Seasonal Forecast for September, October, November 2018

Updated 4 September 2018

Summary of Outlook

The Southwest Monsoon season is expected to prevail until around mid-October before the region transitions to the inter-monsoon season. Prevailing south-westerly or south-easterly winds in the region will gradually weaken and become light and variable in direction. During the inter-monsoon season, lesser rainfall can be expected over the northern ASEAN region as the monsoon rain band gradually migrates south toward the equatorial region. More frequent rain showers are expected for the southern ASEAN region.

During the Sep-Oct-Nov season, most parts of the region are likely to experience near-normal or below-normal rainfall and above-normal temperatures.

Hotspot activities in the northern ASEAN region are expected to remain subdued due to rainy weather.

For the southern ASEAN region, southern parts of Sumatra and Kalimantan could experience occasional increase in hotspot activities during brief periods of dry weather particularly in the September and early-October period. This could lead to an occurrence of transboundary haze in the region. An increase in shower activities with the onset of the inter-monsoon season is expected to help subdue hotspot activities. Isolated hotspots may still emerge occasionally in November but these hotspots are likely to be short-lived.

The 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.

For September 2018, below-normal to near-normal rainfall is favoured over equatorial parts of the region, as well as northwards of 10˚N.

 
For September’s rainfall, all three models – NCEP CFSv2, ECMWF, and UK Met Office (Figures 1a-c) – predict below-normal conditions over parts of the equatorial region (around southern Sumatra, Borneo, Sulawesi, and Papua), as well as over the coastal regions of northern Southeast Asia (north of 10˚N). The spatial extent and magnitude of the likelihood for below-normal conditions differ across the different models. In particular, for NCEP CFSv2, the likelihood for below-normal rainfall is higher than ECMWF or the UK Met Office. Furthermore, only NCEP CFSv2 favours above-normal conditions along the 10˚N latitude stretch, where the other two models predict below-normal or no dominant tercile.

The models’ skill for rainfall prediction are very good over south of the equator and moderate along the 10˚N stretch (see the respective sections of “Model products and verification: NCEP, ECMWF, UK Met Office”).

Rainfall Tercile Summary of NCEP model.

Figure 1a: Rainfall tercile summary predictions of NCEP model for September 2018.

Rainfall Tercile Summary of ECMWF model.

Rainfall Tercile Summary of Met Office model.

Figure 1b: Rainfall tercile summary predictions of ECMWF model for September 2018 (contains modified Copernicus C3S information). Figure 1c: Rainfall tercile summary predictions of UK Met Office model for September 2018 (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 SON 2018, near-normal or below-normal rainfall is favoured over most parts of the Southeast Asia region.

 
For SON 2018, all three models – NCEP CFSv2, ECMWF, and UK Met Office (Figures 2a-c) –differ over the equator and southwards. For this region, NCEP CFSv2 predicts mostly below-normal rainfall, whereas ECMWF and UK Met Office predict a mix of near-normal and below-normal rainfall. For all three models, however, the magnitude of the below-normal rainfall is not expected to be large. All three models have good skill (NCEP, ECMWF, UK Met Office) over the equator and southwards.

Elsewhere, the three models favour below-normal conditions over the northern parts of Southeast Asia. Another consistent prediction across the three models is over northern Sumatra and Peninsular Malaysia where above-normal rainfall is favoured. However, for both these regions, model skill in this season is generally poor, apart from over the northern Philippines.

Rainfall Tercile Summary of NCEP model.

Figure 2a: Rainfall tercile summary predictions of NCEP model for SON 2018.

Rainfall anomaly of ECMWF model.

Rainfall Tercile Summary of Met Office model.

Figure 2b: Rainfall tercile summary predictions of ECMWF model for SON 2018 (contains modified Copernicus C3S information). Figure 2c: Rainfall tercile summary predictions of UK Met Office model for SON 2018 (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 September 2018, above-normal temperature is favoured over many parts of Southeast Asia.

 
All three models – NCEP CFSv2, ECMWF, and UK Met Office (Figures 1a-c) – provide consistent outlook for above-normal temperature conditions over most of the equatorial region. Two of the models, NCEP CFSv2 and UK Met Office, also predict possibility of above-normal conditions over parts of northern Southeast Asia (mainly Myanmar and northern Thailand). There is no dominant category for the ECMWF model for this region.

Over land across the region, the models’ skill for temperature prediction are generally very good (see the respective sections of “Model products and verification: NCEP, ECMWF, UK Met Office”).

Temperature Tercile Summary of NCEP model.

Temperature Tercile Summary of ECMWF model.

Figure 1a: Temperature tercile summary predictions of NCEP model for September 2018. Figure 1b: Temperature tercile summary predictions of ECMWF model for September 2018 (contains modified Copernicus C3S information).

Temperature Tercile Summary of Met Office model.

Figure 1c: Temperature tercile summary predictions of UK Met Office model for September 2018 (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 SON 2018, above-normal temperature is favoured over many parts of Southeast Asia.

 
All three models – NCEP CFSv2, ECMWF, and UK Met Office (Figures 2a-c) – provide a consistent outlook for above-normal temperature conditions over many parts of the region with stronger signals over the equatorial region.

Over land across the region, the models’ skill for temperature prediction are generally very good (see the respective sections of “Model products and verification: NCEP, ECMWF, UK Met Office”).

Temperature Tercile Summary of NCEP model.

Figure 2a: Temperature tercile summary predictions of NCEP model for SON 2018.

Temperature Tercile Summary of ECMWF model.

Temperature Tercile Summary of Met Office model.

Figure 2b: Temperature tercile summary predictions of ECMWF model for SON 2018 (contains modified Copernicus C3S information). Figure 2c: Temperature tercile summary predictions of UK Met Office model for SON 2018 (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.