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Subseasonal Weather Outlook and Review

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.

era5_rainfall_anomalies
cmorph_bld_rainfall_anomalies
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.