Householders in east Belfast were braced for flooding amid warnings of high tides and heavy rain on Friday morning.
Tidal surges are predicted for midday as well as Saturday and Sunday across Northern Ireland and police have 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.
Many other parts of the UK are also braced for the worst as a combination of high tides, heavy rains and strong winds are expected to bring yet more severe flooding to parts of the country.
“Exceptional” weather is expected across the whole of the UK and Northern Ireland, with a high risk of flooding expected around this morning’s high tides posing a significant risk to coastal towns and villages.
The Environment Agency (EA) has issued 21 of the most serious severe flood warnings in place, issued when there is a threat to life or property, affecting the South West, Gloucestershire and Wales.
Homes in Newport were evacuated on Thursday night because of the risk of flooding as Wales prepared for the highest tides in 17 years, ITV News said.
Residents at the Lighthouse Park Estate were taken to a nearby leisure centre as a precaution, and coastguards have been warned of a storm bringing 70mph winds.
People living in Ilfracombe were joined by emergency services during Thursday night as they gathered sand from beaches to build flood defences.
The EA has also issued 188 flood warnings across England and Wales and a further 233 flood alerts.
Heavy rain and winds gusting up to 60mph are due to hit western areas, prompting fears of widespread disruption.
The bad weather is compounded by high tides on Friday morning, with the risk of flooding expected for between two and four hours either side of high water.
Environment Secretary Owen Paterson, who chaired a meeting of the Government’s Cobra emergencies committee, on Thursday warned energy network companies to be prepared, following complaints it took too long to restore electricity to the thousands of homes left without power in the wake of severe weather over Christmas.
In a statement on its website, the EA said: “The flood risk will extend along the UK coastline from north-west England, through Wales and south-west and southern England. Areas particularly at risk include the Isles of Scilly, the north and south coasts of Devon and Cornwall, Dorset and the coastline of Wales.”
The storms have already claimed at least two lives. The body of a 27-year-old man from Surrey was found on Porthleven Sands beach in Cornwall. He had been swept out to sea on New Year’s Eve night having gone for a paddle with friends at nearby Loe Bar.
In a second tragedy on Tuesday, a woman died after being swept out to sea at the popular beauty spot Croyde Bay in north Devon. The woman, who was believed to be on holiday with her family, was rescued from the sea and airlifted to hospital before being confirmed dead by doctors.
Elsewhere, in Dorset a search was carried out for a man who is believed to have fallen into the River Stour, near Iford Bridge in Christchurch.
Speaking on Sky News following the Cobra meeting in London, he said: “We are looking to have a combination of exceptional rain, wind and a surge in sea and high tides and so there are nearly 50 warnings put out around the whole of the west coast and south coast.
“We had a range of ministers from right across government attending the meeting, who will be working very closely with local councils, power companies, utility and transport companies, making sure that all of those organisations are absolutely prepared for the bad weather that is coming.”
The AA, which has attended 1,500 call-outs from those stranded due to floods since December 23, said some drivers were failing to heed warnings.
Darron Burness, head of the AA’s flood rescue team, said: “Our patrols have seen it all in that time - including people ignoring road closure signs, blindly following their sat-nav or other drivers into deep water and 4x4 drivers naively thinking their car has amphibious qualities - and time and again they hear the same excuses that the driver didn’t think the floods were very deep or that their car could deal with it.”