A yellow weather warning has been issued for Swindon and parts of north Wiltshire – with thunderstorms and heavy rain forecast.
The Met Office warning will be in force from 6am until 10pm on Thursday 27 August.
Forecasters say a band of thunderstorms and heavy rain is expected to move north across England and Wales, with conditions potentially becoming slow-moving during the afternoon and evening.
Rainfall totals of between 20mm and 30mm in three hours are expected fairly widely, while some locations could receive up to 50mm over 12 hours.
There is a small chance that more than 50mm could fall within three hours in some areas, with totals exceeding 75mm over 12 hours possible, particularly over east-facing high ground.
Lightning and hail measuring between 1cm and 2cm are also expected to pose hazards.
The Met Office says there is a small chance that homes and businesses could be rapidly flooded, while buildings could be damaged by floodwater, lightning, hail or strong winds.
Fast-flowing or deep floodwater could also pose a danger to life in some locations.
Drivers could face difficult conditions because of spray and sudden flooding, with some roads potentially forced to close.
Train and bus services could also be delayed or cancelled where flooding or lightning strikes occur.
There is a slight chance of power cuts and disruption to other services, while some communities could become cut off by flooded roads.
