Monthly Archives: September 2025

September 24, 2025

Subseasonal Weather Outlook (29 September – 12 October 2025)

Subseasonal Weather Outlook (29 September – 12 October 2025)

Issued: 24 September 2025
First forecast week: 29 September – 5 October 2025
Second forecast week: 6 – 12 October 2025

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Figure 1: Rainfall Outlook

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Figure 2: Temperature Outlook

Wetter conditions are predicted over much of the central and southern Maritime Continent in the next fortnight (29 September – 12 October).

No significant regional temperature anomalies are predicted in the next fortnight (29 September – 12 October).

No significant Madden-Julian Oscillation (MJO) signal was present in the third week of September. Most models predict no significant MJO activity during the forecast period. Subseasonal Weather Outlook (15 – 28 September 2025)

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 15, 2025

Review of Regional Weather for August 2025

Review of Regional Weather for August 2025

 

1. Overview

1.1 During August 2025, many parts of Southeast Asia experienced above-average rainfall (Figure 1). Most of the Maritime Continent recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), although CMORH-Blended (Figure 1, right) recorded negative (drier) anomalies over much of the western Maritime Continent, and parts of Borneo, the southeastern Maritime Continent and northern Philippines. Over Mainland Southeast Asia, positive anomalies were recorded over the east , with a mix of below- to above-average rainfall elsewhere. The largest positive (wetter) anomalies over Southeast Asia were recorded over eastern Mainland Southeast Asia and the southeastern Maritime Continent.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for August 2025 (4 – 17 Aug 2025 and 18 – 31 Aug 2025). The subseasonal weather outlooks also predicted a mix of below-and above-average rainfall over parts of Mainland Southeast Asia, in line with the observations. The observations are also somewhat consistent with the seasonal outlook for August 2025, which predicted an increase in chance of above-normal rainfall in the southern and eastern Maritime Continent.

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Figure 1: Rainfall anomalies for August 2025 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 Near- to above-average temperatures were recorded over Mainland Southeast Asia in August 2025, while below- to near-average temperatures were recorded over most of the Maritime Continent (Figure 2). The coldest anomalies (0.25°C – 1°C below average) were recorded over Borneo and the southeastern Maritime Continent, in line with the above-average rainfall anomalies. The warmest anomalies (0.5°C – 2°C above average) were recorded over parts of central Mainland Southeast Asia, Sumatra, and Java.

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Figure 2: Temperature anomalies for August 2025 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 At start of August, a Madden-Julian Oscillation (MJO) signal was present in the Western Hemisphere (Phase 8) based on the RMM diagram (Figure 3). This signal propagated eastwards, reaching the Indian Ocean (Phases 2 and 3) by the second week of August. However, as the signal approached the Maritime Continent (Phase 4) in the last week of August, the signal rapidly weakened and became indiscernible. At this time of the year, Phases 8 and 1 tend to bring drier conditions to southern Mainland Southeast Asia and the Philippines, and to a lesser extent, Phases 2 and 3 can bring wetter conditions to the western Maritime Continent. However, these patterns are not visible in Figure 1, indicating other drivers likely had a stronger influence on the region’s rainfall.

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 There were signs of a negative Indian Ocean Dipole (IOD) developing in August 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1.

 

September 10, 2025

Subseasonal Weather Outlook (15 – 28 September 2025)

Subseasonal Weather Outlook (15 – 28 September 2025)

Issued: 10 September 2025
First forecast week: 15 – 21 September 2025
Second forecast week: 22 – 28 September 2025

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Figure 1: Rainfall Outlook

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Figure 2: Temperature Outlook

Wetter conditions are predicted over much of the equatorial region and southwestern Maritime Continent in the next fortnight (15 – 28 September).

Drier conditions are predicted over central and northeastern Mainland Southeast Asia in Week 1 (15 – 21 September).

Cooler than usual temperatures are predicted in Week 1 (15 – 21 September) over the southwestern Maritime Continent and parts of the central equatorial region.

No coherent Madden-Julian Oscillation (MJO) signal was present at the start of September.
Most models predict no significant MJO activity during 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.

September 9, 2025

Review of Regional Haze Situation for August 2025

Review of Regional Haze Situation for August 2025

1.1 The ASEAN region experienced Southwest Monsoon conditions during August 2025, characterized by prevailing southeasterly or southwesterly winds in the southern ASEAN region and southwesterly or westerly winds in the northern ASEAN region (Figure 1).

1.2 1.2 While Java and the Lesser Sunda Islands continued to face drier conditions, showers were prevalent over the rest of the southern ASEAN region. Alert Level 2 was downgraded to Level 1 for the southern ASEAN region on 12 August 2025 due to the expected improvement of the hotspot and haze situation, and the reduced risk of transboundary haze over the region under the wet weather conditions. In the northern ASEAN region, wet weather continued, particularly over the northern parts of the Philippines and the eastern parts of Mekong sub-region where there was intense rainfall brought by Typhoons Kajiki and Nongfa (Figure 1).

Figure 1: Average Daily Rainfall and Mean Winds for August 2025. (Source: JAXA GsMaP and GFS, respectively)

 

1.3 1.3 The hotspot counts for the ASEAN region in August 2025 were mostly comparable to the same month in previous years (Figures 2 and 3).

 

Figure 2: Hotspot counts for the southern ASEAN region for August (2021-2025) based on NOAA-20 satellite surveillance.

 

Figure 3: Hotspot counts for the northern ASEAN region for August (2021-2025) based on NOAA-20 satellite surveillance.

 

1.4 Isolated to scattered hotspots were detected in most parts of the southern ASEAN region, with persistent hotspot clusters detected in West Kalimantan, the northern and central parts of Sumatra, and the Lesser Sunda Islands (Figure 4). Moderate but localised smoke plumes were observed to emanate from hotspots in West Kalimantan and the northern parts of Sumatra on some days (Figures 6 and 7).

1.5 In the northern ASEAN region, the hotspot counts remained low with no significant smoke plumes were observed during the month (Figures 4 and 6).

 

Figure 4: Distribution of hotspots detected in August 2025 based on NOAA-20 satellite surveillance.

 

Figure 5: Maximum fire intensity in August 2025 based on NOAA-20 satellite surveillance.

 

Figure 6: Number of days with moderate to dense smoke haze observed in August 2025 based on satellite imagery, ground observations, and air quality reports.

 

Moderate localised smoke plumes were observed in the western parts of Kalimantan on 7 August 2025. (Source: NOAA-20 satellite surveillance) (Source: Himawari-9 satellite surveillance)

 

Figure 8: Moderate smoke plumes were observed in the central parts of Sumatra on 2 August 2025. (Source: Himawari-8 satellite surveillance)