January 14, 2025
SINGAPORE – Cool weather rare for humid Singapore, flash floods, delayed flights and slow business.
All of these happened over three days starting from Jan 10, as a result of a monsoon surge that has been dumping rain on the island, with occasional strong wind gusts.
The mercury fell to 21.6 deg C in the Newton Road area on the morning of Jan 11, almost matching 2024’s lowest temperature of 21.4 deg C.
A stretch of Jalan Seaview in Mountbatten was affected by flash floods on the evening of Jan 10.
Singapore is currently in the wet phase of the north-east monsoon, which typically lasts from December to January.
A monsoon surge that brings prolonged rain is a common phenomenon during this season.
The Straits Times explains the factors behind the prolonged deluge.
What is a monsoon surge?
During the north-east monsoon, winds typically blow from the north-east to the south-west.
A monsoon surge refers to bursts of cold air from Central Asia.
When regions up north, such as Siberia, experience extremely cold weather in the winter months, the cold air occasionally sweeps southwards to the South China Sea, and is blown towards the tropics.
The air then warms and picks up moisture, leading to the formation of dense rain clouds. This brings increased rainfall near the Equator.
There could be moderate to heavy showers and thunderstorms in places such as Singapore, Malaysia and Vietnam. The rainfall can be prolonged – like it has been in Singapore since Jan 10 – lasting from two to several days.
The deluge on Jan 10 and 11 exceeded the amount of rain Singapore typically receives in the entire month of January.
Changi recorded the highest total amount of rainfall at 255.2mm over those two days, exceeding Singapore’s monthly average rainfall of 222.4mm in January.
On average, Singapore experiences two to four monsoon surges each year, according to the Meteorological Service Singapore (MSS) website. Each event can last between one and five days.
This current monsoon surge is expected to last till Jan 13, national water agency PUB said.
Monsoon surges occur more often during the wet phase of the north-east monsoon and are more intense between November and January.
During such periods of prolonged rain, temperatures will dip.
In Malaysia, a monsoon surge is expected to bring heavy continuous rainfall to many states – including Sabah and Sarawak – for three consecutive days from Jan 15, the country’s meteorological department said.
This comes as torrential rain has already caused floods in several states, including Johor, where Kota Tinggi, Kluang and Pontian are affected.
In Johor, close to 4,000 people were seeking refuge in flood relief centres, The Star newspaper reported on Jan 13.
What are other drivers of rainfall in Singapore?
Besides the north-east monsoon, other large-scale and localised weather circulation patterns also influence rainfall here. A major one is the La Nina climate phenomenon, which brings wetter and cooler conditions to the region.
La Nina is forecast to return some time between now and March. On the MSS site, the Republic is currently on “La Nina watch”.
“If a La Nina event occurs, the typically drier months of February and March may experience increased and possibly extended rainfall events,” noted Professor Matthias Roth, who specialises in urban climatology at NUS.
With La Nina, the rainy phase of the north-east monsoon could last longer this season into February, said Dr Koh Tieh Yong, co-chair of the Working Group for Asian-Australian Monsoon at the World Climate Research Programme.
He added: “The raincloud systems (in February) will be like those typical of January, lasting a good part of the day and usually bringing the heaviest rainfall in the afternoon while enveloping most of Singapore.”
According to the US National Oceanic and Atmospheric Administration, there is a 59 per cent chance for La Nina conditions to persist from February to April. Prof Roth added that the phenomenon is currently predicted to be short-lived, lasting until some time between March to May.
The last La Nina episode was a particularly prolonged one, lasting three years between 2020 and early 2023.
It partly led to higher-than-average total rainfall in Singapore in 2022, with the deluge in October that year being the highest for that month in the past four decades.
In Singapore, the phenomenon also tends to moderate the annual mean temperatures.
Other drivers of rainfall are localised weather circulation patterns that affect a smaller area, such as the Sumatra squalls that bring rainfall over Peninsular Malaysia and Singapore.
The Sumatra squall is a line of thunderstorms that develops over Sumatra or the Strait of Malacca, often overnight. It moves to the east, at times producing heavy rain and gusty winds, as it crosses Peninsular Malaysia and Singapore.
Why is forecasting weather in the tropics tricky?
Predicting the weather in the tropics, including Singapore, is particularly challenging. This is because weather processes that lead to thunderstorms, for example, typically develop and dissipate quickly, often within one or two hours.
This short lifespan, along with other reasons, makes forecast models that simulate atmospheric conditions less accurate in the tropics.
These models hence do not perform as well as they do in the mid-latitude, temperate regions.
Prevailing winds in the tropics also make it difficult to track and predict the movement of storms that are limited to a certain geographical area.
However, PUB gave some early warning on Jan 9 that potential flash floods could result from the prolonged weekend rain, so that commuters could plan their routes and avoid flood-prone areas.
What should people do during floods?
To avoid being caught off guard by sudden, intense rain and flooding, commuters can subscribe to PUB Flood Alerts on Telegram or download the myENV app to receive weather forecasts.
It would be best to avoid flooded areas.
For vehicles stranded in flood waters, avoid braking to keep the engine speed higher and constant.
Pedestrians should move to higher ground as flood waters may rise suddenly. Houses and businesses in flood-prone locations should install flood barriers – provided by the authorities – when there is a risk of flood occurring.
Residents and business owners should move essential items to higher ground and avoid touching electrical switches or appliances if they are standing in water.
Each flash flood episode tends to last less than an hour in Singapore.
How will climate change affect rainfall intensity and floods?
The Jalan Seaview flood on Jan 10 may offer a glimpse of what flooding could look like in Singapore amid sea-level rise – expected decades down the road.
That evening, heavy rain coincided with a high tide of 2.8m that temporarily overwhelmed an adjacent canal and surrounding roadside drains.
According to Singapore’s third national climate change study, climate change is expected to bring heavier, more intense and frequent storms. This could lead to more flash flooding.
In the low-carbon emission scenario, the rainy season between November and January is expected to get wetter by up to 58 per cent, said the study, which was released in 2023.
By the end of the century, the daily extreme rainfall during April and May – months which generally do not have much downpour – could almost double.
- Shabana Begum is a correspondent with a focus on environment and science at The Straits Times.