The Met Office has forecast “localised flooding and travel disruption” across County Down and Armagh in Northern Ireland.
As much as 60mm of rain will be seen in some areas.
The flooding may affect homes and businesses, the Met Office said.
Bus and train services are likely to be affected, resulting in longer journey times.
Car travel will also be affected due to spray on the roads.
The extreme weather is expected to begin this afternoon, and continue overnight.
The Met Office said: “Persistent and sometimes heavy rain will develop within the warning area on Christmas Day afternoon, lasting for the rest of the day.
“Rain will become intermittent overnight but a few heavy showers may continue to affect the area until Boxing Day morning.
“Accumulations of 25 to 40 mm are likely, and as much as 60 mm in a few places.
“Given recent rain this may lead to some localised flooding.”