Subseasonal Weather Outlook and Review

1.1 Southwest Monsoon conditions prevailed in the first half of August 2019. In the northern ASEAN region, the prevailing winds were blowing mostly from the southwest or west. During the fortnight, the westerly winds were anomalously strong and stronger-than-usual winds were also observed over the Philippines. The stronger winds could have been due to the presence of Tropical Storm Wipha over the South China Sea in early August 2019 and Typhoon Lekima over the western Pacific Ocean from 4 August till 12 August 2019. Wipha made landfall in northern Viet Nam on 3 August, bringing heavy rain and flash floods to the area. The rainband associated with the storms contributed to the well above-average rainfall recorded over northern Philippines and many parts of the Mekong sub-region.

1.2 In the southern ASEAN region, winds were blowing mainly from the southeast or southwest. The dry conditions observed since July 2019 persisted into the first half of August 2019. Well below-average rainfall was observed over many areas. The dry conditions could be attributed partly to the presence of the Indian Ocean Dipole (IOD) in the positive phase.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 1 – 15 August 2019 (Source: JMA)

Figure 2: Daily average rainfall for the ASEAN region in the first fortnight of August 2019 (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 3: Percent of average rainfall for 1 – 15 August 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.3 There were signs of the Madden-Julian Oscillation (MJO) appearing briefly in Phases 3 and 4 (6th – 11th August), although MJO activity was insignificant by the end of the fortnight. The enhanced shower activities in the northern ASEAN region were consistent with the typical rainfall response from MJO in Phases 3 and 4 during this period.

Figure 4: The MJO phase diagram (blue for August 2019). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology)

1.4 In the first fortnight of August 2019, the El Niño – Southern Oscillation (ENSO) remained neutral (i.e neither El Niño nor La Niña conditions).

1.1 Southwest Monsoon conditions prevailed in the first half of August 2019. In the northern ASEAN region, the prevailing winds were blowing mostly from the southwest or west. During the fortnight, the westerly winds were anomalously strong and stronger-than-usual winds were also observed over the Philippines. The stronger winds could have been due to the presence of Tropical Storm Wipha over the South China Sea in early August 2019 and Typhoon Lekima over the western Pacific Ocean from 4 August till 12 August 2019. Wipha made landfall in northern Viet Nam on 3 August, bringing heavy rain and flash floods to the area. The rainband associated with the storms contributed to the well above-average rainfall recorded over northern Philippines and many parts of the Mekong sub-region.

1.2 In the southern ASEAN region, winds were blowing mainly from the southeast or southwest. The dry conditions observed since July 2019 persisted into the first half of August 2019. Well below-average rainfall was observed over many areas. The dry conditions could be attributed partly to the presence of the Indian Ocean Dipole (IOD) in the positive phase.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 1 – 15 August 2019 (Source: JMA)

Figure 2: Daily average rainfall for the ASEAN region in the first fortnight of August 2019 (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 3: Percent of average rainfall for 1 – 15 August 2019. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.3 There were signs of the Madden-Julian Oscillation (MJO) appearing briefly in Phases 3 and 4 (6th – 11th August), although MJO activity was insignificant by the end of the fortnight. The enhanced shower activities in the northern ASEAN region were consistent with the typical rainfall response from MJO in Phases 3 and 4 during this period.

Figure 4: The MJO phase diagram (blue for August 2019). The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator. The distance of the index from the centre of the diagram is correlated with the strength of MJO. When the index falls within the circle, the MJO is considered weak or indiscernible. (Source: Bureau of Meteorology)

1.4 In the first fortnight of August 2019, the El Niño – Southern Oscillation (ENSO) remained neutral (i.e neither El Niño nor La Niña conditions).