Monthly Archives: May 2021

May 28, 2021

Subseasonal Weather Outlook (31 May – 13 June 2021)

Issued 28 May 2021
First forecast week: 31 May – 6 June
Second forecast week: 7 June – 13 June

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

For Mainland Southeast Asia, an increased chance of drier conditions is predicted over much of the region in Week 1 (31 May – 6 June). In Week 2 (7 – 13 June), wetter conditions may develop over the western coastal region.

Elsewhere in the next fortnight (31 May – 13 June), an increased chance of drier conditions is predicted over much of the southern ASEAN region, particularly over the central equatorial region. Wetter conditions than usual are expected over central and southern Philippines in the next fortnight.

Warmer temperatures than usual are expected over much of the ASEAN region in Week 1 (31 May – 6 June), in particular over Mainland Southeast Asia, northern Philippines and parts of the southern Maritime Continent.

Towards the end of May, an MJO signal was present over the Western Pacific (Phase 6). However, this MJO signal is no longer discernible based on the RMM Index and the models are uncertain as to how the MJO will evolve over the forecast period.

The outlook is assessed for the region in general, where conditions are relative to the average conditions for the corresponding time of year. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.

May 21, 2021

Review of Regional Haze Situation for April 2021

1.1 In April 2021, Northeast Monsoon conditions transitioned to Inter-monsoon conditions as the prevailing winds over most of the ASEAN region became light and variable in direction, except over the Philippines where the prevailing winds blew from the north or northeast.

1.2 During this period, there was an increase in shower activities over most of the northern ASEAN region as the monsoon rainband shifted northwards to be located near the Equator. However, the northern and northwestern parts of the Mekong sub-region experienced drier conditions compared to the other parts of the sub-region. In the southern ASEAN region, wet conditions generally prevailed (Figure 1).

Figure 1: Average Daily Rainfall and Mean Wind for April 2021. (Source: JAXA GsMaP and GFS respectively)

1.3 The hotspot counts over the ASEAN region for April 2021 were either comparable or lower than the previous years in the same review period except for Kalimantan where hotspot counts were higher than previous years (Figures 2 and 3).

Figure 2: Hotspot counts for the northern ASEAN region for the month of April (2017 – 2021). [Note: The hotspot counts from 2019 onwards are based on the NOAA-20 satellite, while those from 2017 – 2018 are based on the Suomi-NPP satellite.]

Figure 3: Hotspot counts for the southern ASEAN region for the month of April (2017 – 2021). [Note: The hotspot counts from 2019 onwards are based on the NOAA-20 satellite, while those from 2017 – 2018 are based on the Suomi-NPP satellite.]

1.4 Hotspot activities were detected in most parts of the northern ASEAN region in April 2021. The hotspot clusters over Myanmar and the northern parts of Lao PDR were more persistent and widespread due to drier conditions over these areas (Figures 4 and 5).

1.5 Moderate to dense smoke haze was observed to emanate from hotspot clusters detected in the northern and southern parts of Lao PDR, and most parts of Myanmar (Figure 6). Smoke haze from the eastern parts of Myanmar and northern parts of Lao PDR was also observed to have been blown into the northern parts of Lao PDR and Viet Nam respectively by the prevailing winds on some days. In view of increased shower activities towards the end of April 2021 helping to improve the hotspot and haze situation over the Mekong sub-region, the Alert Level was downgraded from Alert Level 2 to Alert Level 1 on 30 April 2021.

1.6 In the southern ASEAN region, brief periods of drier conditions led to the development of isolated hotspots mostly in Peninsular Malaysia, Borneo Island, Sumatra and Sulawesi (Figures 4 and 5). No smoke plumes were observed to have emanated from these hotspots (Figure 6).

Figure 4: Distribution of hotspots detected in March 2021 based on NOAA-20 satellite surveillance.

Figure 5: Maximum fire intensity in March 2021 based on NOAA-20 satellite surveillance.

