Subseasonal Weather Outlook and Review

1.1    The Southwest Monsoon season prevailed in September 2015, with low level winds mainly blowing from the southeast or southwest. The monsoon rain band located close to the northern ASEAN region brought shower activities on most days to parts of the region. In contrast, dry weather conditions in the southern ASEAN region prevailed throughout September 2015.

1.2    Tropical Storm Vamco, which developed on 13 September 2015 over the South China Sea, made landfall south of Danang, Vietnam the next day. Vamco caused flooding and damages particularly to the agriculture and fisheries sectors in Vietnam. The Tropical Storm Vamco continued on its westward track and triggered floods across some provinces in Thailand before gradually dissipating on 18 September.

1.3    The prevailing El Niño conditions continued to have an impact on rainfall activities in the ASEAN region. Below-average rainfall was recorded across most parts of the southern ASEAN region. In the northern ASEAN region, near-average to below-average rainfall was received in the Mekong sub-region. The southern part of Philippines recorded above-average rainfall while below-average rainfall was received in the northern part of Philippines. The regional rainfall distribution for September 2015 is shown in Figure Fig. 1A.

2.1    In September, hotspot activities over the northern ASEAN region were generally subdued by the prevailing wet weather conditions. In contrast, the southern ASEAN region was affected by a prolonged transboundary haze situation in recent years. Persistent hotspot activities were observed especially in the fire-prone provinces of central and southern Sumatra, as well as in Kalimantan throughout the month.

2.2    In September 2015, moderate to dense smoke haze blanketed most parts of the central and southern Sumatra, and West and Central Kalimantan. On most days, the air quality readings in many parts of Sumatra including Jambi were in the hazardous range. Hazardous air quality readings were also reported in Palangkaraya in central Kalimantan and Pontianak in western Kalimantan on a number of days. Visibility was reduced to less than 1 km on most days in Jambi, Pekan Baru and Rengat in Sumatra and in Pontianiak and as the haze situation worsened.

2.3    The moderate to dense smoke haze from the fire-prone provinces in Sumatra and Kalimantan were carried by the prevailing winds to affect the region including Brunei Darussalam, Peninsular Malaysia, Sarawak, Singapore and southern Thailand. Some haze had also spread to the South China Sea and was blown by the prevailing winds to affect some islands in southern Philippines.

2.4    Parts of Peninsular Malaysia, East Malaysia and Singapore were affected by transboundary smoke haze since early September 2015. Air quality was in the Unhealthy and/or Very Unhealthy ranges on most days, and on a few days, the air quality entered into the Hazardous range which resulted in school closures, flight diversions or delayed flights amid poor visibility. On 25 September, the 24-hr PSI in Singapore hit a high of 322. On 26 September, visibility at the Subang Airport deteriorated to below 500 m and the airport was closed for several hours. Air quality readings in Port Klang, Shah Alam and Batu Muda also worsened to Very Unhealthy levels of 224-243 the next day morning.

2.3    In Kuching, Samarahan and Sri Aman, the Air Quality Index (AQI) ranged from ~68-~205, with most days in the Unhealthy to Very Unhealthy range between 9 and 28 September 2015. On 10 September, visibility at the Kuching International Airport deteriorated to below 800m and caused disruptions to flight operations and schedules. Satellite images depicting some of the hotspot activities over the ASEAN region in September 2015 are shown in Figs. 2A to 2E.

3.1    The sea surface temperatures (SSTs) in the tropical Pacific Ocean (SST) have increased further in September 2015. A strong El Niño currently prevails in the tropical Pacific Ocean.

3.2    Atmospheric conditions in the ASEAN region have been showing a clear response to the strong El Niño conditions. Large-scale drier-than-normal conditions have been observed particularly in the southern and eastern parts of the ASEAN region. A consensus forecast based on international climate models and expert opinions suggest the continuation of strong El Niño conditions into early-2016, with the possibility of further strengthening in the coming months.

3.3    Typically the impact of El Niño for the Southeast Asia region is drier than average rainfall conditions, especially in the southern parts of the ASEAN region during June to October. More locally-specific impact differs from place to place and for different seasons.

3.4    The region is now gradually transitioning from the Southwest Monsoon season (Jun-Sep/Oct) into the inter-monsoon period (Oct Nov). El Niño can still have a significant impact on the weather conditions (dryness) over the southern part of ASEAN region, in particular, the western part of the Maritime Continent. With the current El Niño conditions, there is still a risk that the prevailing Aug-Sep-Oct (ASO) season could experience extended periods of drier and warmer conditions in this part of Southeast Asia.

