The PSNI is co-ordinating a multi-agency planning team in anticipation of high tides and harsh weather starting at midday on Friday, and lasting through Saturday and Sunday.
Police say residents in the Victoria Park and Sydenham area of east Belfast and the Docks and Corporation Street area are at "high risk" of significant flooding and have been urged to start making preparations.
People queued at Inverary Community Centre, where sandbags were distributed on Thursday night.
Belfast City Council said the centre remained open overnight.
A decision to evacuate residents in east Belfast may be made by the authorities "in the coming hours", police said on Thursday night.
Assistant Chief Constable Stephen Martin said residents living in homes between the Holywood Road and Sydenham Bypass and in streets around the Connswater River area should "move their valuables upstairs" and "pack a case" in the event of an evacuation.
"At present we are not recommending people to evacuate their homes or properties but we are keeping the situation under constant review. I would ask them, where possible, to think about moving valuables upstairs and packing a case so that, if we make a recommendation to evacuate, they are in a position to do so," he said.
"Where possible, we would ask those people who are affected by flooding to go to friends and family outside the flood risk area."
Truckloads of sandbags have already been put in place to protect the area near the Connswater River at Victoria Park.
But the PSNI says the concern is that flood defences could be breached.
"Sandbags have been deployed to protect some key facilities and to bolster the existing defences in Sydenham. We would ask residents to not remove these sandbags for personal use as this will place the homes in the area under an increased risk of flooding," ACC Martin said.
Meanwhile other areas which police said could be affected to lesser extents include Larne, Newry, Newtownards and Clough.
There has also been a warning to stay away from coastal paths and walkways and to drive with extreme caution for anyone who is out and about over the weekend.
The Met Office said three factors - high tides, low pressure and on-shore winds - have come together at the wrong time to create the potential for serious flooding.
A yellow warning for heavy rain is in place for Friday.
Forecasters added: "Another spell of unsettled weather is expected on Friday with further wet and windy conditions likely across Northern Ireland. In addition, there is the risk of a spell of snow above about 300m early in the morning.
"The public should be aware of the risk of localised watercourse or surface water flooding."
In general, over the weekend we would ask all members of the public to stay away from coastal paths and walkways and to drive with extreme caution.
Police said a range of agencies including the Office of First Minister and deputy First Minister, Belfast City Council, NI Fire and Rescue Service, the Rivers Agency, Belfast Health Trust, Road Service, NI Water, NI Electricity, British Telecom and Translink are all working together to ensure plans are in place to deal with all eventualities.
Former Lord Mayor of Belfast Gavin Robinson said residents should remain vigilant and try to keep up-to-date on the latest information from the agencies.
The DUP representative explained: "The potential for this flooding is from a tidal surge as opposed to surface flooding which might have been experienced in the past, and that is why Sydenham and the Docks have been mentioned on either side of Belfast Lough.
"As I understand it the risk at this stage is significant but it's continuing to be evaluated.
"The agencies have been meeting today and are talking throughout the night and so it is important people continue to listen to the news and radio and pick up the messages that are being disseminated, because there's important information emerging for people to follow."
Meanwhile an orange warning is in place in parts of the Republic of Ireland.
Flooding Helpline telephone: 0300 2000 100
Met Éireann said high winds, heavy rain, high tides and localised flooding are affecting coastal areas including counties Donegal, Clare, Cork, Kerry and Limerick.