It almost never happens that a tropical cyclone forms inside the narrow and shallow Malacca Strait, between Peninsular Malaysia and the northern coast of Sumatra. The reason is simple: immediately adjacent to the equator, the Coriolis force is too weak for the initial vortex to gain enough “spin” to organize into a cyclone. However, in late November 2025, an exception occurred. On the night of November 25 to 26, over the strait, Cyclone Senyar developed from a tropical depression – only the second documented case of a tropical cyclone forming in the Malacca Strait since the beginning of reliable measurements.

Meteorologists first spotted a compact cloud of convection on satellite imagery slowly spinning off the northern coast of Sumatra. As winds in the lower layers of the atmosphere organized, the depression strengthened to tropical storm/cyclone status and almost immediately made landfall on northern Sumatra on November 26. Since the strait is narrow and bordered by land on both sides, Senyar did not have “room” for prolonged circulation over open water: after a short episode over water, it made a sharp turn, impacted the coastal belt of Aceh and North Sumatra, then weakened while crossing the mountainous hinterland, but still caused devastating rainfall and floods over a wide area. It briefly found itself over water again towards Malaysia, and then dissipated.

Rare “equatorial” cyclone: what is extraordinary about Senyar

For the formation of tropical cyclones, at least a moderate “spin” of air provided by the Earth's rotation is generally required. This effect is most pronounced in higher tropical latitudes, while within about five degrees of latitude from the equator, it is extremely weak. Because of this, tropical cyclones in the immediate vicinity of the equator are a rarity. Historically, the most famous precedent is storm Vamei from December 2001, which formed near Singapore. Senyar is therefore a meteorological rarity: it developed in almost the same area, in an even narrower and shallower strait, and during the late phase of the northeast monsoon.

Along with the weak Coriolis effect, a combination of factors over the eastern Indian Ocean likely “pushed” the formation: above-average warm seas in the second half of November, intensified northern incursions of colder air bringing dry, fast low-level currents (so-called “cold surge”) to the region, and organized convective bands associated with the monsoon. When all of this coincides with the configuration of the relief – Sumatra to the west and Malaysia to the east “channeling” the flow – even a weak vortex can gain a short-lived momentum sufficient for classification into a tropical storm.

Path and development: from a short “sea window” to devastating land impact

In the morning hours of November 26, Senyar already had a defined circulation center and tropical storm winds. Since the strait at that location is only slightly more than 200 kilometers wide, and the thermal and moisture energy source limited, the system very quickly found itself over land. In northern Sumatra and Aceh, the storm literally “dumped” its energy in the form of torrential downpours. Over the northern edge of the Barisan Mountains, whose slopes fall steeply towards the coast, orography multiplied the precipitation. Satellite estimates based on multi-satellite integrations (IMERG, GPM mission) indicated that in less than two days close to 400 millimeters of rain fell in many locations, with local totals in narrow valleys and on slopes potentially being even higher than the satellite average.

While the circulation weakened, the rainfall increased. Precisely this combination – slow movement and strong orographically enhanced convection – is what most often brings catastrophic floods: rivers and torrential streams from the mountainous hinterland suddenly overflow into densely populated lowland zones. In Aceh and North Sumatra, swollen, sediment-saturated waters rose quickly; torrents carried everything before them and destroyed infrastructure. Certain road routes were cut off by multi-meter drifts of mud and fallen trees, and settlements were left cut off.

Scale of destruction: thousands of landslides, fast torrents, casualties, and displacement

Already by November 27–28, local and national services recorded hundreds of dead and missing in North and West Sumatra and in Aceh. During November 27, an additional earthquake of magnitude about 6.4–6.5 struck the area of northern Sumatra, which further worsened the situation on terrain already soaked with rain: sliding and collapsing triggered new landslides, and the primary priority became logistics – delivery of medical and food supplies and evacuation of the most endangered.

While civil protection, army, and Red Cross teams fought their way to isolated communities, the death toll rose by the hour. By November 29, national estimates spoke of at least hundreds of deaths and tens of thousands evacuated; by December 2–4, hundreds upon hundreds of lost lives and hundreds of thousands displaced were recorded, with a massive impact on the road and power grid. The heaviest losses were concentrated in lowland basins and along river courses, where high-speed torrents carry silt, construction material, and wood – a deadly combination that breaks bridges and demolishes houses.

