Monthly Archives: April 2021

April 30, 2021

Subseasonal Weather Outlook (3 – 16 May 2021)

Issued 30 April 2021
First forecast week: 3 May – 9 May
Second forecast week: 10 May – 16 May

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are expected over parts of the western and central Maritime Continent in Week 1 (3 – 9 May) and may extend northward to coastal regions of southern Viet Nam. These wetter conditions are expected to ease in Week 2 (10 – 16 May) and be confined mainly to northern Sumatra and Peninsular Malaysia.

Drier conditions than usual are expected over much of the southern parts of the Maritime Continent in the next fortnight (3 – 16 May).

Cooler temperatures than usual are expected over parts of the western and central Maritime Continent in the next fortnight (3 – 16 May).

A strong Madden-Julian Oscillation (MJO) signal was present over the Western Hemisphere and Africa (Phase 8) at the end of April. Most models forecast the MJO to weaken in May as it propagates eastwards, decaying over the Indian Ocean towards the end of Week 1.

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.

Alert20210430 – Downgrading of Alert Level 2 to Alert Level 1 for the Mekong Sub-region

Downgrading of Alert Level 2 to Alert Level 1 for the Mekong Sub-region

In the past week, there has been an increase in shower activities over many parts of the Mekong sub-region. The showers have helped to improve the hotspot and haze situation there. Based on surveillance by the NOAA-20 satellite, the number of hotspots has been steadily decreasing in recent days. 210 and 173 hotspots were detected in the Mekong sub-region on 28 and 29 Apr respectively, a significant reduction from the high of over 3000 hotspots detected last week.

More rain is forecast for the Mekong sub-region in the coming weeks. This would help to further subdue the hotspot activities in the region. Occasional isolated hotspots with localised smoke plumes may still be expected during brief periods of dry weather.

April 20, 2021

Review of Regional Haze Situation for March 2021

1.1 In March 2021, Northeast Monsoon conditions gradually weakened over the ASEAN region and the prevailing winds started to turn light and variable on several days. In the northern ASEAN region, the prevailing winds blew mainly from the northeast or southeast, although light and variable winds prevailed over the western parts of the Mekong sub-region. Extended dry conditions persisted over the sub-region in this month as its traditional dry season became established.

1.2 For the southern ASEAN region, the prevailing winds blew mainly from the northeast or east. Northern and central Sumatra as well as parts of Peninsular Malaysia experienced drier conditions in early March 2021 under the influence of the dry phase of the Northeast Monsoon. Elsewhere over the ASEAN region, isolated to scattered showers were observed.

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

1.3 For March 2021, slightly higher hotspot counts were detected in Cambodia compared to previous years for the same period. In other parts of the ASEAN region, the hotspot counts for March this year were lower or comparable to previous years (Figures 2 and 3).

Figure 2: Hotspot counts for the northern ASEAN region for the month of March (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 March (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 Widespread to scattered hotspot activities were detected in most parts of the Mekong sub-region in March 2021, especially in Myanmar, northern Thailand, northern Cambodia and Lao PDR where persistent clusters of hotspots were detected on many days (Figures 4 and 5), contributed by continued dry conditions throughout the month.

1.5 Moderate to dense haze was observed emanating from clusters of hotspots on most days in the Mekong sub-region, particularly over Myanmar and northern parts of Thailand, Lao PDR and Cambodia (Figure 6).

1.6 In the southern ASEAN region, there were brief periods of drier weather in the first half of March 2021 over northern and central Sumatra, Peninsular Malaysia and West Kalimantan. This led to the development of isolated hotspots with localised smoke plumes on several days in the drier areas. As shower activities returned in late March 2021, the hotspot activities and smoke haze situation became more subdued.

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.

April 16, 2021

Subseasonal Weather Outlook (19 April – 2 May 2021)

Issued 16 April 2021
First forecast week: 19 April – 25 April
Second forecast week: 26 April – 2 May

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are expected over northern and central Philippines in Week 1 (19 – 25 April) associated with tropical cyclone activity. There is also a chance of wetter conditions over southern Myanmar, southern Thailand, southern Cambodia, and central and southern Viet Nam in Week 1.

Drier conditions than usual are expected over much of the southern ASEAN region in Week 1 (19 – 25 April). The drier conditions are expected to ease in Week 2 (26 April – 2 May).

Cooler temperatures than usual are expected over central and southeastern Mainland Southeast Asia, as well as northern and central Philippines, in the next fortnight (19 April – 2 May).

A strong Madden-Julian Oscillation (MJO) signal was present over the Western Pacific (Phase 7) in the middle of April. Most models predict this signal to continue propagating eastwards during the next fortnight, entering the Western Hemisphere and Africa (Phase 8) by Week 2.

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.

April 12, 2021

Review of Regional Weather for March 2021

1. Overview

1.1 During March 2021, much of the land masses of the Maritime Continent experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over western coastal parts of Sumatra, eastern coastal parts of Peninsular Malaysia and western Borneo based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). For Mainland Southeast Asia, rainfall anomalies were largely negligible for most of the region, as expected during the dry season for the northern ASEAN region, except for the southern tips of Thailand and Myanmar where the dry conditions are more pronounced.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall over land masses of the Maritime Continent is broadly consistent with the predictions from the subseasonal weather outlooks for March 2021 (22 Feb – 7 Mar, 8 – 21 Mar, 22 Mar – 4 Apr), apart from the anomalies for some parts of southern Sumatra and central Java not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of March 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 Southeast Asia south of 10°N experienced near-average temperatures during March 2021 (Figure 2). In contrast, warmer-than-average temperatures were recorded over most of Southeast Asia north of 10°N, except for some south-eastern parts of Mainland Southeast Asia where below- to near-average temperatures are observed.

Figure 2: Temperature anomalies for the month of March 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 During the first half of March 2021, the Madden-Julian Oscillation (MJO) signal was weak (Figure 3). In the second half of March, it strengthened in the Western Hemisphere (Phases 8 and 1) and propagated eastward through the Indian Ocean (Phases 2 and 3) and western Maritime Continent (Phase 4). Phase 2 and Phase 3 tend to bring wetter conditions to western Maritime Continent and whole of Maritime Continent respectively.

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 much of Southeast Asia during the September to March period.

April 1, 2021

Subseasonal Weather Outlook (5 – 18 April 2021)

Issued 1 April 2021
First forecast week: 5 April – 11 April
Second forecast week: 12 April – 18 April

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are expected over much of the northern ASEAN region as well as northern Borneo, northern Sulawesi, and Maluku Islands in the next fortnight (5 – 18 April). In particular, for Week 1 (5 – 11 April) the highest probability of wetter conditions is over northern Lao PDR and much of Thailand. For Week 2 (12 – 18 April), the highest probability for wetter conditions is over southern Thailand, Cambodia, southern Viet Nam, and the Philippines.

Conversely, slightly drier conditions than usual are expected over much of the southern ASEAN region in Week 1 (5 – 11 April). There is a chance that these drier conditions may continue into Week 2 (12 – 18 April), although the confidence is low.

Cooler temperatures than usual are expected over much of Mainland Southeast Asia in the next fortnight (5 – 18 April), in line with the wetter conditions predicted. Warmer conditions than usual are expected over most of the equatorial region in Week 1 (5 – 11 April) and are expected to return to near-normal in Week 2 (12 – 18 April).

A moderate Madden-Julian Oscillation (MJO) signal was present over the Maritime Continent at the start of April. Models predict this MJO signal to continue propagating eastwards reaching the Western Pacific (Phase 6) at the start of Week 1, before weakening in Phase 7.

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.