Seasonal Forecast for October-November-December 2021

Updated 2 October 2021

Summary of Outlook

Towards the end of September 2021, the monsoon rain band started to shift southwards towards the Equator. Wetter conditions are expected to progressively set in over the ASEAN region as the transition to Intermonsoon conditions takes place in October. During the October to December period, the prevailing southeasterly or southwesterly winds over the ASEAN region are expected to weaken prior to a change in direction to blow from the northeast or northwest.

For the October to December 2021 period, there is an increased chance of above-normal rainfall over much of the ASEAN region, with the highest likelihood over southern and eastern parts of the Maritime Continent. The tropical Pacific has not shown consistent La Niña-like conditions yet, but most models are predicting La Niña-like conditions from October onwards. The negative IOD event is ending, and models predict that the IOD will return to neutral before 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 much of the Maritime Continent during OND 2021. During the same period, the northeastern parts of Mainland Southeast Asia are predicted to experience below- to near-normal temperatures, associated with the northeast monsoon surges.

While the southern ASEAN region is expected to begin transitioning out of its dry season in the coming weeks, an increase in hotspot activity and localised smoke plumes is still possible during dry periods. However, as above-normal rainfall is forecast for most of the southern ASEAN region in October 2021, widespread severe transboundary haze continues to be unlikely. Hotspot and smoke haze activities in the northern ASEAN region are expected to progressively intensify as the Mekong sub-region’s traditional dry season sets in at the end of the year.

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 October 2021, there is an increase in chance of above-normal rainfall over central and eastern parts of the Maritime Continent, and some chance of above-normal rainfall over northeastern parts of Mainland Southeast Asia.

 

For October 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 Peninsular Malaysia with below-normal rainfall. The ECMWF and UK Met Office models (Figures 2 and 3) have the highest likelihood of above-normal rainfall over central and eastern parts of the Maritime Continent, while the NCEP model predicts the highest likelihood over Java (Figure 1). The models’ skill is relatively high for much of the Maritime Continent, apart from northern Sumatra, the Malay Peninsula, and the Philippines where the skill is only low to moderate.

For Mainland Southeast Asia, there is disagreement between the NCEP model and the other two models, with the NCEP model predicting below-normal rainfall over much of the region while the ECMWF and UK Met Office models predicting an increase in chance of above-normal rainfall over central and eastern parts of Mainland Southeast Asia. The models’ skill is low for Mainland Southeast Asia, apart from northeastern parts of Mainland Southeast Asia where there is some moderate skill for the above-normal tercile category.

Rainfall Tercile Summary of ECMWF model.

Figure 1a: Rainfall tercile summary predictions of NCEP model for October 2021. Figure 1b: Rainfall tercile summary predictions of ECMWF model for October 2021 (contains modified Copernicus C3S information).

Figure 1c: Rainfall tercile summary predictions of UK Met Office model for October 2021 (contains modified Copernicus C3S information). Figure 1d: Rainfall tercile summary predictions of multi-model ensemble model for October 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.

 


For OND 2021, there is an increased chance of above-normal rainfall over southern and eastern parts of the Maritime Continent, and some chance of above-normal rainfall over central and eastern parts of Mainland Southeast Asia.

 

For OND 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 southern and eastern parts of the Maritime Continent. No dominant or below-normal terciles are predicted for the Malay Peninsula and northern Sumatra by the three models. The models’ skill is moderate to relatively high for much of the Maritime Continent, apart from northern Sumatra and the Malay Peninsula where the skill is low.

For Mainland Southeast Asia, there is a slight increase in chance of above-normal rainfall predicted by ECMWF and UKMO models for the central and eastern regions, while NCEP model is predicting above-normal rainfall for central Viet Nam and below-normal rainfall for most parts of Mainland Southeast Asia. However, the model skill for Mainland Southeast Asia is low for this season.

Figure 2a: Rainfall tercile summary predictions of NCEP model for OND 2021. Figure 2b: Rainfall tercile summary predictions of ECMWF model for OND 2021 (contains modified Copernicus C3S information).

Figure 2c: Rainfall tercile summary predictions of UK Met Office model for OND 2021 (contains modified Copernicus C3S information). Figure 2d: Rainfall tercile summary predictions of multi-model ensemble model for OND 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.

For October 2021, an increased chance of above-normal temperature is predicted over much of Southeast Asia, except for some parts of the equatorial region and northeastern Mainland Southeast Asia where below- to near-normal temperature is predicted.

 

For October 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1-3) – predict above-normal temperature over much of Southeast Asia, except over southern Sumatra, Java, southern Borneo and northeastern Mainland Southeast Asia where below- to near-normal temperature is predicted. The spatial extent of the regions for below- to near-normal temperature varies between the models.

The models’ skill for predicting above-normal temperature are relatively high across Southeast Asia, except for southern parts of the region where the skill is low to moderate.

Figure 3a: Temperature tercile summary predictions of NCEP model for October 2021. Figure 3b: Temperature tercile summary predictions of ECMWF model for October 2021 (contains modified Copernicus C3S information).

Figure 3c: Temperature tercile summary predictions of UK Met Office model for October 2021 (contains modified Copernicus C3S information). Figure 3d: Temperature tercile summary predictions of multi-model ensemble model for October 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.


For OND 2021, above-normal temperature is favoured over much of the Maritime Continent. Parts of Viet Nam, Lao PDR, Cambodia, and Thailand are predicted to experience below- to near-normal temperatures.

 

For OND 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1-3) – predict above-normal temperature over much of the Maritime Continent (apart from southern Sumatra, Java and southern Borneo where below- to near-normal temperature is predicted).

For Mainland Southeast Asia, below- to near-normal temperature is predicted over much of Mainland Southeast Asia, in particular below-normal temperature for northern Viet Nam, central and southern Lao PDR, northeast Cambodia, and much of Thailand. The exception is over most parts of Myanmar where above-normal tercile is predicted based on the multi-model ensemble (Figure 4).

The seasonal models’ skill for predicting below- and above-normal temperatures are moderate and relatively high for Mainland Southeast Asia and the Maritime Continent respectively.

Figure 4a: Temperature tercile summary predictions of NCEP model for OND 2021. Figure 4b: Temperature tercile summary predictions of ECMWF model for OND 2021 (contains modified Copernicus C3S information).

Figure 4c: Temperature tercile summary predictions of UK Met Office model for OND 2021 (contains modified Copernicus C3S information). Figure 4d: Temperature tercile summary predictions of multi-model ensemble model for OND 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.