Emergency measures were instigated by Government departments as the country was hit by some of the worst flooding in years.
Roads were impassable, homes had to be sandbagged, businesses were forced to close and traffic was gridlocked as up to seven millimetres of rain an hour fell non-stop for four hours on ground that was already saturated yesterday - with more falling overnight.
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The deluge led to rivers including the Lagan bursting their banks and manhole covers being displaced on main roads as the water was left with nowhere to go.
Among the worst-hit areas were Newry, where Bridge Street was submerged in two to three feet of water; Belfast, Lisburn, Antrim, Moira, Lurgan, Banbridge, Donaghadee and parts of Londonderry. Emergency services including the Fire Service were on standby to evacuate people from their homes as the water levels rose throughout the day.
The PSNI had to deal with dozens of road closures as the downpours caused chaos, particularly for commuters.
However, contingency plans involving the PSNI, Fire Service, Department for Regional Development, Rivers Agency, NI Water, Transport NI (formerly the Roads Service), Department of the Environment and the Department of Finance and Personnel swung into operation to deal with the repercussions of the heavy rainfall.
Regional Development Minister Danny Kennedy, speaking from Transport NI headquarters, vowed that budget cuts would not impact on his department's response to the flooding.
He described the situation as "very serious" and said that agencies had "full use of all available resources".
Mr Kennedy has given the go-ahead for outside contractors with specialist pumping equipment to be brought in to alleviate the flooding.Tweets by @TrafficwatchNI
Meanwhile, Environment Minister Mark H Durkan has activated an emergency scheme for affected householders.
He said: "Individual householders who have suffered severe inconvenience can claim from their local council a £1,000 payment as an offer of practical assistance. The payment is meant to ensure that homes are made habitable as quickly as possible. It is not a compensation payment."
The situation was further complicated after the Met Office issued another weather warning for more rain and gale force south-easterly winds in the early hours of today.
Police are urging drivers and pedestrians to take care. Chief Inspector Pat Foy said: "We would advise people to keep themselves safe from harm by not walking or driving in areas of floodwater which can be contaminated with sewage and other hidden hazards such as exposed manholes. It's hard to know how deep the waters are and the dangers that could be hidden below the surface."
Monsoon-like conditions meant rush hour chaos and disruption for thousands of motorists in Belfast and there was a report of a dozen vehicles abandoned after being damaged by a manhole that had its cover blown off by water pressure as drains struggled to cope with the deluge.