The development and movement of heavy rain that affects Hong Kong can be vastly different. Sometimes the heavy rain may be widespread, while on other occasions it may be localised and cause serious flooding only in certain districts but not others.
By making reference to the operation of Special Announcement on Flooding in the northern New Territories, the issuance of the Announcement on Localised Heavy Rain is based on the recorded regional rainfall amount, for example, when rainfall of 70 millimetres or more is recorded in an hour in individual districts of Hong Kong, which may bring serious flooding and risks to the districts. If heavy rain at that time does not extend generally over Hong Kong and reach the criteria for a Red or Black Rainstorm Warning Signal, the Observatory will issue the Announcement on Localised Heavy Rain, indicating the affected districts and the respective rainfall recorded, to alert the public of potential flooding due to heavy rain in the districts, so as to take precautionary measures. The announcement also reminds relevant government departments to take appropriate responsive actions, such as emergency measures to alleviate the impact of flooding in concerned districts.
The Special Announcement on Flooding in the northern New Territories is applicable to the northern part of the New Territories, whereas the Announcement on Localised Heavy Rain is applicable to other areas in Hong Kong. When localized heavy rain persists or there is change in the affected districts, the Observatory will update the announcement appropriately. Furthermore, if heavy rain is forecast to extend to affect widespread areas and reach the criteria of Red or Black Rainstorm Warning Signal, the Observatory will issue the corresponding warning signal.