Lessons From Tropical Storm Senyar: What if Malaysia Received The Rainfall Intensities our Neighbour Experienced?

But Malaysia, Indonesia and Thailand weren’t the only storm-ravaged countries in Asia. While Senyar romped in our backyard, Tropical storm Ditwah, to the west, ravaged Sri Lanka, and Tropical cyclone Koto, to the north-east, devastated central Vietnam. Interestingly, during this period of about a week, these were the only tropical cyclones in the world, as shown in the screen capture below from windy.com

The three storms were even observed to interact, bringing back-to-back pulses of rainfall within their respective low-pressure systems, adding to the the already-saturated conditions from previous events.

Senyar was itself, an oddity, as it formed at a latitude of just 3.8 degrees North of the Equator, making it a rare equatorial cyclone and the first ever tropical storm in recorded history to form in the narrow Straits of Melaka. Normally occurring with a return period of several hundred years, Senyar followed almost on the heels of Tropical storm Vamei, which, just four years ago in 2021, formed at a latitude of just 1.4 degrees N and caused catastrophic flooding in Taman Sri Muda and other parts of Shah Alam.

The relatively brief (four-year) interval between these storms challenges not just one, but two long-held views concerning tropical cyclones; first, that regions close to the equator rarely experience cyclones due to the weak Coriolis effect there; and second, that archipelagic regions, not being adjacent to large expanses of open ocean, are rarely able to access sufficient warm moist air to form cyclones. While these storms have not (thus far) witnessed the wind speeds characteristic of tropical hurricanes and typhoons, the extreme rainfall caused by these storms has already been devastating enough.

Here again, Malaysia despite having the closest proximity to the storm, escaped with the cyclone equivalent of a slap on the wrist. The image below, from Weatherzone, shows rainfall accumulation for the week of the event.

In this map, it is clear that the hardest-hit areas in Indonesia were parts of Aceh, North Sumatera, and the west coast of West Sumatera. Also badly hit were the east coast of Southern Thailand, and the central east coast of Peninsular Malaysia. Weatherzone, in fact, reported that some areas on the map actually experienced rainfall in excess of the 300mm scale, the maximum shown on their colour bar.

The track of the storm is also shown, which immediately shows that cumulative rainfall is not necessarily correlated with proximity to the storm. Medan in North Sumatera, as might be expected, being almost directly in the path of the storm, registered some of the highest rainfall accumulations. But while Dumai, 200km away accumulated only 50mm of rainfall, Padang Panjang, almost 500 km to the south of Medan, received more than 200mm of rainfall.

In the same way, Perlis, located about 200km north of where the storm began, accumulated only between 60mm and 80mm of rainfall, while Songkhla, a further 70km to the north, accumulated more than 200mm of rainfall. On the map above, Perlis and Kedah appear bathed in shades of red, surrounded, above and below by indigo and blue high rainfall intensity concentrations. In this regard, I believe that Perlis was extremely fortunate, as the combination of topography, drainage and development renders the extreme northern section of the state highly vulnerable to high rainfall intensities.

The entire northern section of Perlis is dominated by a single drainage system, drained by the Timah and Pelarit rivers, that empty into the Timah Tasoh reservoir and dam. In an open access article entitled Catchment Delineation and LULC Classification of the Upper Catchment of the Timah-Tasoh Reservoir Using DEM and GIS, published in the International Journal of Integrated Engineering, ISSN: 2229-838X, e- ISSN: 2600-7916, IJIE, Vol. 17, No. 3(2025) 157-164, authors Nor Aisyah Syafiqah Nordin, Zulkarnain Hassan, Norazian Mohamed Noor, Ain Nihla Kamarudzaman, and Ag Shaffie Ag Ahmadni describe the catchment (shown below, left) as covering a total area of 183.34 km2, whereas the reservoir itself has an average surface area of 13.33 km2 and a storage capacity of 40 million cubic metres of water.

As the water level graph (above, right) shows, on November 22nd, the water level crossed the Alert, Warning and Danger levels, and over the course of the two days further increased to more than 1.3 metres above the Danger level by the 25th of November. Recall from the rainfall accumulation map that northern Perlis received mostly between 60mm and 80mm of rainfall, with higher intensities surpassing 100mm, 150mm and even 200mm falling in the Songkhla region, just 70km to the north. With a simple calculation, we can estimate the impact of the Timah Tasoh catchment receiving the 200mm rainfall accumulation that Songkhla received.

If we conservatively assume that the catchment received 100mm of rainfall, which increased the water level in the dam from 28.6m to 30.9m, an increase of 2.3m, and halved the increase to account for the larger surface area of the reservoir when full, we see that doubling the rainfall to 200mm would increase the water level by an additional 1.15m, making the final water level 2.45m above the danger level. Alternatively, we could multiply the depth of the rainfall (200mm) by the area of the catchment (183.34 sq km) to estimate the additional volume of water entering the reservoir, in this case, 36.67 million cubic metres of water, which almost equal to the reservoir’s normal total capacity of 40 million cubic meters.

Our neighbours bore the full brunt of Tropical Storm Senya and paid a heavy price in human casualties, and economic and developmental losses. Only fortune spared us from a similar fate. We must radically change how we implement tropical cyclone preparedness before the next one arrives.