Householders in East Belfast were braced for flooding amid warnings of high tides and heavy rain.
Tidal surges are predicted for midday as well as Saturday and Sunday across Northern Ireland and police asked members of the public to avoid coastal paths and to drive with extreme caution.
High risk areas include the densely-packed residential streets of Sydenham in the east of Belfast and around the docks.
Other towns that could be affected to a lesser extent than Belfast include Larne, Newry, Newtownards and Clough in counties Antrim and Down.
Police are co-ordinating a major planning operation involving many public services.
The Office of First Minister and Deputy First Minister, Belfast City Council, Northern Ireland Fire and Rescue Service, the Rivers Agency, Belfast Health Trust, Road Service, Northern Ireland Water, Northern Ireland Electricity, British Telecom and Translink are co-operating.