December 2, 2025 | 07:45 pm
TEMPO.CO, Jakarta - After a brief respite from heavy rainfall, Aceh and North Sumatra must prepare for renewed severe weather. The Meteorology, Climatology, and Geophysics Agency (BMKG) forecasts a potential for heavy rain in these areas from December 2–4, 2025, with Aceh urged to be on full alert for the entire week.
The weekly weather prospects from BMKG for the period of December 2–8 identify several regions at risk of potential heavy rain. Aside from these areas, other regions requested to be on high alert for the upcoming week include Bangka Belitung, West Sulawesi, and Papua Highlands.
Jakarta, Banten, West Java, and Central Java, similar to North Sumatra, are advised to be on alert from December 2–4. Meanwhile, East Java is targeted for the period spanning December 5–8.
According to BMKG, the atmospheric dynamics in the coming days indicate that the Equatorial Rossby Wave and Kelvin Wave will be active, influencing the atmosphere across several Indonesian regions. These two classifications of tropical atmospheric waves possess the capacity to increase convergence zones and intensify the process of air mass lifting, thus triggering the formation and growth of convective clouds that produce heavy rainfall.
"The regions within the influence of these waves include the southern part of Sumatra, the central and southern parts of Kalimantan, the western part of Java, Sulawesi, Maluku, North Maluku, and Papua," states the BMKG official announcement.
Among other factors across the global, regional, and local atmosphere, BMKG cited the reinforcement of the Asian Monsoon, a monsoonal wind recognized for carrying moist air, as a significant contributor.
Furthermore, the prevalence of the western zonal wind component within the Indonesian region further escalates the supply of water vapor from the Indian Ocean, subsequently triggering the formation of rain clouds across the majority of Indonesia.
Meanwhile, the Madden-Julian Oscillation (MJO) is predicted to be situated in Phase 7, specifically in the Western Pacific, and is therefore not expected to contribute to the increased proliferation of rain clouds in the Indonesian region. The same applies to the ex-Tropical Cyclone Koto, which is currently located in the South China Sea and is forecast to move southwest toward the Vietnamese mainland.
"The presence of this tropical cyclone is also predicted not to have any discernible effect on the weather and sea conditions in the Indonesian region," the BMKG noted.
What Is the Weather Outlook in West Sumatra, North Sumatra, and Aceh?
After Tropical Cyclone Senyar dissipated, the northern part of Sumatra experienced a period free from heavy rain. Daily rainfall data from BMKG showed wide areas without rain on November 28, 29, and 30. Rain returned to North Sumatra on Monday, December 1, but only as light-to-moderate intensity.
Based on the latest prospects, rain intensity is likely to increase in North Sumatra and spread to Aceh in the coming days. West Sumatra, however, is not included in this immediate alert.
These three regions are still recovering from the severe floods, flash floods, and landslides caused by heavy and extreme rain that occurred around ten days prior to November 28. That event, which began around November 17, saw storm seeds grow in the Malacca Strait and strengthen into a rare tropical cyclone event impacting Sumatra, causing the most severe destruction among tropical cyclones this year.
Editor’s Choice: BMKG Forecasts Afternoon Rain Across Most of Jakarta
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