Tropical cyclones almost never form over the Strait of Malacca. The narrow waterway separating Peninsular Malaysia from the Indonesian island of Sumatra is so close to the equator that the Coriolis effect is generally too weak to allow storms to rotate enough to organize into cyclones. But on November 25, 2025, meteorologists observed a tropical depression intensifying into Cyclone Senyar, the second documented case of a tropical cyclone forming in the strait.
Surrounded by land on both sides, Senyar made landfall in Sumatra later that day as it turned around and headed east toward Malaysia. As the storm moved slowly over Sumatra’s mountainous terrain, it dropped nearly 400 millimeters (16 inches) of rain in many areas, according to satellite estimates from NASA’s Global Precipitation Measurement (GPM) mission. (Due to averaging of satellite data, local precipitation amounts may differ when measured from the ground.)
The torrent caused significant flash floods and landslides across Sumatra’s rugged terrain. Streams and rivers quickly overflowed with water laden with sediment and debris which swept through villages, towns and villages. Media reports suggest the damage was compounded by an earthquake on November 27 and the abundance of loose woodpiles in the area that became destructive battering rams in high water. As of December 4, Indonesian authorities reported several hundred deaths and more than 700,000 displaced people.
Landsat 9’s OLI-2 (Operational Land Imager-2) captured this image of flooding in Aceh and North Sumatra provinces on November 30, 2025. Muddy waters filled with sediment appear to have inundated much of Lhoksukon, a town of 40,000, and several surrounding villages.
Other tropical cyclones and monsoon rains that hit Sri Lanka, Thailand, Malaysia and Vietnam around the same time also caused significant destruction throughout the region. According to an estimate by the United Nations Office for the Coordination of Humanitarian Affairs, the floods have affected more than 10.8 million people in the region and displaced more than 1.2 million.
NASA Earth Observatory image by Lauren Dauphin, using Landsat data from the US Geological Survey. Story by Adam Voiland.
- BNPB (2025) News Index. Accessed December 4, 2025.
- India Today (December 2, 2025) What made Cyclone Senyar a unique weather anomaly in the Strait of Malacca. Accessed December 4, 2025.
- The Malaysian reserve (December 3, 2025) Indonesia’s death toll from floods, landslides rises to 811. Accessed December 4, 2025.
- NASA Earthdata (2025) Tropical cyclones. Accessed December 4, 2025.
- The New York Times (December 3, 2025) Where floodwaters have turned piles of wood into floating rams. Accessed December 4, 2025.
- ReliefWeb (December 3, 2025) Asia and the Pacific: Humanitarian overview of cyclones and floods in Southeast and South Asia (covering the period from November 17 to December 3, 2025. Accessed December 4, 2025.
- Tsunami and Disaster Mitigation Research Center (November 29, 2025) Extreme rainfall from Tropical Cyclone Senyar triggers widespread flooding and infrastructure damage across Aceh. Accessed December 4, 2025.
