As the clean-up operation continues in the flood-hit north west, questions are being asked about the response of the authorities.
Roads and bridges crumbled, cars were washed away and homes and businesses were destroyed in storms.
More than 100 people were rescued from their cars and homes after 63% of August's average rainfall fell within nine hours.
Politicians have queried preparations ahead of the weather warnings.
In other developments:
- Derry City Airport was temporarily closed but check-in desks were due to reopen at 13:00 BST
- Help centres set up at the YMCA in Drumahoe and the Eglinton Community Centre reopened at 08:00
- Derry City and Strabane District Council said its officers dealt with more than 230 requests for assistance on Wednesday
- The Department of Infrastructure says more than 30 roads have been affected
- Senior Irish government figure, Joe McHugh, has called for emergency funding to help those affected by flooding in County Donegal
- 55,000 chickens at the Moy Park processing farm were killed in flash flooding, the News Letter reports
Sinn Féin's northern leader Michelle O'Neill said the process of rebuilding would require a "robust and concerted approach across several different government departments".
"Clearly questions need to be answered about the state of preparedness and the response of some statutory agencies," she added.
SDLP leader Colum Eastwood tweeted: "Families and businesses struggling to cope and big questions for statutory agencies. Where was the preparation?"
DUP leader Arlene Foster travelled to Drumahoe in Londonderry to meet victims.
She also tweeted that she had spoken to David Sterling, the interim head of the Northern Ireland Civil Service, about the government response.
The Department of Infrastructure has said it will investigate any problems with the NI Direct floodline, after many complaints callers could not get through to the emergency helpline during the crisis.
Owen McGivern, head of operations for the Department of Infrastructure and Rivers, said there were questions to be answered, and that the flood line number's performance would be reviewed.
John Kelpie, chief executive of Derry City and Strabane District Council, said the amount of rainfall was much higher than had been forecast.
"We have established a council presence in the two major areas affected - Drumahoe and Eglinton," he said.
"We have up to 30 environmental health officers doing door to door knocks, checking on properties."
As torrential downpours led to flash floods, the rescue services dealt with more than 400 emergency calls overnight on Tuesday.
Families have been left homeless after their houses were damaged by flood water and raw sewage, and numerous roads remain blocked in the worst-affected areas of counties Derry, Tyrone and Donegal.
The Department for Infrastructure said hundreds of staff from across several agencies were working to reopen roads, pump water, and clean up residential homes and business properties.
An emergency payment has been made available to flood victims, which will provide affected householders with up to £1,000 for refurbishments, following inspections by council officials.
But the SDLP say the scheme should be extended to include non-domestic properties like businesses, community and sporting organisations.
Meanwhile, a senior Irish government figure has called for emergency funding to help those affected by flooding in County Donegal.
Joe McHugh, the Irish chief whip and Minister of State for Gaeilge, Gaeltacht and the Islands, visited some of those affected in Buncrana, Bridgend and Burnfoot.
"It is essential that an emergency aid package is put together so we can help families rebuild their lives and so that bridges and roads washed away in the floods are restored as soon as possible," he said.
Mr McHugh said he would work with cabinet colleagues to arrange an emergency funding package for those hit by the floods.
On Thursday, the Department of Social Protection in the Republic of Ireland said that emergency income support payments have been made to some of those affected by flooding in County Donegal under the Humanitarian Assistance Scheme.
Thirty soldiers are expected to arrive in Inishowen on Friday to help with the clean-up operation.
The Irish weather service Met Éireann has issued a yellow weather warning for County Donegal until 18:00 local time on Friday, with "further spells of heavy rain expected".