor to moderate elsewhere.
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| Figure 1: Rainfall tercile summary predictions of NCEP model for July 2024 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for July 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Rainfall tercile summary predictions of UK Met Office model for July 2024 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for July 2024 (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological rainfall’s lower tercile boundary for July based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for July 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 JAS 2024, above-normal rainfall is predicted for the equatorial region.
For JAS 2024, above-normal rainfall is predicted for the equatorial region based on the multi-model ensemble (Figure 4). The NCEP and UK Met Office models (Figures 1 and 3) predict above-normal rainfall for this region with higher confidence over Borneo and the eastern equatorial region. The ECMWF model (Figure 2) shows a mix of near- to above-normal rainfall over this region, with higher confidence of above-normal rainfall over the eastern equatorial region. Models’ skill is overall good for above-normal rainfall over the central to eastern equatorial region, and moderate to good for the western region.
Elsewhere in the Maritime Continent, a mix of below- to above-normal rainfall is predicted over the Philippines based on the multi-model ensemble (Figure 4), while there is disagreement between the models for the southern Maritime Continent. Models’ skill is poor to moderate over Philippines and good over the southern Maritime Continent.
Over Mainland Southeast Asia, the models predict above-normal rainfall for much of the region. Model skill is generally low for Mainland Southeast Asia at this time of the year, apart from over the southern parts, where model skill is low to moderate.
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| Figure 1: Rainfall tercile summary predictions of NCEP model for JAS 2024 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for JAS 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Rainfall tercile summary predictions of UK Met Office model for JAS 2024 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for JAS 2024 (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological rainfall’s lower tercile boundary for JAS based on CHIRPS (reference period: 1991-2020). | Figure 6: Average climatological rainfall’s upper tercile boundary for JAS 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 July 2024, above-normal temperature is predicted over Southeast Asia.
For July 2024, above-normal temperature is predicted over Southeast Asia based on the multi-model ensemble (Figure 4). All three models agree with high confidence on above-normal temperature over most of the region. The NCEP model (Figure 1) predicts pockets of near-normal temperature over the western and central equatorial region, while the ECMWF model (Figure 2) predicts near-normal temperature for the southernmost part of the Maritime Continent. Models’ skill for predicting above-normal temperature is good over much of Southeast Asia, except for northern parts of Mainland Southeast Asia with moderate to good model skill.
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| Figure 1: Temperature tercile summary predictions of NCEP model for July 2024 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for July 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Temperature tercile summary predictions of UK Met Office model for July 2024 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for July 2024 (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological temperature for July 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 JAS 2024, above-normal temperature is predicted over Southeast Asia.
For JAS 2024, above-normal temperature is predicted over Southeast Asia based on the multi-model ensemble (Figure 4). All three models agree with high confidence on above-normal temperature over most of the region. Lower confidence of above-normal temperature is predicted over parts of Borneo and Papua by the NCEP model (Figure 1), over the southernmost part of the Maritime Continent by the ECMWF model (Figure 2), and over the southwestern part of the Maritime Continent by the UK Met Office model (Figure 3). Models’ 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 NCEP model for JAS 2024 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for JAS 2024 (contains modified Copernicus C3S information). |
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| Figure 3: Temperature tercile summary predictions of UK Met Office model for JAS 2024 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for JAS 2024 (contains modified Copernicus C3S information). |
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| Figure 5: Average climatological temperature for JAS 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.