Monthly Archives: September 2026

September 24, 2026

Subseasonal Weather Outlook (28 September – 11 October 2026)

Subseasonal Weather Outlook (28 September – 11 October 2026)

Issued: 23 September 2026
First forecast week: 28 September – 4 October 2026
Second forecast week: 5 – 11 October 2026

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Figure 1: Rainfall Outlook

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Figure 2: Temperature Outlook

Drier than usual conditions are predicted over most of the southern ASEAN region in the next fortnight (28 September – 11 October). Drier than usual conditions are also predicted for parts of the northeastern Maritime Continent and southern Mainland Southeast Asia in Week 1 (28 September – 4 October).

Warmer than usual conditions are predicted over most of the ASEAN region in Week 1 (28 September – 4 October). These warmer conditions are predicted to persist over most of the southern ASEAN region in Week 2 (5 – 11 October).

A Madden-Julian Oscillation (MJO) signal was present over the Western Hemisphere (Phases 8 and 1) in the third week of September, based on the RMM index. Models predict the signal to weaken and become inactive by the start of the forecast period. During the forecast period (28 September – 11 October), most models predict no significant MJO activity.

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.

September 18, 2026

Review of Regional Weather for August 2026

Review of Regional Weather for August 2026

 

1. Overview

1.1 During August 2026, the southern ASEAN region recorded generally below-average rainfall, while much of the northern ASEAN region recorded above-average rainfall (Figure 1). The largest positive (wettest) anomalies were recorded over western coastal and northeastern Mainland Southeast Asia along with parts of the northern Philippines in both datasets (GSMaP-NRT and CMORPH-Blended). The largest negative (drier) anomalies were recorded over Borneo, southern Philippines and western Papua. There were some differences between the datasets, particularly over southeastern Mainland Southeast Asia where GSMaP-NRT recorded wetter conditions over most of Cambodia apart from the coast and drier conditions over southern Viet Nam, while CMORPH-Blended recorded a mix of wetter and drier conditions over both regions.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the northern ASEAN region and the below-average rainfall over the southern ASEAN regions are consistent with the predictions from the subseasonal weather outlooks for August 2026 (3 – 16 August 2026 and 17 – 30 August 2026). The observations are also consistent with the seasonal outlook for August 2026, which predicted below normal rainfall over most of the southern ASEAN region and above normal rainfall over parts of the northern ASEAN region.

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Figure 1: Rainfall anomalies for August 2026 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

 

1.3 Most of the southern ASEAN region experienced above-average temperature, while much of the northern ASEAN region experienced near-average temperature (Figure 2). The warmest anomalies (2°C – 3°C above average) occurred over parts of the western Maritime Continent. Negative (cool) anomalies were only recorded in isolated areas.

era5_temperature_anomalies
Figure 2: Temperature anomalies for August 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

 

2. Climate Drivers

2.1 While a Madden-Julian Oscillation (MJO) signal was present over the Western Pacific (Phases 6 and 7) during much of August based on the RMM, there was little eastward propagation. This lack of clear MJO signal may be due to the strong El Niño conditions in the Pacific. Phases 6 and 7 typically bring drier conditions to the central and eastern Maritime Continent, while Phase 6 can bring wetter conditions over parts of Mainland Southeast Asia and the Philippines. This is in line with wetter conditions over the northern ASEAN region and drier conditions over the southern ASEAN region.

mjo_phase_diagram

 

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 The El Niño conditions were present in August 2026, with the relative Nino3.4 index supporting the strength of the El Niño to be strong. El Niño conditions typically bring drier conditions to the Maritime Continent at this time of the year.

 

September 14, 2026

Review of Regional Haze Situation for August 2026

Review of Regional Haze Situation for August 2026

1.1 During August 2026, the ASEAN region continued to experience the Southwest Monsoon conditions. The prevailing winds over the southern ASEAN region blew from the southeast or south, while the prevailing winds over the northern ASEAN region blew from the southwest to west (Figure 1).

1.2 Dry weather was observed over most of the southern ASEAN region during the review period, except for some showers over the northern parts of Peninsular Malaysia and Sumatra, and the northeastern parts of Borneo (Figure 1). Alert Levels 2 and 3 for the southern ASEAN region were raised on 3 August 2026 and 26 August 2026 respectively, in view of an increased risk of transboundary haze occurrence, amid persistent dry weather and an escalating hotspot and smoke haze situation. On the other hand, showers prevailed over the northern ASEAN region (Figure 1).

Figure 1: Average Daily Rainfall and Mean Winds for August 2026. (Source: JAXA GsMaP and GFS, respectively)

 

1.3 Over the southern ASEAN region, hotspot counts were mostly higher in August 2026 compared to previous years, with Kalimantan, Sumatra, Sabah and Sarawak having the highest counts recorded in five years (Figure 2). Over the northern ASEAN region, the hotspot counts for August 2026 were generally lower or comparable to previous years, except over the Philippines where the hotspot count was the highest recorded in five years (Figure 3).

Figure 2: Hotspot counts for the southern ASEAN region for August (2022-2026) based on NOAA-20 satellite surveillance.

 

Figure 3: Hotspot counts for the northern ASEAN region for August (2022-2026) based on NOAA-20 satellite surveillance.

 

1.4 Scattered to widespread persistent and intense hotspots were detected in parts of Kalimantan and the southern parts of Sumatra (Figures 4 and 5). Towards the end of the review period, moderate to dense transboundary smoke plumes were observed to cover North, West and Central Kalimantan, Brunei, the western and central parts of Sarawak and the southern parts of Sabah (Figure 6 and 9). Moderate to dense smoke plumes were also observed to emanate from hotspots detected in other parts of Kalimantan, the central and southern parts of Sumatra, the western parts of Peninsular Malaysia and the eastern parts of Java (Figures 6, 7, 8 and 9).

1.5 Over the northern ASEAN region, the overall hotspot and smoke haze situation remained subdued (Figures 4 and 5).

 

Figure 4: Distribution of hotspots detected in August 2026 based on NOAA-20 satellite surveillance.

 

Figure 5: Maximum fire intensity in August 2026 based on NOAA-20 satellite surveillance.

 

Figure 6: Number of days with moderate to dense smoke haze observed in August 2026 based on satellite imagery, ground observations, and air quality reports.

 

Figure 7: Moderate smoke plumes observed over the eastern parts of Java on 11 August 2026. (Source: Himawari-9 satellite surveillance)

 

Figure 8: Moderate to dense smoke plumes observed over the central and southern parts of Sumatra and the western parts of Peninsular Malaysia on 23 August 2026. (Source: Himawari-9 satellite surveillance)

 

Figure 9: Moderate to dense transboundary smoke plumes observed to cover North, West and Central Kalimantan, Brunei, the western and central parts of Sarawak and the southern parts of Sabah on 30 August 2026. Moderate to dense smoke plumes were also observed in other parts of Kalimantan. (Source: Himawari-9 satellite surveillance)