Hydrometeorological and Watershed Drivers of the November 2025 Floods in Aceh, North Sumatra, and West Sumatra under Tropical Cyclones Senyar and Koto

Tropical Cyclones Senyar and Koto triggered one of the most destructive flood episodes in recent decades in northern and western Sumatra, with extreme rainfall causing widespread flooding and landslides across Aceh, North Sumatra, and West Sumatra in late November 2025.Satellite and situational analyses describe Senyar as a rare cyclone that formed in the Strait of Malacca, delivering torrential rains to the rugged Barisan Mountains belt, where steep slopes, short river response times, and populated floodplains combine to generate high-impact flash floods.Reports from international and national agencies indicate hundreds of deaths, hundreds of thousands of displaced people, and extensive damage to housing and infrastructure, with Mandailing Natal, Sibolga, Pesisir Selatan, and several districts in Aceh and Langkat among the hardest hit [1].At the hydro-meteorological level, the 2025 Sumatra floods fit a broader pattern of increasing exposure to extreme rainfall in Indonesia, where intense events are projected to become more frequent as the climate warms.Northern and western Sumatra are particularly vulnerable because moist air masses from the Indian Ocean and the equatorial Pacific can interact with regional circulation anomalies, topography, and