Subseasonal Weather Outlook and Review

1.1 Inter-Monsoon conditions prevailed in the second fortnight of October 2018. The monsoon rainband along the equatorial ASEAN region continued to bring rainy weather to the southern ASEAN region. The passage of super typhoon Yutu across northern Philippines brought heavy rainfall to the Philippines and parts of the northern ASEAN region. Central parts of Sumatra and Kalimantan, and Luzon Island received above normal rainfall.

1.2 Parts of the Mekong sub-region experienced dry weather, in particular in areas such as northern Myanmar, southern parts of Lao PDR and Viet Nam, Cambodia and western Thailand, received below normal rainfall. Due to the occasional incursion of dry air from the southern hemisphere, areas around the Java Sea, including southern parts of Sumatra and Kalimantan and eastern Java, also received below normal rainfall.

1.3 The daily average rainfall and the percentage of average rainfall for the second fortnight of October 2018 are shown in Figure 1 and Figure 2 respectively.

1.4 On 21 October 2018, a tropical depression to the east of Guam strengthened into tropical storm Yutu, which subsequently tracked westwards and intensified into a super typhoon by 24 October 2018. After making landfall on the island of Luzon, the Philippines on 30 October 2018, Yutu traversed across the island, bringing torrential rainfall to northern Philippines. It later tracked north-westward into the South China Sea and weakened into a tropical depression. The track of Super Typhoon Yutu from 21 October 2018 to 2 November 2018 is shown in Figure 3.

1.5 The persistent high pressure system over north Asia resulted in anomalous north-easterly winds over the northern ASEAN region and South China Sea. To the south, anomalously strong south-easterly winds were observed over the Java sea area. Elsewhere in the region, winds were close to normal conditions.

1.6 There were signs of El Niño conditions developing, including the warming of sea surface temperature over the tropical Pacific Ocean, warmer than average sea sub-surface temperatures and weakening of trade winds. In addition, the sea surface temperature difference between the western and eastern Indian Ocean continued to indicate a positive Indian Ocean Dipole (IOD) event.

1.7 The Madden Julian Oscillation (MJO) weakened in Phase 3 during the third week of October 2018. MJO Phase 3 typically brings wetter weather over the western half of the ASEAN region and drier weather over the Philippines and the surrounding vicinity. This was broadly consistent with the regional rainfall patterns observed.

1.1 Inter-Monsoon conditions prevailed in the second fortnight of October 2018. The monsoon rainband along the equatorial ASEAN region continued to bring rainy weather to the southern ASEAN region. The passage of super typhoon Yutu across northern Philippines brought heavy rainfall to the Philippines and parts of the northern ASEAN region. Central parts of Sumatra and Kalimantan, and Luzon Island received above normal rainfall.

1.2 Parts of the Mekong sub-region experienced dry weather, in particular in areas such as northern Myanmar, southern parts of Lao PDR and Viet Nam, Cambodia and western Thailand, received below normal rainfall. Due to the occasional incursion of dry air from the southern hemisphere, areas around the Java Sea, including southern parts of Sumatra and Kalimantan and eastern Java, also received below normal rainfall.

1.3 The daily average rainfall and the percentage of average rainfall for the second fortnight of October 2018 are shown in Figure 1 and Figure 2 respectively.

1.4 On 21 October 2018, a tropical depression to the east of Guam strengthened into tropical storm Yutu, which subsequently tracked westwards and intensified into a super typhoon by 24 October 2018. After making landfall on the island of Luzon, the Philippines on 30 October 2018, Yutu traversed across the island, bringing torrential rainfall to northern Philippines. It later tracked north-westward into the South China Sea and weakened into a tropical depression. The track of Super Typhoon Yutu from 21 October 2018 to 2 November 2018 is shown in Figure 3.

1.5 The persistent high pressure system over north Asia resulted in anomalous north-easterly winds over the northern ASEAN region and South China Sea. To the south, anomalously strong south-easterly winds were observed over the Java sea area. Elsewhere in the region, winds were close to normal conditions.

1.6 There were signs of El Niño conditions developing, including the warming of sea surface temperature over the tropical Pacific Ocean, warmer than average sea sub-surface temperatures and weakening of trade winds. In addition, the sea surface temperature difference between the western and eastern Indian Ocean continued to indicate a positive Indian Ocean Dipole (IOD) event.

1.7 The Madden Julian Oscillation (MJO) weakened in Phase 3 during the third week of October 2018. MJO Phase 3 typically brings wetter weather over the western half of the ASEAN region and drier weather over the Philippines and the surrounding vicinity. This was broadly consistent with the regional rainfall patterns observed.