Thunderstorms are expected to develop during Friday afternoon, lasting well into the evening.
Although temperatures continue to rise, this hot and humid weather is a trigger for heavy rain and thunderstorms, both of which are set to hit Sheffield this weekend.
On the roads, motorists are warned ahead of lightning, flash flooding and large hailstones, with a thunderstorm warning in place for Friday afternoon until just before midnight.
It is officially the hottest day of the year so far in the United Kingdom with temperatures soaring to 35.1C in Surrey.
The July record for the highest overnight minimum temperature is 23.3C - and the Met Office says that could be surpassed in south-east England on Thursday.
While Ireland's cooler, wetter weather of recent days is set to stretch into the weekend and next week, the United Kingdom has continued to bake.
In the west, Glasgow is already cloudy, with maximum temperatures of around 22C.
The all-time British record is 38.5 degrees Celsius set in August 2003 near Faversham in Kent, southeast England, while the highest July temperature was 36.7 degrees Celsius in 2015 at Heathrow airport.
"Large hail, frequent lightning and strong and gusty winds will be additional hazards".
With potentially record-breaking high temperatures in the coming days, the Government is being warned it must do more to protect vulnerable people in extreme heat.
Inverness will experience extremely hot temperatures today (up to 27C), but very little sunshine, staying dry throughout.
The warning that there is a 90% possibility of heatwave conditions came into force on Monday morning and will last until 9am on Friday in the Midlands, south-east England and eastern England.
Some places could see as much as 30mm of rainfall in an hour and 60mm in three hours.