Satellites and field reports: how we know where it was worst

In the first days after the strike, the most reliable overview was from orbit. Tools like Zoom Earth and meteorological geostationary satellites (Himawari-9) tracked the formation and path of Senyar. For precipitation estimation, IMERG estimates from the Global Precipitation Measurement (GPM) mission were used, which merge measurements from multiple satellites every half hour and provide a precipitation amount field with a spatial resolution of approximately 10×10 km. For mapping flooded surfaces and turbid, silt-rich river flow, high-resolution optical images were particularly valuable – for example, Sentinel-2 (ESA) and Landsat 9 (OLI-2). Images from late November clearly show flooded zones in the lowlands around the coast of northern Sumatra and Aceh: water covered agricultural areas, roads, and fringe parts of settlements. In the wider area of Lhoksukon, Lhokseumawe, and river alluvial fans north of Barisan, the reflection of light on turbid water creates a recognizable “brown” signature that experts use to differentiate flooded zones from saturated soil.

At the same time, data piled up from field reports, local media, and humanitarian organizations about destroyed bridges, collapsed drainage canals, and telecommunication interruptions. The electrical grid, which in these provinces passes through hilly and forested areas, suffered a series of damages: from toppled poles to undermined routes. Air assistance – helicopters for medical evacuations and food delivery – became crucial in the first week after the storm.

Wider regional context: when cyclones and monsoon coincide

Senyar did not remain an “isolated story”. Almost simultaneously, in the wider Asian environment and over the western Pacific, a series of tropical systems and monsoon episodes unfolded that multiplied the damage: Malaysian federal states in the peninsular part were hit by heavy rains and floods, southern Thailand (especially Songkhla) recorded severe consequences, and further to the east and north, cyclonic and monsoon waves in Vietnam and the Philippines brought additional precipitation extremes. At the same time, a separate cyclone with devastating consequences for the population and infrastructure was taking place across the Bay of Bengal and Sri Lanka. Humanitarian balances for the wider region during the last week of November and the first days of December spoke of thousands dead and millions affected.

Such spatial and temporal overlap of dangerous phenomena is not unknown, but such a strong synthesis of monsoon flows and tropical cyclones – likely intensified due to global ocean warming – leaves little “room” for protection and rescue systems. One major crisis triggers another: at the moment when resources are concentrated in one country, neighboring ones are already entering an alarming state, so regional cooperation and timely data exchange become crucial.

Why the Malacca Strait is almost “immune” – and why it wasn't this time

For a weak depression to turn into a tropical cyclone, a combination of conditions is needed: sufficiently warm sea (usually at least 26–27 °C), moisture in the troposphere, low wind shear aloft, and – a key component – spin that enables the organization of a spiral. At the equator, this spin is practically zero. That is why classic textbooks state that tropical cyclones do not form within approximately 5° of latitude from the equator. The only way to bypass this “ban” is for the synoptic situation to compensate for the lack of Coriolis: strong northern incursions (cold surge) over warm seas can create and enhance vorticity in the lower layers of air, while relief (wind channeling) and forced convection over warm water surfaces ensure additional support. In this sense, Senyar is a textbook example of how rare, but possible, combinations can briefly “break” climatic statistics.

Hotspots of suffering: North Sumatra, Aceh, and the West Sumatra edge

On the line from the coast of northern Sumatra towards the interior, there are numerous settlements situated along river terraces and alluvial fans. Here flood waves are fastest and most dangerous, because they have little room to spread – rivers quickly cross banks and enter residential districts. In North Sumatra and Aceh, flood and landslide waves hit multiple districts almost simultaneously, and in some, dozens of landslides occurred at the same time. Due to collapsed roads and bridges, part of the municipalities was cut off for up to several days, without electricity and telecommunications. Health institutions and schools received evacuees, while sports halls and religious buildings were converted into temporary shelters.

In West Sumatra, along the slopes of Barisan and at the exits of river valleys towards the coast, “turbid” torrents carried huge amounts of sediment. When such water spills onto the plain, it deposits silt tens of centimeters thick, which subsequently poses both a health risk (contaminated water, disabled drainage, increased risk of infections) and an economic problem (damaged arable land, greenhouses, warehouses, and workshops).

