The Observatory will introduce a new rainstorm warning system after criticism by the Ombudsman that the current one is inadequate.
The amber, red and black signals will remain, but from March 23 they will predict hourly rainfall rather than stating actual measurements.
The amber signal, predicting or measuring 20 millimetres an hour, has been issued only to government departments until now, but will be given to the public under the new system.
The red signal will warn of rainfall exceeding 50 mm an hour and the black signal for 70 mm or above.
Flooding announcements in the northern New Territories will also be introduced.
But the Observatory admitted rainstorm predictions would be confined to between a few minutes and an hour beforehand.