The Met Office has issued a severe warning for large parts of the UK, including Sheffield.
Today is set to be the hottest day of the year but overnight and through Wednesday the very humid air will trigger some severe thunderstorms which are likely to cause flash flooding in places and disruption to transport networks and power supplies.
The weather warning states: "There is potential for some severe thunderstorms to develop across Wales, northern England and the Midlands.
"Here, the risk is greatest during Wednesday, by which time the risk of storms will be reducing across the west of the yellow area - and then finally also reducing across the east of the area by Wednesday evening as storms tend to clear away northeastwards.
"Whilst most places will miss the worst, please be aware of the risk of localised flash flooding.
"Frequent lightning, large hail and strong winds could be additional hazards.
"All of this could lead to some flooding of homes, businesses and transport networks. Disruption to power is also possible."