Review of Regional Weather for April 2026

1.1 During May 2026, a mix of near- to above-average rainfall was recorded over most of Mainland Southeast Asia with a mix of below- to above-average rainfall for the Maritime Continent (Figure 1). Parts of southwestern and central Mainland Southeast Asia recorded above-average rainfall based on both datasets (GSMaP-NRT and CMORPH-CRT). The western and southeastern Maritime Continent recorded mostly above-average rainfall, with most of the northeastern part recording below-average rainfall and the central Maritime Continent a mix of below- to above-average rainfall. The largest positive (wetter) anomalies were recorded over the southern Malay Peninsula and Papua for both datasets, with the largest negative (drier) anomalies over the central and northern parts of the Philippines.

1.2The observed rainfall anomaly pattern of above-average rainfall over southwestern Mainland Southeast Asia to a lesser extent over the southern Malay Peninsula is consistent with the predictions from the subseasonal weather outlooks for May 2026 (27 April – 10 May 2026, 11 – 24 A 2026) and 25 May – 7 June 2026). The wet anomalies over the western Maritime Continent and the dry anomalies over the Philippines are consistent with the seasonal outlook for May 2026, however, the seasonal outlook predicted below-normal rainfall as the dominant tercile for southwestern Mainland Southeast Asia.

Figure 1: Rainfall anomalies for May 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT 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 Above-average temperature was recorded eastern Mainland Southeast Asia, with below- to near-average temperature for northwestern Mainland Southeast Asia (Figure 2). Near-to above-average temperature was recorded for much of the Maritime Continent. The warmest anomalies (1°C – 2°C above average) were recorded over parts of northern Cambodia, southern Lao PDR and northern Philippines. The coolest anomalies (-1°C – -2°C below average) were recorded over parts of central and northern Myanmar. Cooler anomalies (-0.5°C – -1°C below average) were also recoded over central Thailand and Papua.

Figure 2: Temperature anomalies for May 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 The Madden-Julian Oscillation (MJO) was active over the Indian Ocean and Western Pacific during much of May. During the first two weeks of May, an MJO signal was present over the Indian Ocean (Phases 2 and 3), before decaying and becoming inactive during the third week of the month. An MJO signal then emerged over the Western Pacific (Phases 6 and 7) in the last week of May. At this time of the year, Phases 2 and 3 tend to bring wetter conditions to the western and central Maritime Continent, with Phases 6 and 7 tending to bring drier conditions to the eastern and central Maritime Continent. This is somewhat in line with wetter conditions over parts of the western Maritime Continent and drier conditions over the northwestern Maritime Continent 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 The El Niño – Southern Oscillation (ENSO) was neutral in April, with the Indian Ocean Dipole (IOD) also in the neutral state.