Category Archives: Fortnightly Weather Review

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.

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.

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.

1.1 The Southwest Monsoon season transitioned into the inter-monsoon period around mid-October 2018. During the first half of October 2018, the monsoon rainband shifted southward to the equatorial ASEAN region. This southward shift was driven by stronger-than-usual northeasterly and easterly winds over the South China Sea, due to an outflow of dry air from a high pressure system over north Asia. As a result, most parts of the northern ASEAN region received below normal rainfall, except for Myanmar where above normal rainfall was contributed by rain associated with a low pressure weather system that developed over the Andaman Sea and Bay of Bengal.

1.2 The monsoon rainband brought above-normal rainfall to the equatorial ASEAN region, viz northern half of Sumatra, Indonesia, Peninsular Malaysia, and parts of Kalimantan. Below normal rainfall was received in southern Sumatra and Java due to the incursion of dry air from the southern hemisphere.

1.3 The rainfall distribution and the percentage of average rainfall for the first fortnight of October 2018 are shown in Figure 1 and 2, respectively.

1.4 Under the influence of an intense high pressure system over north Asia, there were anomalous easterly winds over the northern ASEAN region and the South China Sea. Elsewhere in the region, winds were close to near normal conditions.

1.5 In the first fortnight of October 2018, there are indications of El Niño conditions developing in the tropical Pacific Ocean. For instance, the sea surface temperatures were observed to be warmer than average along the equator in the Pacific Ocean, and trade winds were also observed to have weakened. Over the Indian Ocean, sea surface temperatures were observed to be cooler than usual in the eastern Indian Ocean – signs of a nascent positive Indian Ocean Dipole (IOD) event.

1.6 The Madden Julian Oscillation (MJO) which was active and propagated through Phases 1 and 2 during the first fortnight of October 2018, significantly influenced the weather in the region. The drier than usual conditions over parts of the northern ASEAN region and around the Java Sea area, as well as the anomalous easterly winds over the South China Sea were characteristic of the MJO signal in Phases 1 and 2 over the ASEAN region.

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.

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.

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.

1.1 Southwest Monsoon conditions prevailed in the first fortnight of September 2018. During the period, the monsoon rainband situated between latitudes 5°N and 15°N, brought rainy conditions to the northern ASEAN region. Near- to below normal rainfall was recorded over most parts of the northern ASEAN region, except for parts of Myanmar and southern Viet Nam where above-normal rainfall was received. The northern parts of Philippines received above-normal rainfall, contributed partly by the tropical storm activities during the fortnight.

1.2 In the southern ASEAN region, the incursion of dry air from the southern hemisphere brought dry conditions over most parts of the region. Most parts of the southern ASEAN region experienced below-normal rainfall, except for Peninsular Malaysia and the northern half of Sumatra where near-normal rainfall was received.

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

1.4 During the second week of September 2018, Typhoon Mangkhut, developed over the western Pacific Ocean and tracked west to northwestwards toward the Philippines. The storm intensified rapidly into a Super Typhoon and brought heavy rains accompanied by strong winds as it made landfall over Luzon, the Philippines on 15 September 2018. The rainband associated with the Super Typhoon brought wet weather over most parts of the northern ASEAN region, northern Peninsular Malaysia and northern Sumatra.

1.5 During the first half of September 2018, anomalous northwesterly winds were observed over southern Thailand and Peninsular Malaysia while over the South China Sea, slightly stronger than usual westerly winds were observed. The anomalous winds were due to the influence of Super Typhoon Mangkhut, as well as Typhoon Jebi (Typhoon Jebi developed in the Pacific Ocean and affected Japan). Figure 4 shows the average and anomalous winds at 5000 feet.

1.6 During the period, the sea surface temperatures were observed to be warmer than average in the central and eastern Pacific Ocean, and the trade winds were observed to be slightly weaker in the western Pacific Ocean. However, the El Niño-Southern Oscillation (ENSO) continued to remain in the neutral state (neither El Niño nor La Niña).

1.7 The Madden Julian Oscillation (MJO) signal was generally weak and non-discernible during the first fortnight of September 2018, and had little or no influence on the weather in the region.

