Up to 30mm of rain could fall in just an hour in Sheffield today as forecasters warn of heavy showers and thunderstorms.
The Met Office has issued a yellow 'be aware' warning for rain for Friday, which is valid until 10pm.
It said: "Scattered heavy showers and thunderstorms are likely to develop during Friday, particularly the afternoon and then easing only slowly through the evening.
"Some places will miss the worst of the heavy showers and thunderstorms, but where they do occur they are likely to bring torrential rain with the potential for 20-30 mm of rain to fall in an hour and perhaps as much as 50-60 mm in 3 hours in a few locations.
"Lightning and large hail are also possible."
The Met Office warned homes and businesses 'could be flooded quickly' and fast flowing or deep floodwater posed a 'danger to life'.
Forecasters predicted a murky start with extensive low cloud and areas of mist slow to clear. Once cloud moves towards the coast, it will leave warm sunny spells, sparking the slow-moving and potentially thundery downpours this afternoon.
Temperatures could reach 25°C.
Showers will fizzle out overnight to leave most places dry but low cloud and mist will spread inland once again with temperatures dropping no lower than 14°C.