Review of Regional Weather Conditions for Second Fortnight of September 2018

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1. Review of Regional Weather Conditions for Second Fortnight of September 2018

1.1 Southwest Monsoon conditions continued to prevail in the second fortnight of September 2018. During the period, the northern ASEAN region experienced rainy weather on most days. The southward shift of the monsoon rainband brought an increase in rainfall in the equatorial region including southern Thailand, Peninsular Malaysia, northern Sumatra, Sabah and Sarawak, which is typical for this period of the year.

1.2 In southern Sumatra and Kalimantan, there were showers interspersed with brief periods of dry weather. The Java region continued to remain generally dry. Generally, the observed conditions were consistent with the outlook provided for the period.

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


Figure 1: Daily average rainfall for the ASEAN region in the second fortnight of September 2018. (Source: JAXA Global Satellite Mapping of Precipitation)

Figure 2: Percentage of Normal Rainfall for 16 – 30 September 2018. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

1.4 Light and variable winds prevailed over the northern ASEAN region. In the southern ASEAN region, prevailing winds blew generally from the southeast or south. There were no significant wind anomalies observed in the region during the review period. Figure 3 shows the average and anomalous winds at 5000 feet.



Figure 4: 5000 ft average winds (left) and anomalies (right) for 16 – 30 September 2018. (Source: JMA)

1.5 The sea surface temperatures of the eastern Pacific Ocean reflected neutral El Niño-Southern Oscillation (ENSO) conditions (neither El Niño nor La Niña). The key atmospheric indicators also suggested neutral conditions, although weak westerly low-level wind anomalies have developed recently.

1.6 The Madden Julian Oscillation (MJO) was largely weak and non-discernible during the review period. Nonetheless, the MJO (Phase 8) began to strengthen and develop toward the end of the fortnight. The MJO had no significant influence on the weather in the region during this period.

Figure 5: The MJO phase diagram for September 2018 (blue for September). 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)