Seasonal Forecast for September-October-November 2021
Updated 3 September 2021
Summary of Outlook
In September 2021, the Southwest Monsoon conditions are forecast to persist as the monsoon rain band remains north of the Equator. During this period, the northern and southern ASEAN regions continue to experience their traditional wet and dry season respectively. The Southwest Monsoon conditions are expected to gradually transit into the inter-monsoon period typically in the latter part of October where the equatorial ASEAN region experiences an increase in shower activities. The prevailing winds during the Southwest Monsoon are expected to blow from the southeast or southwest and gradually weaken to become light and variable in direction during the transition into the inter-monsoon period.
For the September to November 2021 period, there is an increased chance of above-normal rainfall over much of the ASEAN region, with the highest likelihood over central and southern parts of the Maritime Continent. There were signs of La Niña-like conditions developing in August, although the prediction for the rest of 2021 is still split between ENSO-neutral and La Niña conditions developing. The Indian Ocean Dipole (IOD) is currently in the negative phase, with models forecasting the IOD to return to neutral in November or December 2021. A negative IOD tends to bring above-average rainfall for the southern ASEAN region for this time of the year. Warmer-than-usual temperature is expected for most parts of the ASEAN region during SON 2021.
Despite the traditional dry season over the southern ASEAN region, the likelihood for widespread severe transboundary haze is low due to the expected above-normal rainfall condition over the region. However, isolated hotspots with localised smoke plumes can still occur during dry periods. With the increase in shower activities during the inter-monsoon period, hotspot activities in the region are expected to be suppressed. In the northern ASEAN region, the hotspot activities are expected to remain subdued under the prevailing wet weather conditions during the forecast period.
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 – September 2021For September 2021, there is an increase in chance of above-normal rainfall over much of the Maritime Continent.
For September 2021, based on the multi-model ensemble (Figure 4), there is an increase in chance of above-normal rainfall over much of the Maritime Continent, apart from northern Sumatra and the Malay Peninsula with below-normal rainfall and northern Philippines with no dominant terciles predicted. The ECMWF and UK Met Office models (Figures 2 and 3) have the highest likelihood of above-normal rainfall over central and southern parts of the Maritime Continent, while the NCEP model predicts the highest likelihood over southern Sumatra, Java and the Philippines with no dominant terciles over central Maritime Continent (Figure 1). The models’ skill is relatively high for much of the Maritime Continent, apart from northern Sumatra, the Malay Peninsula, and northern Philippines where the skill is only low to moderate.
For Mainland Southeast Asia, there is disagreement between NCEP model and the other two models, with NCEP model predicting below-normal rainfall over much of the region while ECMWF and UK Met Office models predicting a small increase in chance of above-normal rainfall or no dominant rainfall for much of the region. The models’ skill is low for Mainland Southeast Asia.
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| Figure 1a: Rainfall tercile summary predictions of NCEP model for September 2021. | Figure 1b: Rainfall tercile summary predictions of ECMWF model for September 2021 (contains modified Copernicus C3S information). |
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| Figure 1c: Rainfall tercile summary predictions of UK Met Office model for September 2021 (contains modified Copernicus C3S information). | Figure 1d: Rainfall tercile summary predictions of multi-model ensemble model for September 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 – September-October-November 2021 (SON 2021)For SON 2021, there is an increased chance of above-normal rainfall over much of Southeast Asia.
For SON 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1-3) predict an increased chance of above-normal rainfall for much of the Maritime Continent, with highest likelihood over central and southern parts of the Maritime Continent. No dominant and below-normal terciles are predicted for parts of Peninsular Malaysia and Sumatra, respectively, by the three models. The model skill for the above-normal tercile category is moderate to relatively high for much of the Maritime Continent, with higher skill where the probability of above-normal rainfall is highest in Figure 4.
For Mainland Southeast Asia, there is slight increase in chance of above-normal rainfall predicted by ECMWF and UKMO models for much of the region, while NCEP model is predicting above-normal rainfall for Cambodia and Viet Nam and below-normal rainfall for northern parts of Mainland Southeast Asia. However, the model skill for Mainland Southeast Asia is only low for this season.
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| Figure 2a: Rainfall tercile summary predictions of NCEP model for SON 2021. | Figure 2b: Rainfall tercile summary predictions of ECMWF model for SON 2021 (contains modified Copernicus C3S information). |
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| Figure 2c: Rainfall tercile summary predictions of UK Met Office model for SON 2021 (contains modified Copernicus C3S information). | Figure 2d: Rainfall tercile summary predictions of multi-model ensemble model for SON 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 – September 2021For September 2021, an increased chance of above-normal temperature is predicted over much of Southeast Asia.
For September 2021, there is an increased chance of above-normal temperature predicted over much of Southeast Asia based on the multi-model ensemble (Figure 4), except for southern Sumatra and southern Borneo with near-normal temperatures predicted. The UK Met Office (Figure 3) model predicts the highest spatial coverage of above-normal temperature, followed by the NCEP and ECMWF models (Figures 1 and 2). The NCEP model predicts a lower chance of above-normal temperature conditions over southern Mainland Southeast Asia, while the ECMWF model predicts near-normal temperatures over parts of the equatorial region, in particular over southern Sumatra, and Borneo.
The models’ skill for above-normal temperature for the Maritime Continent is low to moderate for the three models, and for Mainland Southeast Asia is moderate to relatively high for the ECMWF and UK Met Office models and low for NCEP.
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| Figure 3a: Temperature tercile summary predictions of NCEP model for September 2021. | Figure 3b: Temperature tercile summary predictions of ECMWF model for September 2021 (contains modified Copernicus C3S information). |
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| Figure 3c: Temperature tercile summary predictions of UK Met Office model for September 2021 (contains modified Copernicus C3S information). | Figure 3d: Temperature tercile summary predictions of multi-model ensemble model for September 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 – September-October-November 2021 (SON 2021)For SON 2021, above-normal temperature is favoured for most of Southeast Asia.
For SON 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1 – 3) – predict a high likelihood of above-normal temperature for most of the Maritime Continent, except for near-normal temperature over southern Borneo and southern Sumatra predicted by the three models and near-normal temperatures for parts of Java predicted by NCEP and ECMWF models.
For Mainland Southeast Asia, above-normal temperatures are predicted for most of the area with higher likelihood in the western region. The exceptions are over parts of eastern Thailand, Lao PDR, and northern Viet Nam where there is no dominant terciles based on the multi-model ensemble (Figure 4).
The models’ skill for predicting above-normal temperature for Southeast Asia is moderate to high for this season.
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| Figure 4a: Temperature tercile summary predictions of NCEP model for SON 2021. | Figure 4b: Temperature tercile summary predictions of ECMWF model for SON 2021 (contains modified Copernicus C3S information). |
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| Figure 4c: Temperature tercile summary predictions of UK Met Office model for SON 2021 (contains modified Copernicus C3S information). | Figure 4d: Temperature tercile summary predictions of multi-model ensemble model for SON 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.