November 23, 2018
1.1 Dry conditions gave rise to isolated hotspot activities in parts of the Mekong sub-region (Figure 1) but the hotspots were generally short-lived. In the southern ASEAN region, hotspot activities were generally subdued due to the rainy weather.
Figures 1: Isolated hotspots detected in Cambodia and Viet Nam in early November
1.1 Northeast Monsoon conditions prevailed over the northern ASEAN region in the first half of November 2018. In most parts of the Mekong sub-region and the northern Philippines, conditions were drier with a decrease in shower activities as compared to the previous fortnight. Below-average rainfall was also received in many parts of the northern ASEAN region. In contrast, there were wetter-than-average conditions over many parts of the southern ASEAN region as a result of the Madden – Julian Oscillation (MJO) and wind convergence.
1.2 The daily average rainfall and the percentage of average rainfall for the first fortnight of November 2018 are shown in Figure 1 and Figure 2 respectively.
Figure 1: Daily average rainfall for the ASEAN region in the first fortnight of November 2018. (Source: JAXA Global Satellite Mapping of Precipitation)
Figure 2: Percentage of Average Rainfall for 1 – 15 November 2018. The rainfall data may be less representative for areas with a less dense rainfall network. (Source: IRI NOAA/NCEP CPC Unified Precipitation Analyses)
1.3 Tropical Storm Yutu, traversed across the Philippines in October 2018 and track northwestwards before it weakened into a low pressure system over the South China Sea in early November 2018. In the second week of the fortnight, Tropical Cyclone Gaja developed over the Bay of Bengal and subsequently tracked westwards away from the ASEAN region. The presence of Tropical Storm Yutu and Tropical Cyclone Gaja in the region during the fortnight influenced the winds in the region.
1.4 North-easterly or easterly winds prevailed over the northern ASEAN region, however there were anomalous south-westerly winds over the South China Sea to the northwest of the Philippines with the presence of Tropical Storm Yutu in the surrounding vicinity. In the southern ASEAN region, Tropical Cyclone Gaja over the Bay of Bengal led to anomalously strong westerly winds over the Indian Ocean to the west of Sumatra, Indonesia, and gave rise to the convergence of winds over the equatorial ASEAN region.
Figure 4: 5000 ft average winds (left) and anomalies (right) for 1 – 15 November 2018. (Source: JMA)
1.5 The Madden – Julian Oscillation (MJO) transitioned from Phase 1 to Phase 5 in the first half of November 2018. The MJO activities contributed to the rainy conditions experienced in the southern ASEAN region.
Figure 5: The MJO phase diagram for Oct-Nov 2018 (blue for November). 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)
1.6 The warming of sea surface temperatures in the tropical Pacific Ocean brought near El Niño thresholds to the region. However, the lack of sustained signs of El Niño in the trade winds, cloudiness and the Southern Oscillation Index (SOI) indicates that the tropical ocean and the atmosphere remained uncoupled. Hence, El Niño conditions have yet to be fully established.
November 14, 2018
Issued 14 November 2018
Isolated hotspot activities may emerge in the Mekong sub-region due to the dry and warmer-than-usual conditions in some parts of the sub-region. In the southern ASEAN region, hotspot activities are expected to remain generally subdued.
Issued 14 Nov 2018
valid for weeks 3 and 4: 16 Nov – 30 Nov
Climatologically, the northern ASEAN region experiences drier weather in the second half of November compared to the southern ASEAN region, where there would typically be more shower activities during this period. For the coming fortnight, drier-than-average conditions are expected over parts of Malaysia, Sumatra and Kalimantan. This is particularly so in the first week of the fortnight when the MJO is forecast to propagate eastwards through Phases 5 and 6. However, models’ predictions of the MJO development differ, with some models forecasting a weakening of the MJO during this period.
Slightly warmer temperatures are expected for the Mekong sub-region except for Myanmar. Near normal temperatures are predicted elsewhere.
The outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.