Subseasonal Weather Outlook and Review

1.1 Northeast Monsoon conditions over the ASEAN region persisted into the second fortnight of December 2019. During the preview period, the prevailing winds blew from the northeast or east over the northern ASEAN region and from the northwest or northeast over most parts of equatorial ASEAN region.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 16 – 31 Dec 2019 (Source: JMA)

1.2 Showers fell over many areas in the southern ASEAN region. East Malaysia, southern Sumatra, Kalimantan and Java recorded above-average rainfall while rainfall was below average in Peninsular Malaysia, and northern and central Sumatra. Parts of the Philippines received above-average rainfall due to the passage of Tropical Storm PHANFONE across central Philippines in late December 2019. Over the Mekong sub-region, dry weather persisted in the second fortnight of December 2019.

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

Figure 3: Percent of average rainfall in the second fortnight of December 2019. The rainfall data is less representative for areas with a less dense rainfall network. Hatched areas indicate climatology dry mask (average daily rainfall below 1 mm). (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

Figure 4: Historical Track of Tropical Storm PHANFONE (Source: JAXA)

1.3 Despite the strong Madden–Julian Oscillation (MJO) signal in Phases 6 and 7 in the last week of December 2019, its contribution to the rainfall in the eastern Maritime Continent was not discernible and could have been masked by the presence of Tropical Storm PHANFONE.

Figure 5: The MJO phase diagram for November (green) – December 2019 (blue). 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 The El-Niño Southern Oscillation (ENSO) remains neutral (i.e neither El Niño nor La Niña conditions). The Indian Ocean Dipole (IOD) is currently slightly positive but continues to weaken and a return to neutral state by early 2020 is likely.

1.1 Northeast Monsoon conditions over the ASEAN region persisted into the second fortnight of December 2019. During the preview period, the prevailing winds blew from the northeast or east over the northern ASEAN region and from the northwest or northeast over most parts of equatorial ASEAN region.

Figure 1: 5000 ft average winds (left) and anomalies (right) for 16 – 31 Dec 2019 (Source: JMA)

1.2 Showers fell over many areas in the southern ASEAN region. East Malaysia, southern Sumatra, Kalimantan and Java recorded above-average rainfall while rainfall was below average in Peninsular Malaysia, and northern and central Sumatra. Parts of the Philippines received above-average rainfall due to the passage of Tropical Storm PHANFONE across central Philippines in late December 2019. Over the Mekong sub-region, dry weather persisted in the second fortnight of December 2019.

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

Figure 3: Percent of average rainfall in the second fortnight of December 2019. The rainfall data is less representative for areas with a less dense rainfall network. Hatched areas indicate climatology dry mask (average daily rainfall below 1 mm). (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)

Figure 4: Historical Track of Tropical Storm PHANFONE (Source: JAXA)

1.3 Despite the strong Madden–Julian Oscillation (MJO) signal in Phases 6 and 7 in the last week of December 2019, its contribution to the rainfall in the eastern Maritime Continent was not discernible and could have been masked by the presence of Tropical Storm PHANFONE.

Figure 5: The MJO phase diagram for November (green) – December 2019 (blue). 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 The El-Niño Southern Oscillation (ENSO) remains neutral (i.e neither El Niño nor La Niña conditions). The Indian Ocean Dipole (IOD) is currently slightly positive but continues to weaken and a return to neutral state by early 2020 is likely.