As much as three-quarters of the August rainfall fell in a single day
Torrential rain has caused widespread flooding across Northern Ireland, with rivers bursting their banks, landslides and roads cut off.
Police also said two bridges had collapsed and 37 major roads were shut down, including part of the M1 motorway, and a train had derailed.
The Fire and Rescue Service had helped people from their homes in Belfast, counties Down, Armagh and Antrim.
NI Water said its resources had been "overwhelmed" by the deluge.
Some places in central and eastern Northern Ireland saw up to three-quarters of the August rainfall in a single day.
Portglenone in County Antrim had 62mm of rain in 12 hours
The newly constructed Broadway Underpass in west Belfast was under 15ft of flood water, and a train derailed in Country Laois after a landslide on the track.
A section of the central barrier of the Westlink in west Belfast was cut with angle-grinders to allow traffic to be turned away from deep flood water.
Police warned drivers in south Belfast that manhole covers had been lifted out of place and carried along by flood water.
They have also urged motorists not to undertake any journeys unless they were extremely urgent.
"The floods are all over Northern Ireland," said a police spokeswoman.
Motorists battle the floods in Castlereagh, east Belfast
"Portglenone and Aghadowey are practically cut off, it seems. There have been several landslides, including one about one mile from the Spelga Dam on the Slievemaman Road in Newcastle, Co Down."
Newcastle Town Hall and the town's Presbyterian church were opened as shelters for those affected by the floods, as was Watty Grahams GAA club in Maghera, Co Derry.
One of the collapsed bridges is at Fairy Glen, Rostrevor, Co Down and the other is on the Lurgan to Cahone road at Rathisland.
The Northern Ireland Fire and Rescue Service said there had been ground-floor flooding in Belfast, Down and Armagh, and that every single one of its fire appliances was in use.
Crews have been pumping water away from homes in Carrington Street in East Belfast and residents were taken to emergency accommodation in the nearby Avoneil Leisure Centre.
The Ballygawley Road and the Cookstown area of County Tyrone were among those flooded, and part of the Shore Road on the outskirts of north Belfast was closed after floods brought down a wall.
Sporting fixtures, including several Carnegie Irish Premier Division football matches, were also hit, and the Ulster Grand Prix at Dundrod, Co Antrim was cancelled.
Organisers earlier had to cancel Saturday's Ulster Grand Prix at Dundrod.
NI Water said their personnel had received almost 3,000 calls which they were dealing with along with the Rivers Agency and Roads Service.
A spokesperson said persistent and heavy rain following three weeks of exceptional climatic conditions had caused the flooding.
The new Broadway underpass is under 20 feet of flood water
"No system is equipped to deal with such intense rainfall which means that systems have become overwhelmed in some areas.
"Staff and contractors from all three drainage agencies (Rivers Agency, Roads Service and NI Water) are working with the Fire and Rescue Service and are responding to all reported incidents as soon as possible.
"However, resources have been overwhelmed due to the scale of this event. All our sympathies are with any residents affected in what is clearly a distressing experience for everyone."
The Northern Ireland Water helpline telephone number is 08457 440088.
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