The Met Office has issued a yellow rain warning and said that thunderstorms overnight could bring flash floods.
The official forecaster said: “Thundery showers are expected to push north across southern parts of the UK through Tuesday evening (18 July).
“Although many places won’t see these showers, there is a chance of localised flooding of homes, businesses and susceptible roads.
“Frequent lightning may be an additional hazard with possible disruption to power networks.
“Similarly, but very locally, hail may cause impacts.
“Southwest England looks like it will be first to see scattered thunderstorms, the threat then expanding northeastwards through the course of the evening.
“There is relatively low confidence in the areas more at risk of intense downpours.
“Some places will miss them altogether but, where they do occur, there is the potential for locally 20mm to 30mm of rain within an hour.
“This may result in surface water flooding, particularly should these amounts fall across urban areas.”