Sun-seekers planning outdoor activities could be forced to dive for cover as showers take hold as the day wears on.
Wales, the Midlands and south Pennines have weather warnings in force as sharp thundery spells could dump up to 20mm.
But temperatures are still likely to surpass those in the Mediterranean, particularly in southern and eastern parts of the UK, with Sunday on track to be the hottest day of the year so far.
Bookies have been slashing their odds for Sunday’s forecast, which could see the mercury rise to 26C (78F), around 10C (18F) higher than the monthly average.
Ladbrokes said that bets have been pouring in and are offering 2/1 for the mercury to reach 30C or higher before the weekend is out.
Jessica Bridge of Ladbrokes said: “It’s getting hotter by the day and punters are convinced the sunshine’s here to stay all summer.”
A spokeswoman for the Met Office, Emma Boorman, said: “It’s likely that today is going to be the hottest day of the year but likely that tomorrow will then beat today. Sunday is looking like it is going to be the peak of the warmth.
“The temperature is very mild across parts of the UK through the course of today. There’s some sunshine on offer particularly for southern and eastern parts of England but we will see showery outbreaks of rain, potentially some thunderstorms developing, as we go into this afternoon.”
But she added: “It is not the same everywhere though, much cooler, cloudier and with some outbreaks of rain further north and west, particularly Northern Ireland and western Scotland and misty and murky conditions along the north-east coast of both England and Scotland suppressing the temperatures back in to the low to mid-teens.”
Thunderstorms will carry on through the night but by Sunday it will feel even warmer with the sunshine more widespread.
Temperatures are likely to get into the high teens in most places, with expectations that the south-east part of the UK will see temperatures of 26C - the hottest day yet.
Pollution levels are expected to peak on Sunday, with high levels possible locally and widespread moderate air pollution.
Those heading outdoors to enjoy the fine weather over the weekend are also warned to be aware that UV levels will be high.
As the week progresses temperatures are still likely to be mild but will fall day on day.