Emergency response teams have been alerted across Northern Ireland amid fears the overnight flooding that swamped Belfast could be repeated today.
Further downpours are forecast, as thousands struggle to clean-up damage caused by flooding across the city and parts of County Antrim.
Emergency services reported receiving more than 700 call-outs linked to flooding in Belfast, while the region's water authority said it handled nearly 3,000 flood calls, and 1,000 homes were hit by power cuts.
Major roads were left impassable at the height of the crisis, while scenes of rising sewage water flooding homes sparked complaints that government agencies were slow to respond.
Environment minister Alex Attwood said he had now instructed councils in Northern Ireland to immediately convene their emergency response group to address the flooding.
He said: "There is the potential for more flooding and flash flooding in many places in Northern Ireland in the coming hours.
"I spent a large part of last night in the emergency room in Belfast City Council monitoring the ongoing situation and doing what I could to assist.
"As a result, I have activated the emergency financial assistance scheme for local government.
"This will provide £1,000 of emergency funding for households damaged by flood water.
"I would also like to acknowledge the work of the emergency services and councils."
The Met Office has issued a further amber weather warning valid from 4am to 9pm today and the roads service has brought in extra staff.
An Executive spokeswoman said: "There are expected to be further slow moving torrential downpours and further flooding can be expected."
A Northern Ireland Water (NIW) spokeswoman said its call centre had received 2,800 calls about localised flooding.
She added that areas affected included Lisburn, Dunmurry, Belfast and Bangor.
The spokeswoman said: "NI Water have received reports of blocked sewers, reports of external flooding and reports of internal flooding which are being responded to by staff and contractors.
"Every effort has been made to alleviate the flooding in the affected areas as quickly as possible and assist with the clean-up operation."
Cars were submerged during the massive downpour yesterday evening, and scenes of floods pouring down from high ground onto houses were reported across the city.
Even a police vehicle in West Belfast was submerged, with officers rescued from the rooftop.
Neighbours rallied to help each other as the rain hit, with a group of pensioners stranded on their way to a funeral assisted on the Stewartstown Road.
A breakdown lorry helped get the mourners to church, before the vehicle was taken away to be repaired after becoming swamped on a flooded road.
Politicians urged NIW to spend more money improving drainage systems.
Ulster Unionist representative Michael McGimpsey, whose south Belfast constituency was one of those hit by the floods, said the same areas were being repeatedly affected in heavy rain.
"I am calling upon the Executive to take steps to alleviate the perennial flooding which occurs in South Belfast following a short period of heavy rain," he said.
"The situation as it affects the people of South Belfast is absolutely intolerable.
"The Executive simply must find the money to ensure that the drainage system is upgraded so that local people are not living in fear of having their homes and businesses destroyed every time it rains.
"Previous estimates have put the cost of the required works at £35 million and if that is the case then so be it.
"It will be money well spent."
Meanwhile, parts of Cork city and county in the Irish Republic were left under several feet of water after flash floods overnight.
Met Eireann reported that 50mm of rain fell between 8pm and 3am on already saturated ground as thunderstorms swept in from the south.
Locally, the figure may have been as high as 70mm.
The worst affected areas included Douglas on the outskirts of the city, Carrigaline, Blackpool, Togher, the Kinsale Road and a stretch of the South Ring Road.
Council chiefs have put a flood response plan in place.
Forecasters warned there would be further rain today but that it was not expected to be as heavy as overnight.
Elsewhere there were reports of more than 1,000 homes in Cork without electricity.
The Met Eireann forecast for today is for a band of heavy rain, with embedded thunderstorm activity and hail, to continue moving northwards in the morning producing further flooding.
Drier weather would follow with some sunny spells developing, it said.