Figure 6: Number of days with moderate to dense smoke haze observed in March 2021 over an area based on satellite imagery, ground observations and air quality reports.

Alert20210521 – End of Dry Season for the Northern ASEAN Region

End of Dry Season for the Northern ASEAN Region

Over the past week, there have been increased shower activities over the northern ASEAN region. Based on surveillance by the NOAA-20 satellite, the total hotspot count in the region has remained low, with 30 and 29 hotspots detected on 19 and 20 March 2021 respectively.

With rainy weather forecast over the northern ASEAN region in the coming days, hotspot activities are expected to be subdued. However, there could still be isolated hotspots with localised smoke plumes during brief periods of drier weather.

May 14, 2021

Subseasonal Weather Outlook (17 – 30 May 2021)

Issued 14 May 2021
First forecast week: 17 May – 23 May
Second forecast week: 24 May – 30 May

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

An increased chance of wetter conditions is expected over parts of the eastern Maritime Continent in Week 1 (17 – 23 May). Models are also predicting wetter conditions over southern Viet Nam and Cambodia, although with a lower confidence compared to the eastern Maritime Continent.

Drier conditions than usual are expected over much of northwestern Mainland Southeast Asia in Week 1 (17 – 23 May).

Warmer temperatures than usual are expected over much of northwestern Mainland Southeast Asia in Week 1 (17 – 23 May). These warmer temperatures are expected to ease and become confined mainly to northern parts of Myanmar and Lao PDR in Week 2 (24 – 30 May).

A weak Madden-Julian Oscillation (MJO) signal was present over the Indian Ocean (Phases 2 and 3). While the MJO signal is expected to be less coherent at the start of the forecast period, most models forecast the MJO to propagate eastwards and move into the Maritime Continent (Phase 4) by middle of Week 1. By end of May, some models predict the MJO to reach the Western Pacific (Phases 6 and 7).

Review of Regional Weather for April 2021

1. Overview

1.1 During April 2021, much of the Mainland Southeast Asia and the Malay Peninsula experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over northern parts of Thailand and Lao DPR for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended), with GSMaP-NRT also recording larger anomalies over Peninsular Malaysia and eastern side of northern Sumatra. Large positive anomalies were also recorded over the eastern part of central Philippines and the region around Nusa Tenggara due to Typhoon Surigae and Tropical Cyclone Seroja, respectively. Much of the rest of the Maritime Continent experienced below-average rainfall.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall over the northern ASEAN region and drier conditions in the southern ASEAN region) is broadly consistent with the predictions from the subseasonal weather outlooks for April 2021 (5 – 18 Apr, 19 Apr – 2 May).

Figure 1: Rainfall anomalies for the month of April 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most parts of the northern ASEAN region experienced below-average temperatures during April 2021, apart from northern parts of Myanmar and Viet Nam where above-average temperatures were recorded (Figure 2). In contrast, most parts of the southern ASEAN region experienced near- to above-average temperatures.

Figure 2: Temperature anomalies for the month of April 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of April 2021, a Madden-Julian Oscillation (MJO, Figure 3) signal was present in Phase 5 (Maritime Continent). The signal then propagated eastward into the Western Pacific (Phases 6 and 7). The signal became less coherent for a time in Phase 7, before strengthening again and propagating eastwards into the Western Hemisphere (Phase 8) at the end of the month. Phase 5 tends to bring wetter conditions to the eastern half of the Maritime Content, while Phase 6 tends to bring drier conditions to the western half. Phases 7 and 8 tend to bring drier conditions to much of the Maritime Continent, similar to what is observed from the rainfall anomalies in Figure 1.

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

2.2 Over the tropical Pacific Ocean, La Niña conditions were present but weakening. Overall, the sea surface temperatures in the central and eastern Pacific Ocean remained cool, with atmospheric indicators (cloudiness and wind anomalies) remaining consistent with the weakening La Niña conditions. La Niña events tend to bring wetter-than-average conditions to parts of Southeast Asia during the March to May period.