The Hanoi Drainage Company said widespread rain swept across the capital overnight on August 20 and into the morning of August 21 due to the combined effects of the intertropical convergence zone and a tropical depression over the northern East Sea.
Rainfall data recorded by 6 am showed 117.8mm in Hai Ba Trung Ward, 104.6mm in Phuc Loi, 101.6mm in Hoang Mai and 96.2mm in Hoang Liet.
In outlying areas, My Duc Commune recorded the highest rainfall at 136.6mm, followed by Phu Xuyen at 86.9mm, Thanh Tri at 79mm and Dong Anh at 76.8mm.
Despite hours of heavy rain, inspections on Friday morning found no flooded locations across the city, the company said.
Drainage workers were deployed overnight to clear rubbish from drains and keep water flowing. Floodgates at retention lakes were opened and major pumping stations put into operation to lower water levels across the drainage system.
The company said personnel and equipment would remain on standby throughout Friday, with pumping stations operating as required and drainage inlets being cleared to maintain water flow.
The overnight downpour and thunderstorms raised concerns about rush-hour flooding, but residents reported roads remained clear, including in flood-prone areas.
Locations closely watched during heavy rain, including roads around Aeon Mall Long Bien, Vo Chi Cong Street, An Khanh, My Dinh and Nguyen Trai Street, were reported to be free of flooding on Friday morning.
Hanoi has identified urban flooding as one of five major bottlenecks requiring urgent action and has accelerated drainage projects ahead of the 2026 rainy season.
The city has been implementing 10 emergency infrastructure projects, including five drainage system projects and five retention lake projects, with the aim of bringing them into operation before this year's rainy season.