Seasonal Forecast for May-June-July 2021
Updated 6 May 2021
Summary of Outlook
The prevailing Inter-monsoon conditions are expected to persist in May 2021 as the monsoon rain band moves northwards over the Equator. During this period, the prevailing winds over the region are typically light and variable in direction for much of the ASEAN region. Inter-monsoon conditions typically transition into the Southwest Monsoon in June with the prevailing winds strengthening and blowing from the southeast or southwest. The Southwest Monsoon season is the traditional dry season for the southern ASEAN region, which brings persistent dry conditions over the region. For the northern ASEAN region, the Southwest Monsoon is the traditional wet season.
For the May to July 2021 period, models predict below-normal rainfall over parts of the western and central Maritime Continent, in particularly over parts of Sumatra, Peninsular Malaysia and Borneo. Elsewhere, there is no consistent prediction for the rainfall outlook between the models consulted. La Niña conditions are present over the tropical Pacific Ocean at the end of April, with climate models predicting its continued weakening to neutral conditions during May – June.
Warmer-than-usual temperature is expected for much of the Maritime Continent, with near to warmer-than-usual temperature elsewhere.
The increase in shower activities is expected to help subdue the hotspot and haze situation over the Mekong sub-region. In the southern ASEAN region, isolated hotspots with localised smoke plumes may develop in areas with below-normal rainfall over Sumatra, Peninsular Malaysia and Borneo. As the traditional dry season typically sets in during June/July, the hotspot and haze situation is expected to elevate with increased risk of transboundary smoke haze in the region.
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.
Monthly Rainfall Outlook – May 2021For May 2021, there is a small increase in chance of below-normal rainfall for the western Maritime Continent and a small increase in chance of above-normal rainfall for much of the northern ASEAN region.
For May 2021, based on the multi-model ensemble there is an increased chance of above-normal rainfall over the Philippines and much of Mainland Southeast Asia (Figure 4). Based on the individual models: the UK Met Office model (Figure 3) predicts above-normal rainfall over most Mainland Southeast Asia and the Philippines, the ECMWF model (Figure 2) predicts above-normal rainfall for western and central parts of Mainland Southeast Asia and the Philippines, while the NCEP model (Figure 1) only predicts small parts of Mainland Southeast Asia to receive above-normal rainfall with the Philippines and Myanmar predicted to experience below-normal rainfall. The models’ skill is moderate to relatively good for the Philippines, with low to moderate skill for Mainland Southeast Asia.
All three models predict below-normal rainfall for much of the western Maritime Continent, although the model skill is only low to moderate for this region.
Elsewhere (Java region and eastern Indonesia), there is either disagreement between the models or no dominant tercile predicted.
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| Figure 1a: Rainfall tercile summary predictions of NCEP model for May 2021. | Figure 1b: Rainfall tercile summary predictions of ECMWF model for May 2021 (contains modified Copernicus C3S information). |
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| Figure 1c: Rainfall tercile summary predictions of UK Met Office model for May 2021 (contains modified Copernicus C3S information). | Figure 1d: Rainfall tercile summary predictions of multi-model ensemble model for May 2021 (contains modified Copernicus C3S information). |
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.
Seasonal Rainfall Outlook – May-June-July 2021 (MJJ 2021)For MJJ 2021, there is an increased chance of below-normal rainfall over parts of the western and central Maritime Continent.
For MJJ 2021, all three models – NCEP, ECMWF, and UK Met Office (Figures 1-3) – predict an increased chance of below-normal rainfall for parts of Sumatra, Peninsular Malaysia, and Borneo in the western and central Maritime Continent. The ECMWF model (Figure 2) predicts below-normal rainfall over most of Sumatra, Peninsular Malaysia, and western and central Borneo, while the UK Met Office model (Figure 3), only predicts below-normal rainfall for northern Sumatra, southern Peninsular Malaysia, and western Borneo. The NCEP model (Figure 1) shifts the highest likelihood for below-normal rainfall further south and east, covering much of the region south of the equator. The model skill for below-normal rainfall is low to moderate for the western Maritime continent, and moderate to high for the central Maritime Continent.
