Subseasonal Weather Outlook and Review

1.1 Southwest Monsoon conditions continued to prevail in the second fortnight of July 2018. The monsoonal rain band brought rainy weather to many parts of the northern ASEAN region. In general, there was above normal rainfall, except for southern Thailand and southern Viet Nam, where rainfall was below normal. In contrast, dry weather prevailed over the southern ASEAN region. Rainfall was below normal in southern Sumatra, southern Kalimantan, Sabah and Java. Nonetheless, there was a gradual increase in shower activities towards the end of July.

1.2 The daily average rainfall and the percent of normal rainfall for the second fortnight of July 2018 are shown in Figure 1 and Figure 2.

1.3 On 17 July 2018, Tropical Storm Son-Tinh developed over the South China Sea and tracked westwards before making landfall at northern Viet Nam on 19 July 2018. The tropical storm brought torrential rain to parts of Viet Nam and Lao PDR. The track of Tropical Storm Son-Tinh from 17 July 2018 to 19 July 2018 is shown in Figure 3.

1.4 Broad scale anomalous westerly winds were observed over the northern ASEAN region, brought about by the development of tropical low pressure systems over the South China Sea. Meanwhile, the winds over the southern ASEAN region were predominantly from the southeast or southwest.

1.5 Neutral conditions of the El Niño-Southern Oscillation (ENSO) persisted, and the sea surface temperature over the tropical Pacific Ocean decreased slightly but continued to remain higher than average. Other atmospheric indicators were also indicative of neutral conditions.

1.6 The Madden Julian Oscillation (MJO) transitioned from Phase 5 to Phase 6 in the third week of July. The MJO subsequently weakened and became indiscernible by the end of July. MJO Phases 5 and 6 typically brought wetter weather over the Western Pacific Ocean and drier weather over the surrounding region of Java Sea. This was consistent with the regional rainfall patterns observed.

1.1 Southwest Monsoon conditions continued to prevail in the second fortnight of July 2018. The monsoonal rain band brought rainy weather to many parts of the northern ASEAN region. In general, there was above normal rainfall, except for southern Thailand and southern Viet Nam, where rainfall was below normal. In contrast, dry weather prevailed over the southern ASEAN region. Rainfall was below normal in southern Sumatra, southern Kalimantan, Sabah and Java. Nonetheless, there was a gradual increase in shower activities towards the end of July.

1.2 The daily average rainfall and the percent of normal rainfall for the second fortnight of July 2018 are shown in Figure 1 and Figure 2.

1.3 On 17 July 2018, Tropical Storm Son-Tinh developed over the South China Sea and tracked westwards before making landfall at northern Viet Nam on 19 July 2018. The tropical storm brought torrential rain to parts of Viet Nam and Lao PDR. The track of Tropical Storm Son-Tinh from 17 July 2018 to 19 July 2018 is shown in Figure 3.

1.4 Broad scale anomalous westerly winds were observed over the northern ASEAN region, brought about by the development of tropical low pressure systems over the South China Sea. Meanwhile, the winds over the southern ASEAN region were predominantly from the southeast or southwest.

1.5 Neutral conditions of the El Niño-Southern Oscillation (ENSO) persisted, and the sea surface temperature over the tropical Pacific Ocean decreased slightly but continued to remain higher than average. Other atmospheric indicators were also indicative of neutral conditions.

1.6 The Madden Julian Oscillation (MJO) transitioned from Phase 5 to Phase 6 in the third week of July. The MJO subsequently weakened and became indiscernible by the end of July. MJO Phases 5 and 6 typically brought wetter weather over the Western Pacific Ocean and drier weather over the surrounding region of Java Sea. This was consistent with the regional rainfall patterns observed.