Heavy rain persists as dry season begins in May, BMKG explains
The Meteorology, Climatology, and Geophysics Agency (BMKG) has explained the potential for rain during the onset of the dry season. BMKG noted that the activity of tropical cyclone seed 99W and atmospheric waves at the start of the dry season has triggered significant weather potential in parts of Indonesia.
BMKG’s Public Meteorology Director, Andri Ramdhani, explained that current climate analysis shows some regions of Indonesia have entered the dry season, while others are in a transitional period. This is indicated by minimal cloud cover from morning to midday, leading to intense surface heating.
“This has led to maximum temperatures exceeding 35.0°C in Aceh, Riau, Bali, most of Kalimantan, Central Sulawesi, West Papua, and East Java between 21 and 24 May 2026. In regions still in transition, daytime heat accumulation causes convection, leading to atmospheric instability and rain clouds forming in the afternoon and evening,” Andri stated in a report on Wednesday, 27 May 2026.
Andri said the atmospheric instability is validated by high rainfall intensity, ranging from heavy to very heavy, recorded in parts of Indonesia, including West Kalimantan (129.5 mm/day), Jakarta (102.4 mm/day), Bengkulu (96.4 mm/day), Maluku (88.4 mm/day), West Java (86.9 mm/day), Bangka Belitung Islands (77.7 mm/day), Central Kalimantan (73.3 mm/day), North Sumatra (70.2 mm/day), East Kalimantan (68.8 mm/day), Central Papua (67.2 mm/day), North Sulawesi (64.5 mm/day), Jambi (61.8 mm/day), Banten (58.6 mm/day), South Sulawesi (55.6 mm/day), Lampung (55.4 mm/day), Southeast Sulawesi (55.2 mm/day), North Kalimantan (52.2 mm/day), and Central Java (50.0 mm/day).
Andri added that the increased rainfall during this period is also influenced by interactions of several atmospheric phenomena.
“The Madden-Julian Oscillation (MJO) is currently in phase 5 (Maritime Continent). Additionally, Equatorial Rossby waves and Kelvin waves are contributing to the growth of rain clouds in several Indonesian regions,” he said.
The formation of cyclonic circulation in several Indonesian regions has also triggered areas of wind speed reduction (convergence). Such atmospheric conditions further support cloud formation, increasing the likelihood of rain in some areas.
Atmospheric Dynamics Over the Coming Week
Current global climate indicators show signs of El Niño conditions in the Pacific Ocean, confirmed by a Nino 3.4 index of +0.68 and a Southern Oscillation Index (SOI) of -12.5. However, regional atmospheric dynamics still have the potential to trigger rain cloud growth, particularly in northern Indonesia.
Tropical cyclone seed 99W is forecast to be located in the northern Pacific Ocean near Papua New Guinea. This system creates convergence and confluence areas from West Papua to Cendrawasih Bay, along the western coast of Southwest Papua, and around the system. Additionally, cyclonic circulation is predicted to form in the waters west of West Sumatra, the southwestern Indian Ocean off Banten, and the Sulu Sea. These systems form convergence and confluence areas in the waters west of Bengkulu, west of North Sumatra, and around the cyclonic circulation.
Furthermore, MJO activity projected towards phase 6 (Western Pacific) is expected to remain spatially active in North Kalimantan, East Kalimantan, North Sulawesi, Gorontalo, Central Sulawesi, North Maluku, and northern West Papua and Southwest Papua. Easterly-moving Kelvin waves are also forecast to be active across Aceh, northern West Kalimantan, East Kalimantan, North Kalimantan, North Sulawesi, Gorontalo, and Central Sulawesi.
Meanwhile, westward-propagating Equatorial Rossby waves are forecast to be active in East Kalimantan, North Kalimantan, North Sulawesi, Gorontalo, and Central Sulawesi. Mixed Rossby-Gravity (MRG) wave activity is also expected in northern Aceh. MJO and atmospheric wave activities will help increase moisture supply and support rain cloud formation in the affected regions.
Rainfall Potential Over the Coming Week
Andri said Indonesia’s weather is generally dominated by light to heavy rain. During 26-28 May, he warned of moderate rainfall increases in Aceh, West Sumatra, Riau, Riau Islands, Jambi, South Sumatra, Bangka Belitung Islands, Bengkulu, Lampung, Banten, Central Java, East Java, West Kalimantan, Central Kalimantan, East Kalimantan, South Kalimantan, North Sulawesi, Gorontalo, Central Sulawesi, West Sulawesi, South Sulawesi, Southeast Sulawesi, Maluku, Southwest Papua, West Papua, Central Papua, Papua, and South Papua.
Additionally, heavy rain accompanied by lightning, thunderstorms, and strong winds may occur with the following early warning levels:
• Caution (heavy to very heavy rain): North Kalimantan, North Maluku, and Mountainous Papua.
• Strong winds: Aceh, East Java, East Kalimantan, West Nusa Tenggara, East Nusa Tenggara, Maluku, North Maluku, Southwest Papua, West Papua, South Papua.
For the period 29 May to 1 June, moderate rainfall increases are expected in Aceh, Riau, Riau Islands, East Java, West Kalimantan, North Kalimantan, North Sulawesi, Central Sulawesi, West Sulawesi, South Sulawesi, Southeast Sulawesi, North Maluku, Maluku, Southwest Papua, Central Papua, Papua, and South Papua.
Heavy rain accompanied by lightning, thunderstorms, and strong winds may occur with the following early warning levels and