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

When surface winds blowing from two different directions meet in this manner, air is forced upwards and rain clouds form.

The convergence of two wind systems, one of which was from a line of thunderstorms originating either over Sumatra island or the Strait of Malacca, caused the intense rainfall, said weather and climate scientist Koh Tieh Yong from the Singapore University of Social Sciences (SUSS).

The upward movement of air also resulted in an area of low atmospheric pressure, which brought the Sumatra squall close to Singapore.The result was thedownpour which began at 5am, unleashing flash floods and traffic jams around the island.

Temperatures on Friday also plummeted to a low of 21.9 deg C at 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 at Hougang. This was followed by 111.0mm at Choa Chu Kang and 107.8mm at Paya Lebar.

The highest daily total rainfall so farthis month was at 121.8mm at 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 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 was 95.8mm, recorded at Choa Chu Kang. The highest ever 60-minute rainfall intensity reported here, 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.

More research, for example, on squall and thunderstorm formationscould 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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