Violent storms lashed Singapore early yesterday morning, toppling trees and causing traffic jams.

The Sumatra squall hit at about 6am and lasted five hours.

From 6.10am to 7.10am, 96.8mm of rain fell at Mandai, the Meteorological Service Singapore said. The highest recorded one-hour rainfall for the month of July was 109.6mm in 2010.

It said that the top wind speed yesterday morning was 59.8kmh, near Changi. The highest recorded wind speed for the month of July was 84.6kmh in 2012.

Sumatra squalls are common in the ongoing south-west monsoon season, and rainfall for the first fortnight of this month is expected to be average.

Yesterday, flash floods hit Commonwealth Lane and the Kranji Expressway slip road to Woodlands Road. Fallen trees or branches were reported in several areas including Mandai and Ang Mo Kio.

"A tree fell along Clementi Road, right across from SIM University. Most of my students were late as a result," said university lecturer Chang Qizhong, 31.

But there were bright spots too. Ms Grace Chia, 40, and her daughter were drenched on their way to school yesterday.

But the Primary 1 pupil's school, Pei Tong Primary in Clementi, offered pupils a change of socks and uniforms. "It's a neighbourhood school with a big heart," she said.

GRACE CHUA AND AW CHENG WEI