The Met Office has issued a yellow weather warning covering large parts of South Wales and the south-west of England. The warning began at midday and lasts until 8am the following day.
Rain is expected to become heavier and more widespread in these regions before easing early Wednesday. Forecasters predict up to 40mm of rain in the affected areas, with some exposed south-facing hills in the Brecon Beacons and Dartmoor possibly receiving more than 50mm.
"If so, consider preparing a flood plan and an emergency flood kit. Give yourself the best chance of avoiding delays by checking road conditions if driving, or bus and train timetables, amending your travel plans if necessary."
The forecast for Wednesday predicts continued cloudy skies with outbreaks of rain.
The Met Office warns residents in 20 locations to prepare emergency flood kits and essential supplies ahead of heavy rain and potential flooding impacting South Wales and south-west England.