Flash floods around S'pore caused by clash of winds

Police push a mini-bus off after flood waters receded on Aug 20, 2021. ST PHOTO: KEVIN LIM
Personnel from SCDF and PUB communicate on site after cars were left stranded on Aug 20, 2021. ST PHOTO: KEVIN LIM
Cars left stranded in flood waters at the junction of Pasir Ris Drive 12 and Tampines Avenue 10, on Aug 20, 2021. ST PHOTO: KEVIN LIM
Workers opening drain covers to help flood waters recede at the junction of Pasir Ris Drive 12 and Tampines Avenue 10, on Aug 20, 2021. ST PHOTO: KEVIN LIM

SINGAPORE - The deluge of rain that flooded parts of Singapore on Friday morning (Aug 20) was brought about by a clash of winds blowing from the north and the south.

When surface winds blowing from two different directions meet, some air is forced upwards. This creates conditions that facilitate the formation of rain clouds.

The convergence of two wind systems, one of which was from a line of thunderstorms originating over Indonesia and the other a land breeze blowing from Johor, caused the intense rainfall, said weather and climate scientist Koh Tieh Yong from the Singapore University of Social Sciences (SUSS).

After daybreak, the Sumatra squall closed in on Singapore. This compounded the downpour that had begun at 5am, unleashing flash floods and traffic jams around the island.

Temperatures on Friday also plummeted to a low of 21.9 deg C in Changi, Pulau Ubin and Woodlands.

The National Environment Agency (NEA) said the highest daily total rainfall recorded as at 1pm on Friday was 112.1mm in Hougang. This was followed by 111.0mm in Choa Chu Kang and 107.8mm in Paya Lebar.

The highest daily total rainfall so far this month was 121.8mm on Sentosa on Thursday.

Pasir Ris experienced the fourth heaviest rainfall on Friday, with 105.2mm.

This caused flash floods at the junction of Tampines Avenue 10 and Pasir Ris Drive 12 that affected motorists and passengers, some of whom had to be helped by civil defence officers.

The downpour was heaviest over the central, northern and eastern parts of Singapore, said NEA.

The highest rainfall intensity over 60 minutes on Friday was 95.8mm, recorded in Choa Chu Kang. The highest-ever 60-minute rainfall intensity reported in Singapore was on Nov 2, 1995, when it hit 147mm.

The collision of the two wind systems behind Friday's downpour would have been virtually impossible to predict reliably more than a few hours in advance, said Associate Professor Koh.

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More research, for example, on squall and thunderstorm formations could help Singapore better predict and prepare for sudden storm formations, he said.

Over the next few days, thundery showers are forecast mainly from morning to early afternoon.

The Meteorological Service Singapore said on Monday that more thundery showers are expected for the rest of August due to prevailing south-west monsoon conditions.

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