Swathes of England are set to be hit by thunderstorms and heavy rainfall following the driest start to summer on modern record.
Flash flooding could lead to power cuts, hazardous driving conditions and transport delays across stretches of the country on Friday afternoon, the Met Office said.
Yellow weather warnings are in place across the east midlands and east of England, London and the south-east, and the south-west from 2pm until the end of the day.
Some areas could see up to 30mm (3cm) of rainfall in an hour, with the risk of flooding increased as hard ground conditions mean water is initially less absorbed into the soil.
Rain is also forecast across Scotland and Northern Ireland through Friday.
However, no rain is forecast in Sheffield for the rest of today with temperatures remaining around 20C.
The Met Office has predicted a cloudy day in the city before sunny intervals for Tramlines punters on Saturday and Sunday.
Temperatures are set to peak over the weekend at 24C on Sunday before the first bit of rain is forecast for late Monday afternoon.
Across the UK temperatures are set to remain in the high-20s across southern parts and could climb even higher going into the weekend.
Meanwhile United Utilities, which supplies the north-west, is introducing a hosepipe ban from August 5 amid concern over low reservoir levels.
It has asked for permission to take more water from three lakes in Cumbria to safeguard supplies, applying for drought permits for Ullswater and Windermere and a drought order for Ennerdale.
Elsewhere, the Environment Agency has taken to aeration to sustain oxygen levels for fish in some rivers during the hot spell.
The UK as a whole saw only 19.5% of expected summer rainfall between June 1 and July 16, according to Met Office figures.
As of Wednesday, the UK having just 47mm (1.85in) of rain so far means it is the driest start to summer in modern records which date back to 1961, followed by 2013 with 59mm (2.3in) of rain.
Aidan McGivern, Met Office meteorologist, said: "The hottest air will be in the south-east with the heat and humidity combining to set off thunderstorms for Friday evening rush hour.
"Tricky driving conditions, spray on roads, flash flooding, 30mm (3cm) of rainfall in the space of an hour for some but hit-and-miss thunderstorms.
"There are also some outbreaks of rain for the south-west and Wales as we wake up on Saturday some useful rain for the gardens and the farms, but it soon starts to brighten up and dry up across large parts of the country.
"The sun does break through by the afternoon and sunny spells in the afternoon leading to again heat and humidity in the south-west and once more that could set off a few thunderstorms as we end Saturday."