Elsewhere, there is either disagreement between the models (eastern Maritime Continent), or the skill is low for this season (Mainland Southeast Asia).
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| Figure 2a: Rainfall tercile summary predictions of NCEP model for MMJ 2021. | Figure 2b: Rainfall tercile summary predictions of ECMWF model for MMJ 2021 (contains modified Copernicus C3S information). |
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| Figure 2c: Rainfall tercile summary predictions of UK Met Office model for MMJ 2021 (contains modified Copernicus C3S information). | Figure 2d: Rainfall tercile summary predictions of multi-model ensemble model for MMJ 2021 (contains modified Copernicus C3S information). |
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.
Monthly Temperature Outlook – May 2021For May 2021, above-normal temperature is favoured over much of the Maritime Continent, with near-normal temperature over the central and eastern parts of Mainland Southeast Asia.
For May 2021, an increased chance of above-normal temperature is predicted over much of the Maritime Continent, with the highest likelihood in the eastern Maritime Continent (apart from the northern part of the Philippines) by the multi-model ensemble (Figure 4). The likelihood for the above-normal temperature varies among the three models, with the NCEP model (Figure 1) being the most confident, followed by the UK Met Office model (Figure 3), while the ECMWF model (Figure 2) predicts near- to above-normal temperature for Sumatra, Malaysia and the Philippines. Models’ skill for above-normal temperature for the Maritime Continent is moderate to high.
For Mainland Southeast Asia apart from Myanmar, temperature is predicted overall to be near-normal (Figure 4). Looking at the individual models, the UK Met Office model (Figure 3) predicts slightly cooler temperature of between below- and near-normal, while the NCEP (Figure 1) and ECMWF (Figure 2) models predict temperature more likely to be near-normal. Over Myanmar, there is disagreement between the models. For below- and near-normal temperatures, the models’ skill for this region is low to moderate for this month.
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| Figure 3a: Temperature tercile summary predictions of NCEP model for May 2021. | Figure 3b: Temperature tercile summary predictions of ECMWF model for May 2021 (contains modified Copernicus C3S information). |
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| Figure 3c: Temperature tercile summary predictions of UK Met Office model for May 2021 (contains modified Copernicus C3S information). | Figure 3d: Temperature tercile summary predictions of multi-model ensemble model for May 2021 (contains modified Copernicus C3S information). |
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.
Seasonal Temperature Outlook – May-June-July 2021 (MJJ 2021)For MJJ 2021, an increased chance of above-normal temperature is predicted for much of the Maritime Continent, with near- to above-normal temperature predicted for much of Mainland Southeast Asia.
For MJJ 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1 – 3) – predict an increased chance of above-normal temperature for much of the Maritime Continent, apart from the northern Philippines where near- to above-normal temperature is predicted. The model skill for predicting above-normal temperature for the Maritime Continent is moderate to high.
For Mainland Southeast Asia, near- to above-normal temperature is predicted by the multi-model ensemble (Figure 4). While the ECMWF (Figure 2) and UK Met Office (Figure 3) models predict near-normal temperature for much of this region, the NCEP (Figure 1) model predicts a higher likelihood of above-normal temperature over Myanmar, Lao PDR, and Viet Nam, with near-normal temperature elsewhere. The model skill for predicting above-normal temperature for Mainland Southeast Asia is generally moderate, and low for near-normal temperature (near-normal temperature generally has the lowest predictive skill of the three terciles).
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| Figure 4a: Temperature tercile summary predictions of NCEP model for MMJ 2021. | Figure 4b: Temperature tercile summary predictions of ECMWF model for MMJ 2021 (contains modified Copernicus C3S information). |
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| Figure 4c: Temperature tercile summary predictions of UK Met Office model for MMJ 2021 (contains modified Copernicus C3S information). | Figure 4d: Temperature tercile summary predictions of multi-model ensemble model for MMJ 2021 (contains modified Copernicus C3S information). |
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.