An amber rain warning was put in place by the NI Weather and Flood Advisory Service overnight.
Heavy downpours are continuing across Northern Ireland with the advisory in place until 6pm this evening.
The Met Office warning – in place due to Storm Francis – will remain in place until early Wednesday morning.
A Medium (Amber) level warning of Rain has been issued for Northern Ireland.
A spokesperson for the NI Weather and Flood Advisory Service said: “We can expected some very heavy rainfall with rainfall amounts of up to 40mm (1.57inches) are possible widely across Northern Ireland.
“However there is a high risk that some areas will exceed this, with up to 90mm (3.54inches) possible.
“This will lead to a significant risk of flooding, as such we have issued flood alerts ahead of the expected rainfall and flood risk.
“Strong winds up to 40-50mph may be an additional hazard at times during Tuesday. We will monitor wind speeds and issue warnings accordingly.
“The public should be aware of heavy rainfall with an increasing risk of flooding.
“The public are advised to take extra care if travelling, avoid flooded roads, slow down and use caution, especially during this morning’s rush hour.”