The Met service issued a heavy rainfall warning on Thursday that will be in effect from 1am until 12pm on Friday.
The service said rain and storms will mainly affect the south coast and inland areas. The rainfall could exceed 50mm per six hours in some areas, it added.
The public were urged to be aware of the possibility of flooding and potential problems in outside activity.
Driving conditions are expected to be difficult due to low visibility and slippery surfaces.