The Met Office has issued a yellow warning for rain covering parts of Kent and Surrey.
The 24-hour long alert, due to come into force at 06:00 GMT on Friday, also covers swathes of England and part of Wales.
“Impacts appear more probable across south-east Wales, the Midlands and parts of southern England,” the meteorological service said.
According to the alert, outbreaks of rain are expected to develop from Thursday, become prolonged and heavy throughout Friday, then slowly ease on Saturday morning.