1.1    The Southwest Monsoon season prevailed in September 2015, with low level winds mainly blowing from the southeast or southwest. The monsoon rain band located close to the northern ASEAN region brought shower activities on most days to parts of the region. In contrast, dry weather conditions in the southern ASEAN region prevailed throughout September 2015.

1.2    Tropical Storm Vamco, which developed on 13 September 2015 over the South China Sea, made landfall south of Danang, Vietnam the next day. Vamco caused flooding and damages particularly to the agriculture and fisheries sectors in Vietnam. The Tropical Storm Vamco continued on its westward track and triggered floods across some provinces in Thailand before gradually dissipating on 18 September.

1.3    The prevailing El Niño conditions continued to have an impact on rainfall activities in the ASEAN region. Below-average rainfall was recorded across most parts of the southern ASEAN region. In the northern ASEAN region, near-average to below-average rainfall was received in the Mekong sub-region. The southern part of Philippines recorded above-average rainfall while below-average rainfall was received in the northern part of Philippines. The regional rainfall distribution for September 2015 is shown in Figure Fig. 1A.

2.1    In September, hotspot activities over the northern ASEAN region were generally subdued by the prevailing wet weather conditions. In contrast, the southern ASEAN region was affected by a prolonged transboundary haze situation in recent years. Persistent hotspot activities were observed especially in the fire-prone provinces of central and southern Sumatra, as well as in Kalimantan throughout the month.

2.2    In September 2015, moderate to dense smoke haze blanketed most parts of the central and southern Sumatra, and West and Central Kalimantan. On most days, the air quality readings in many parts of Sumatra including Jambi were in the hazardous range. Hazardous air quality readings were also reported in Palangkaraya in central Kalimantan and Pontianak in western Kalimantan on a number of days. Visibility was reduced to less than 1 km on most days in Jambi, Pekan Baru and Rengat in Sumatra and in Pontianiak and as the haze situation worsened.

2.3    The moderate to dense smoke haze from the fire-prone provinces in Sumatra and Kalimantan were carried by the prevailing winds to affect the region including Brunei Darussalam, Peninsular Malaysia, Sarawak, Singapore and southern Thailand. Some haze had also spread to the South China Sea and was blown by the prevailing winds to affect some islands in southern Philippines.

2.4    Parts of Peninsular Malaysia, East Malaysia and Singapore were affected by transboundary smoke haze since early September 2015. Air quality was in the Unhealthy and/or Very Unhealthy ranges on most days, and on a few days, the air quality entered into the Hazardous range which resulted in school closures, flight diversions or delayed flights amid poor visibility. On 25 September, the 24-hr PSI in Singapore hit a high of 322. On 26 September, visibility at the Subang Airport deteriorated to below 500 m and the airport was closed for several hours. Air quality readings in Port Klang, Shah Alam and Batu Muda also worsened to Very Unhealthy levels of 224-243 the next day morning.

2.3    In Kuching, Samarahan and Sri Aman, the Air Quality Index (AQI) ranged from ~68-~205, with most days in the Unhealthy to Very Unhealthy range between 9 and 28 September 2015. On 10 September, visibility at the Kuching International Airport deteriorated to below 800m and caused disruptions to flight operations and schedules. Satellite images depicting some of the hotspot activities over the ASEAN region in September 2015 are shown in Figs. 2A to 2E.

3.1    The sea surface temperatures (SSTs) in the tropical Pacific Ocean (SST) have increased further in September 2015. A strong El Niño currently prevails in the tropical Pacific Ocean.

3.2    Atmospheric conditions in the ASEAN region have been showing a clear response to the strong El Niño conditions. Large-scale drier-than-normal conditions have been observed particularly in the southern and eastern parts of the ASEAN region. A consensus forecast based on international climate models and expert opinions suggest the continuation of strong El Niño conditions into early-2016, with the possibility of further strengthening in the coming months.

3.3    Typically the impact of El Niño for the Southeast Asia region is drier than average rainfall conditions, especially in the southern parts of the ASEAN region during June to October. More locally-specific impact differs from place to place and for different seasons.

3.4    The region is now gradually transitioning from the Southwest Monsoon season (Jun-Sep/Oct) into the inter-monsoon period (Oct Nov). El Niño can still have a significant impact on the weather conditions (dryness) over the southern part of ASEAN region, in particular, the western part of the Maritime Continent. With the current El Niño conditions, there is still a risk that the prevailing Aug-Sep-Oct (ASO) season could experience extended periods of drier and warmer conditions in this part of Southeast Asia.