The working week got off to a delayed start yesterday after the second black rainstorm of the year flooded streets and interrupted services. Schools, courts, banks, the stock exchange, clinics and social welfare centres failed to open on time after the signal went up at 6.20am. It was lowered at 8.50am and all had reopened by 11am. Between 100mm and 150mm fell over Hong Kong Island and the New Territories, causing 28 flood reports. The Hang Seng Bank Building on Nathan Road, Mongkok, briefly lost electricity at 2pm and the Good Hope Building, also on Nathan Road, experienced a power cut when its ground-floor meter room flooded. It was restored at 8.30pm. Ferry services to the outlying islands were halted temporarily but services at Chek Lap Kok were uninterrupted. Flooding shut three roads on Hong Kong Island, 23 in West Kowloon and two in the New Territories. 'The flooding was mainly confined to Mongkok and Shamshuipo,' a government spokeswoman said. The Hong Kong Observatory had little advance warning the overnight thunderstorms would intensify into a black rainstorm. 'We issued the thunderstorm warning at 11.10pm on Sunday, but the rain intensified very rapidly in the early hours,' said scientific officer C. C. Lam. The amber warning was hoisted at 3am, the red at 5.30am, followed by the black. Amber means more than 30mm of rain is expected to fall within an hour, red 50mm and black 70mm. The first black warning of the year occurred on April 23, when as much rain fell in 24 hours as usually falls in the entire month. April was the wettest since weather records began more than a century ago. So far this year, 894.8mm of rain has fallen, 45 per cent above normal. Mr Lam said the low-pressure trough that caused the storms had moved south, and the Observatory expected mainly cloudy weather for the rest of the week, with the possibility of morning showers.