A severe weather warning is in place for Devon, with thunderstorms and heavy rain forecast until Saturday.
The warning, in place from 4am on Friday through until 9pm on Saturday, forecasts showers becoming heavy and thundery across southwestern parts of England and Wales early on Friday, moving northeast to affect other areas south of The Wash to North Wales during the day.
Some reduction in activity is expected overnight, before further slow moving heavy and thundery downpours develop during Saturday, easing from the north later.
There is a warning that the "downpours may lead to localised flooding and potential disruption to transport and outdoor events."
The Met Office chief forecaster said: "A slack area of low pressure is expected to extend eastwards across southern UK on Friday and Saturday, bringing with it moist and deeply unstable air.
"Daytime heating will assist the development of heavy showers and thunderstorms. These are likely to become slow-moving and organised into bands giving some places 15-20 mm of rain in an hour or less and perhaps 40 mm in 2 or 3 hours.
"Hail and lightning will be additional hazards. As with the nature of showers, some places will escape the heavier ones."