The alert is in force from 4pm on Thursday to 4pm on Friday.
Shower rates may exceed 35 millimetres per hour, it added.
In the evening on Thursday temperature will fall to 12C inland, 15C in the coastal areas and 7C in the higher mountain areas.
On Friday temperature will rise to 21C inland and in the coastal areas and 10C in the mountainous areas.
Temperatures are expected to remain at the same levels on Saturday with scattered showers and the possibility of thunderstorms....