1.1 Southwest Monsoon conditions prevailed in the second fortnight of August 2018. During the period, rainy weather persisted over most parts of the northern ASEAN region. For the northern parts of Myanmar, Lao PDR and Vietnam and parts of the Philippines, above-average rainfall was recorded.

1.2 In the southern ASEAN region, Sumatra and Kalimantan experienced drier conditions during the first half of the fortnight. This was followed by a gradual increase of shower activities in the latter half of the fortnight. The Java region remained generally dry throughout the period.

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

1.4 Stronger-than-usual southwesterly winds prevailed over the northern ASEAN region. In the southern ASEAN region, there were no significant wind anomalies observed, and the prevailing winds generally blew from the southeast or southwest. Figure 3 shows the average and anomalous winds at 5000 feet.

1.5 The El-Southern Oscillation (ENSO) remained in the neutral state(neither El Niño nor La Niña), and other atmospheric indicators continued to show neutral conditions.

1.6 The Madden Julian Oscillation (MJO), which started in Phase 6, weakened and became non-discernible throughout the review period. The weakening of the MJO Phase 6 partly contributed to the cessation of drier conditions over the southern ASEAN region in the second fortnight of August 2018.

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 The Southwest Monsoon conditions prevailed in the first fortnight of July 2018. During the period, the equatorial region received more rainfall than the other parts of the Southeast Asia in the first week of July. In the second week of the month, with the strengthening of the monsoon winds and movement of the monsoon rain band away from the equatorial region, wetter weather was felt in the northern ASEAN region while the southern ASEAN region transitioned from wet to dry weather conditions by the end of the fortnight. These conditions were consistent with model forecasts, as indicated in the subseasonal outlook for 2 – 16 July 2018.

1.2 During the review period, parts of the Mekong sub-region, including Myanmar, Lao PDR and Viet Nam, received above-normal rainfall. Rainfall was below normal over most parts of the western equatorial Maritime Continent, particularly over Peninsular Malaysia, Sumatra, Java, and West and Central Kalimantan. The rainfall distribution and the percent of normal rainfall for the first fortnight of July 2018 are shown in Figures 1 and 2.

1.3 Stronger-than-usual southwesterly winds prevailed over the northern ASEAN region and the South China Sea. In areas south of the Equator, winds generally blew from the southeast or south, typical of the wind flow for this time of the year. Figure 3 shows the average and anomalous winds at 5000 feet.

1.4 The El Niño-Southern Oscillation (ENSO) remained in the neutral state (neither El Niño nor La Niña). While there were signs of further increase in sea surface temperatures in the eastern Pacific Ocean, other atmospheric indicators continued to show neutral conditions.

1.5 In the second week of July 2018, the Madden Julian Oscillation (MJO) strengthened and progressed through Phases 4 and 5 . While its strength was stronger than expected, its movement was largely in line with model forecasts. This was consistent with the increased rainfall activities and a strengthening of south-westerly or westerly winds observed over the northern ASEAN region, and the transition from wet to dry conditions over the southern ASEAN region.

1.1 The southwest monsoon conditions prevailed in the second fortnight of June 2018. Shower activities were observed over many parts of the region.

1.2 During this period, there were isolated showers over parts of the northern ASEAN region but wetter weather was observed over coastal areas of Myanmar. In southern Myanmar, heavy rains reportedly triggered floods and landslides during the beginning of the second fortnight. Based on climatology, the wet conditions over coastal Myanmar is typical during this period of the season. Most parts of the Mekong sub-region experienced below-normal to near-normal rainfall, while northern and coastal areas of Myanmar, eastern Cambodia and southern Vietnam received above-normal rainfall.

1.3 In the southern ASEAN region, large scale convergence of winds caused by a circulation west of Borneo brought widespread shower activities over the region. Most areas experienced wetter than usual conditions, except for central Sumatra, central Kalimantan and Java where below-normal rainfall were received.

1.4 The rainfall distribution and the percentage of normal rainfall for the second fortnight of June 2018 are shown in Figures 1 and 2.

1.5 During the second fortnight of June 2018, weather in the region was influenced by Tropical Storm “Gaemi” and Typhoon “Prapiroon” as well. Rainbands associated with “Gaemi” brought widespread showers over parts of the northern ASEAN region, and persistent wet weather over Sumatra, Peninsular Malaysia and Singapore in the southern ASEAN region. Toward the end of June 2018, rain bands from “Prapiroon” also brought widespread showers over most parts of the ASEAN region.

