Now, the storms serve up oceans, not just rain. In the last week of November, slow-moving, meandering storms struck Indonesia, Thailand and Sri Lanka that left climatologists wondering and people at large not knowing what to make of it and what to do.

The “largest and most challenging natural disaster in our history”, as Sri Lankan President Anura Kumara Dissanayake put it, started when Cyclone Ditwah made landfall on November 28, unleashing days of intense rain, flooding communities and causing landslides. The death toll, according to this report, tops 639 people, with  the majority of fatalities in Kandy district, 234. Additionally, 203 people remain missing. The storm has affected more than 1.8 million people, with over 90,000 houses damaged or destroyed.

Since late October, south-central Vietnam has been pounded by heavy, relentless rainfall. According to this report in The Guardian, more than 1,900mm (74.8in) was recorded in some parts of Vietnam. The region is a major coffee production belt and home to popular beaches, but it is also prone to storms and floods. Ninety-eight people died, with 12 missing.

Between November 26 and 28, extreme rainfall in Indonesia’s northern and western Sumatra island triggered severe flooding across Aceh, followed by a strong earthquake. More than three million people were affected, with nearly 1,000 reported dead and many more injured or missing, as per this report.

Flooding was an “extinction-level disturbance” for the world’s rarest great ape, the Tapanuli orangutan, causing catastrophic damage to its habitat and survival prospects, scientists warned, according to this story.

In early November, back-to-back typhoons—Kalmaegi, which killed more than 220 people and affected an estimated 2.4 million—and  Fung-Wong (Uwan) slammed the Philippines, according to this report.

Dr. Ravindra Jayaratne, Reader, University of East London, says that for Sri Lanka and much of South Asia, the emerging risk-relevant climatology is increasingly characterised by :

• Higher intensity rainfall extremes, especially during active monsoon and intra-seasonal periods
• More frequent compound flood contexts—river flooding + landslides + coastal surge risk in some events
• Greater sensitivity to slow-moving systems, because multi-day totals dominate hazard outcomes
• Large uncertainty in where the worst impacts occur, because mesoscale rainbands interacting with terrain can shift the damage footprint dramatically

An expert in coastal engineering and specialist in modelling sediment transport processes, tsunamis, storm surges, extreme waves and glacier lake outburst floods, Jayaratne says when multiple risks align, the probability of widespread heavy rain and flooding rises well above reference levels.

The aligned:   

• Active convergence zones (monsoon trough or the Intertropical Convergence Zone (ITCZ), the region that circles the earth near the equator, where the trade winds of the northern and southern hemispheres come together, as per this;
• A favourable Madden-Julian Oscillation (MJO) state (MJO is the major fluctuation in tropical weather on weekly to monthly timescales. It could be characterised as an eastward moving “pulse” of cloud and rainfall near the equator that typically recurs every 30 to 60 days, as per this;
• Anomalously warm seas;
• Forecasts indicating slow-moving systems.

It requires long-term records and attribution to know whether a completely new cyclone regime is governing the whole region. However, Jayaratne notes that a warmer baseline climate increases the threshold for rainfall intensity and flood hazards. Natural variability—monsoon dynamics + MJO + waves—still governs when storms cluster, but the clustered episodes now have a higher chance of producing record-breaking rainfall intensities and damaging profiles.

Warmer air holds more water vapour, leading to heavy rainfall and calamitous floods. Added to that are environmental degradation that can lead to saturated soil which is susceptible to landslides, land degradation due to deforestation and flood plain exposure due to rapid and poor urbanisation.

Jayaratne, along with an international team of coastal engineers, social scientists and policy makers has attempted to produce cyclone-driven compound flood hazard maps for three vulnerable landfall cities in Sri Lanka; Batticaloa, Mullaitivu and Mannar. They have successfully simulated Mundeni Aru river basin catchment in Batticaloa which has shown how destructive storm surge-tide-sea level rise-rainfall coupled floods can be.

In their research, they show that Sri Lanka lies at the convergence of multiple storm tracks in the Bay of Bengal. Jayaratne explains that when weather forecasters say storm tracks are converging, they usually mean that different disturbances such as depressions and cyclones generated over a broad area ending up following similar paths because of the large-scale winds that steer them, typically in the lower-mid troposphere, funnel them into the same corridor.

On top of that, winds from different directions bring moist air into the same region, where it piles up, rises, and produces deep convection, resulting in a thunderstorm with intense rain. This is the classic convergence mechanism, and it can be dramatically enhanced by local topography.

Sri Lanka sits in a part of the Indian Ocean climate system where different disturbances and winds from different directions can occur in the same day or week. The island can be in the pathway of a cyclonic vortex and within or near an active convergence zone associated with the monsoon trough or the intertropical convergence zone (IPCZ) with terrain effects amplifying the rainfall.

Moreover, its central highlands—Kandy, Matale, Nuwara Eliya, Badulla—can increase rainfall via “orographic lifting”—forcing air upward—and interaction with local circulations.

As to why these systems move towards Sri Lanka, Jayaratne says storm motion and clustering near Sri Lanka is mainly governed by two mechanisms: Steering by surrounding pressure systems, that is, subtropical ridges, monsoon trough position, and mid-level winds set the preferred corridors.

“If the pattern repeats or stays locked, multiple systems can follow similar tracks,” he tells Hot Rock.

Secondly, slow translation, slow movement. Even a modest storm can become catastrophic if it moves slowly and keeps the rain shield over the same catchments for days. This is explicitly noted for late-November 2025 impacts: prolonged rainfall occurred because the system’s slow movement kept rainfall over the island, saturating soils and overwhelming catchments.

Cyclone Ditwah should be the turning point in how the country and  the region approaches warming-driven storms.