Residents have been advised to take shelter and be on guard for severe flooding. HONG KONG: Hong Kong's observatory raised its highest black rain signal on Thursday (Jun 18) at 12.55pm local time, prompting schools to close and some businesses to halt, as it asked residents to take shelter and be on guard for severe flooding. Heavy rain exceeding 70mm in an hour is likely to continue, the observatory said on its website. It is the second time the black warning signal has been hoisted this year;