ST Explains: What’s a Sumatra squall, and how does it affect S’pore’s weather?
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Trees were uprooted in Choa Chu Kang Grove after a heavy storm battered Singapore on the night of Sept 17.
ST PHOTO: NG SOR LUAN
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SINGAPORE – The storm that swept into Singapore on Sept 17 lasted barely an hour, but left its mark across the island.
From Tampines in the east to Mount Faber in the south and Yishun in the north, netizens took to Facebook, Instagram and TikTok to document felled trees and broken branches strewn over roads.
There were also other unusual incidents, such as a pair of trousers seen flying among high-rise buildings, pool furniture flying across and landing in the pool and various tableware pieces flying at an outdoor restaurant as patrons attempted to enjoy their meal.
Elsewhere in the region, Malaysia and Brunei were also affected by the extreme winds and rainfall. The Malaysian island of Penang had more than 200 toppled trees Brunei’s fire and rescue department personnel attended to 72 emergency calls
In a statement on Sept 18, the National Environment Agency’s Meteorological Service Singapore (MSS) confirmed that the widespread thundery showers and gusty winds on Sept 17 were caused by a Sumatra squall.
The squall had developed over the Strait of Malacca and swept across the island rapidly from about 7pm to 8.30pm, with East Coast Park recording the highest wind gust of 83.2kmh at 7.33pm.
The Straits Times looks at what Sumatra squalls are, how this weather phenomenon affects Singapore and the rest of South-east Asia and if Singapore can expect to see more of such weather in the upcoming months.
What are Sumatra squalls and what causes them?
Sumatra squalls are a common and vigorous line of thunderstorms travelling rapidly towards the South China Sea.
“The causes of this phenomenon are complex and include convection, which is an upward transport of heat from the surface into the atmosphere, over the warm waters of the Malacca Strait, and converging land breezes passing between Sumatra and the Malay Peninsula,” said Dr Matthias Roth, a professor of urban climatology at NUS’ department of geography.
“Besides intense convection, Sumatra squalls also possess strong vertical wind shear – a variation in wind speed at a height which, in combination, results in heavy rainfall and strong winds.”
Sumatra squalls typically last for a few hours after forming early in the morning or at night and may occur at any time of the year, resulting in heavy rainfall. The squalls are most common during inter- and south-west monsoon seasons, such as April to May, and October to November.
There can be an average of seven squalls a month during such periods, said Associate Professor Steve Yim from the Asian School of the Environment and Lee Kong Chian School of Medicine at NTU.
During this period, severe flooding, fallen trees, damage to objects and properties and power outages can happen.
The disruptive weather system can affect up to 85 million people in the region, according to a study published in 2018.
When was the last time Singapore had such intense storms?
The Republic faced its worst wind gust in Tengah on April 25, 1984 – at a speed of 144.4kmh.
In 2018, ST reported the record was almost broken as wind speed hit a high of 133.3kmh in Tengah
Heavy rainfall was detected over western parts of the island such as Jurong and Choa Chu Kang, said the MSS at the time.
In 2010, there was another occurrence where wind speed hit a high of 90.7kmh on Nov 29.
However, climate incidents such as the Sept 17 rainstorm may happen again in the near future.
“Latest research shows that extreme climate events such as convection may increase in the near future due to climate change. This is because climate change can cause more energy to be released to the atmosphere, enhancing atmospheric instability,” said Prof Yim, who is also the principal investigator of the Earth Observatory of Singapore at NTU.
Dr Koh Tieh Yong, co-chair of the Working Group for Asian-Australian Monsoon within the World Climate Research Programme, noted that as climate changes, short-term equatorial storm systems are uncertain and under-studied, as they may always change in terms of frequency, intensity and propagation speed.
“Reports of increased extreme weather in authoritative reports like those of Intergovernmental Panel on Climate Change typically refer to studies made in temperate countries where low-pressure storm systems operate on different principles from tropical squalls like those in our region, or tropical cyclones which are a different weather system altogether,” Dr Koh said. “Unfortunately, this is the state of the science in this regard.”
How can I protect myself and my belongings from such wild weather?
During the inter-monsoon season, which is expected to take place from October to November, you can expect several rainy thunderstorms at any time of the day.
Members of the public can subscribe to an SMS alert service by the MSS which issues warnings on rising water levels in canals or drains and or/ heavy rain in Singapore.
The Singapore Civil Defence Force said in an advisory that residents can secure or move items placed outdoors – such as toys, bicycles, potted plants and heavy objects – indoors and away from the windows.
In the event of a thunderstorm or heavy rainfall, people are advised to seek shelter indoors, avoid open spaces, tall objects and water bodies, and refrain from using electrical devices, said Prof Yim.
The public should also stay away from flooded areas and avoid driving or walking through standing water, he said.
When the storm is over, check the surroundings for broken glass, fallen trees, broken power cables and other hazards.