Early loss estimates and humanitarian picture

In the first days after the passage of Senyar, disaster services and international humanitarian organizations published frequent balance updates. In Indonesia, reports cited hundreds dead and missing and hundreds of thousands displaced just on Sumatra; in Thailand, the southern part of the country experienced one of the deadliest flood episodes in recent years; in Malaysia, material damage was great, although human casualties were significantly lower. Wider, regional totals – which also included a separate cyclone over Sri Lanka and strong monsoon/typhoon episodes over Vietnam and the Philippines – already amounted to more than a thousand deaths and millions affected by early December.

The Red Cross and Red Crescent, together with national agencies and the army, deployed field teams for clean water, temporary accommodation, and medical aid. On Sumatra, along with road and air logistics, ships were also used to supply isolated coastal communities. Fundamental priorities were: (1) stabilization of critical infrastructure (bridges, key road junctions, power transmission lines), (2) ensuring drinking water and sanitary conditions in reception centers, (3) restoration of communications so that the coordination of rescue and supply activities would be more effective.

Role of forests, land use, and “secondary” effects

Although the basic cause of the crisis is extreme precipitation and the unusual path of the tropical cyclone, scientists and civil society organizations point out that the consequences are amplified due to land use methods: deforestation, unregulated logging, and erosion-sensitive slopes increase surface runoff and create conditions for fast, destructive landslides. In lowland zones, filling of river terraces, unmaintained canals, and covering of natural absorption surfaces by urbanization further worsen flood risk. Thus, Senyar, alongside meteorological exceptionality, also exposed structural weaknesses in spatial planning and maintenance of water management infrastructure.

Safety and development issues: how to reduce risk before the next storm

The lesson is harsh, but clear. Early warning systems and rapid communication save lives – but preparation and resilient planning are equally important. For northern and western Sumatra, this means more than a meteorological forecast: up-to-date landslide risk maps (based on topography, geology, and satellite precipitation estimates), retention and restoration of forest cover on steep slopes, controlled forest exploitation, and reconstruction of drainage systems alongside settlements and roads. In rural areas, high slopes above settlements need an early warning system to trigger rapid evacuation when precipitation thresholds are exceeded. In urban zones, green retention spaces and impermeable protection of critical infrastructure (water pumping stations, substations, hospitals) are key.

At the level of the entire Southeast Asian region, Senyar reminds us that transnational data exchange and intervention coordination are not a “luxury” but a necessity. Tropical cyclones and monsoons know no borders; satellite data, hydraulic models, and unified protocols for aid mobilization must be immediately available to everyone who decides and intervenes.

What Senyar told us about the “new normal”

One rare storm does not make a trend, but Senyar fits into the broader picture of strengthening precipitation extremes in a warmer climate. Evidence from observations and models shows that the rise in sea surface temperature increases the potential amount of water vapor in the atmosphere, so episodes of convection and tropical systems yield more rain in a shorter time. In places like the Malacca Strait, where coastline geometry is extremely “sensitive” to wind channeling, rare constellations of synoptic factors – like those in late November – can result in events that go down in history. This is a reliable and most important warning: rare does not mean impossible, and community resilience is built on the assumption that even very rare scenarios will recur sooner or later.

Methodological note: how to read early numbers

In such crises, numbers on the dead, missing, and displaced necessarily change from day to day. Early data come from multiple sources – local agencies, international organizations, media – and are often not perfectly aligned. This does not mean they are wrong, but that they reflect a dynamic situation on the ground. Therefore, below we list key sources confirming the following facts: (1) Senyar is an extremely rare case of a tropical cyclone that developed in the Malacca Strait right next to the equator; (2) the path included rapid landfall on Sumatra on November 26 and a disastrous rainfall effect over the mountainous hinterland; (3) satellite estimates indicate precipitation totals of the order of hundreds of millimeters in a short time; (4) the humanitarian balance at the level of Indonesia and the region reached thousands of victims and millions affected already in the first days of December; (5) on November 27, a strong earthquake was also recorded near northern Sumatra, which further complicated rescue and recovery operations.

Looking ahead

Late November and early December 2025 in Southeast Asia will be remembered for a cascade of disasters. But even now it is possible to single out a fraction of good news. In the few days after the catastrophe, protocols for the exchange of high-resolution satellite imagery among agencies were significantly improved, which accelerated the mapping of flood zones; humanitarian organizations activated mechanisms for the rapid supply of drinking water and sanitation; and meteorological services stepped up the communication of precipitation thresholds and landslide hazards to local authorities and the population. If these improvements are incorporated into permanent procedures, communities will be more resilient when the next time – whether in five, ten, or twenty years – some “rare” constellation appears again over the Malacca Strait.