Subseasonal Weather Outlook and Review

1.1    Southwest Monsoon conditions continued to prevail in September 2016, and the prevailing winds in the region blew mainly from the southwest or west. The monsoon rain belt was north of the Equator between latitudes 5 N and 15 N on most days in September 2016.

1.2    During the review period, showers affected most parts of the northern ASEAN region. On 12 September 2016, Tropical Depression “Rai”, which developed over the South China Sea, east-northeast of Ho Chi Minh City, made landfall in central Vietnam. “Rai” brought heavy rainfall to the Mekong sub-region, in particular over Vietnam and northern Thailand, causing the rivers to break its banks which led to severe flooding and loss of 12 lives. Over the southern ASEAN region, it was generally wet with occasional brief periods of dry weather conditions. This wetter than normal conditions is atypical of the traditional dry season of the southern ASEAN region in September 2016.

1.3    For September 2016, most parts of the northern ASEAN region except Cambodia and southern Vietnam recorded near-normal to above-normal rainfall. In the southern ASEAN region, near-normal rainfall was recorded in Malaysia, Brunei Darussalam and most parts of Sumatra. Above-normal rainfall prevailed over large areas of the Indonesian Archipelago, including southern Sumatra, Kalimantan and Java. The regional rainfall distribution for September 2016 is shown in Fig. 1A.

2.1    In September 2016, shower activities in the northern ASEAN region continued to subdue the hotspot activities there. For the southern ASEAN region, hotspot activities in the near-equatorial region were mostly subdued due to the higher than usual shower activities in the region.

2.2    In mid-September 2016, the influence of Super Typhoon Meranti over the western Pacific Ocean brought drier weather conditions over parts of Kalimantan. This led to an escalation of hotspot activities in West Kalimantan where more than 100 hotspots were detected between 12 and 14 September 2016. Visible smoke plumes and haze were observed mainly in West Kalimantan over the two days. The dry weather conditions were short-lived and an improvement in the hotspot and haze situation was experienced in the subsequent days as shower activities returned to Kalimantan.

2.3    In the last week of September 2016, the brief periods of dry weather conditions in parts of Sumatra brought an increase in hotspot activities but there were no significant smoke plumes or haze observed in the vicinity of the hotspots. Hotspot activities over parts of the ASEAN region during September 2016 are shown in Fig. 2A – Fig. 2E

3.   Status of El Niño/La Niña

1.1    Southwest Monsoon conditions continued to prevail in September 2016, and the prevailing winds in the region blew mainly from the southwest or west. The monsoon rain belt was north of the Equator between latitudes 5 N and 15 N on most days in September 2016.

1.2    During the review period, showers affected most parts of the northern ASEAN region. On 12 September 2016, Tropical Depression “Rai”, which developed over the South China Sea, east-northeast of Ho Chi Minh City, made landfall in central Vietnam. “Rai” brought heavy rainfall to the Mekong sub-region, in particular over Vietnam and northern Thailand, causing the rivers to break its banks which led to severe flooding and loss of 12 lives. Over the southern ASEAN region, it was generally wet with occasional brief periods of dry weather conditions. This wetter than normal conditions is atypical of the traditional dry season of the southern ASEAN region in September 2016.

1.3    For September 2016, most parts of the northern ASEAN region except Cambodia and southern Vietnam recorded near-normal to above-normal rainfall. In the southern ASEAN region, near-normal rainfall was recorded in Malaysia, Brunei Darussalam and most parts of Sumatra. Above-normal rainfall prevailed over large areas of the Indonesian Archipelago, including southern Sumatra, Kalimantan and Java. The regional rainfall distribution for September 2016 is shown in Fig. 1A.

2.1    In September 2016, shower activities in the northern ASEAN region continued to subdue the hotspot activities there. For the southern ASEAN region, hotspot activities in the near-equatorial region were mostly subdued due to the higher than usual shower activities in the region.

2.2    In mid-September 2016, the influence of Super Typhoon Meranti over the western Pacific Ocean brought drier weather conditions over parts of Kalimantan. This led to an escalation of hotspot activities in West Kalimantan where more than 100 hotspots were detected between 12 and 14 September 2016. Visible smoke plumes and haze were observed mainly in West Kalimantan over the two days. The dry weather conditions were short-lived and an improvement in the hotspot and haze situation was experienced in the subsequent days as shower activities returned to Kalimantan.

2.3    In the last week of September 2016, the brief periods of dry weather conditions in parts of Sumatra brought an increase in hotspot activities but there were no significant smoke plumes or haze observed in the vicinity of the hotspots. Hotspot activities over parts of the ASEAN region during September 2016 are shown in Fig. 2A – Fig. 2E

3.   Status of El Niño/La Niña