December 11, 2025 | 08:27 am
TEMPO.CO, Jakarta - The web-based cyclone prediction application owned by the Indonesian National Innovation and Research Agency (BRIN), Kamajaya, shows that the cyclone 91S is approaching the southern region of Sumatra during the second decade of this month, or on December 11 to 20, 2025. BRIN's climate and atmosphere researcher, Erma Yulihastin, said the core of the wind vortex was near Bengkulu.
"The entire west coast of Sumatra will be affected by the increasing rainfall that has been consistently occurring since today (Wednesday)," she told Tempo on Wednesday night, December 10, 2025.
Erma also stated that apart from affecting Sumatra, 91S will have an indirect impact in the form of increased rainfall on the west side of Java. This prediction is based on data entered on November 30. "This means that it has accommodated the weather conditions after the tropical cyclone event of Senyar," she said.
The Head of the Meteorology, Climatology, and Geophysics Agency (BMKG), Teuku Faisal Fathani, confirmed that his agency continues to monitor the mobility of 91S, which is currently located in the Indian Ocean west of Lampung. Based on BMKG's analysis, 91S will increase the potential for moderate to heavy rain in parts of Aceh, North Sumatra, West Sumatra, Bengkulu, and Lampung.
"The public should also be aware of the potential for high waves in the Indian Ocean from the west of Nias to the south of Banten, as well as in the southern part of the Sunda Strait," said Faisal in Sibolga, North Sumatra, as quoted from BMKG's official statement on Wednesday night.
The movement of cyclone 91S monitored by BMKG on Wednesday night at 19:00 WIB, Wednesday night, December 10, 2025. (BMKG Documentation)
The Deputy of Meteorology at BMKG, Guswanto, stated that the cyclone is tending to move towards the south and southwest. He ensured that his agency has coordinated with the National Disaster Management Agency (BNPB) and several Regional Disaster Management Agencies (BPBD).
"To ensure that mitigation runs optimally according to the potential weather conditions influenced by the presence of 91S," said Guswanto. The public is urged to remain calm because the chance of 91S becoming comparable to Tropical Cyclone Senyar - a storm that formed in the waters and moved inland - is still low.
North First, South Later
Sumatra seems to be surrounded by cyclones from the north and south over the past few weeks. The cyclone 91S, which is currently being monitored by BMKG, emerged on December 7 in the southwest of Lampung in the Indian Ocean. Two days after its appearance, its maximum wind speed reached 20 knots or 37 kilometers per hour, with a minimum air pressure of 1,008 hPa.
The cyclone 91S formed while 93W was still present in the North Pacific Ocean region, specifically northeast of Papua, but 93W disappeared first due to a decrease in atmospheric parameters.
Before sweeping the southern area, the northern region of Sumatra had already been hit by a storm. Towards the end of November 2025, Tropical Cyclone Senyar, initially a cyclone 95B, triggered a major flood in the Malaysian Peninsula. This cyclone also caused extreme rainfall in the northern Sumatra region, resulting in floods and landslides in three provinces.
Erma once referred to Cyclone Senyar as a very rare and unusual phenomenon. "Usually tropical cyclones develop in open seas, such as the Indian Ocean and the South China Sea," she said.
Read: Tropical Cyclones: Why Are Some Countries More at Risk?
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