The Met Office has issued a yellow weather warning for rain, advising residents in 20 locations to prepare an emergency flood kit along with essential supplies.
The warning took effect at midday and will last until 8am the next day. It covers large parts of South Wales and the south-west of England, where heavy and widespread rain is expected before easing early Wednesday.
"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."
Residents should assess their flood risk and prepare accordingly. To cope with possible power outages, people are encouraged to gather torches, spare batteries, and mobile phone power packs.
The weather is expected to remain cloudy with intermittent rain showers continuing through Wednesday.
The Met Office urges people in vulnerable areas to prepare flood kits and essentials to stay safe and minimize disruption amid heavy rain and travel disturbances.