1.1 Inter-monsoon conditions prevailed in May 2018, and most parts of the ASEAN region experienced an increase in shower activities. The increased shower activities brought an end to the dry season in the northern ASEAN region.

1.2 In May 2018, most parts of the Mekong sub-region received below-normal to near-normal rainfall except for the northern parts of Myanmar, eastern Cambodia and southern Vietnam where above-normal rainfall was recorded.

1.3 In the southern ASEAN region, below-normal rainfall was experienced over the northern parts of Sumatra, eastern Peninsula Malaysia, Java, and parts of Kalimantan. Elsewhere, near-normal to slightly above-normal rainfall were received.

1.4 The rainfall distribution for May 2018 and the percentage of normal rainfall for May 2018 are shown in Figure 1 and 2.

1.5 In the last week of May 2018, a low-pressure system developed over the Bay of Bengal and tracked north to northeast-wards toward Myanmar before making landfall around end May. The rain band associated with the low pressure system brought strong winds and heavy rain over many parts of Myanmar, including Yangon and Mandalay.

1.6 Over the South China Sea and the southern part of the Mekong sub-region, the northeasterly/easterly wind component was anomalously strong. Prevailing winds typically blow from the southwest or west in May, and the wind anomaly in May 2018 could be due to a persistent ridge over the southern coast of China from which there was an outflow of northeasterly and easterly winds. Figure 3 shows the average and anomalous winds at 5000 feet.

1.7 The El Niño-Southern Oscillation (ENSO) remained neutral (neither El Niño or La Niña) in May 2018. The atmospheric and oceanic indicators of ENSO such as sea surface temperature and trade winds were also indicative of neutral conditions.

1.8 In May 2018, the Madden Julian Oscillation (MJO) progressed from Phase 1 to Phase 4 from the second week of the month. The MJO in Phase 3 and 4 typically brings slightly enhanced rainfall over the southern ASEAN region during this time of the year. In the last week of May 2018, an increase in rainfall was observed over parts of the southern ASEAN region.

2. Review of Land/Forest Fires and Smoke Haze Situation

3. Status of El Niño/La Niña and Indian Ocean Dipole

1.1 The Northeast Monsoon conditions gave way to the inter-monsoon conditions in April 2018. The inter-monsoon period is characterised by afternoon showers with winds that are generally light and variable in direction. In April 2018, a gradual increase in shower activities was observed over parts of the Mekong sub-region, while in the southern ASEAN region, conditions were generally wet.

1.2 In the northern ASEAN region, most parts of the Mekong sub-region received near-normal to above-normal rainfall, except for parts of Myanmar, eastern Thailand and southern Vietnam.

1.3 In the southern ASEAN region, Peninsula Malaysia and Sumatra received below-normal rainfall. Near normal to below-normal rainfall were received over southern Kalimantan and Java.

1.4 The rainfall distribution for April 2018 and the percentage of normal rainfall for April 2018 are shown in Figure 1 and 2.

1.5 In April 2018, prevailing winds in the region were predominantly blowing from the northeast or southeast, with comparatively weaker winds within the equatorial belt between 5ºN and 5ºS. Stronger northeasterly winds were observed over the South China Sea, while anomalously strong westerly winds continued to be observed to the east of Sulawesi since March 2018. Figure 3 shows the average and anomalous winds at 5000 feet.

1.6 During the month, the El Niño-Southern Oscillation (ENSO) remained at neutral (neither El Niño or La Niña) conditions. The sea surface temperature over the central tropical Pacific Ocean, as well as the cloudiness and trade wind conditions were also indicative of neutral conditions.

1.7 The Madden Julian Oscillation (MJO) progressed from Phase 7 to Phase 2 during the first half of the month before propagating to Phase 3 during the third week of April 2018. The MJO in Phase 2 and 3 typically brings suppressed rainfall over the area around the Philippines, and slightly enhanced rainfall over the southern ASEAN region during this time of the year. However, the MJO event in April 2018 did not have a significant influence on the weather patterns in the ASEAN region. For the rest of the month, the MJO signal remained weak.

2. Review of Land/Forest Fires and Smoke Haze Situation

3. Status of El Niño/La Niña and Indian Ocean Dipole