The UK has so far fallen short of seeing its hottest day of the year, though forecasters are “hesitant” to rule out the possibility later on.
Temperatures in Santon Downham, in Suffolk, reached a high of 28.2C (82.76F) at just before 1pm on Wednesday, the Met Office said.
But this is just below the 29.7C (85.46F) recorded in Teddington, Middlesex, on Monday, which was the hottest day of the year so far.
It comes ahead of thunderstorms set to move in across large parts of England, including London and the South East, from 6pm.
The Met Office has issued a yellow warning meaning people could see flooding to homes and businesses, communities becoming “cut off”, power cuts and delays to public transport following heavy rainfall.
The thunderstorms are the result of a small, low-pressure system moving in from Europe, with warmer surface-level air meeting colder upper air.
Forecaster Simon Partridge told the PA news agency: “The UK has so far seen a high of 28.2C in Santon Downham in Suffolk at 12.45pm, which unfortunately falls just short of being the hottest day of the year.
“But I’d be hesitant to rule out it becoming hotter later on in the day as temperatures could reach the 30C point, it’s possible.
“The higher cloud hanging above parts of the country will make it difficult, as it blocks out the sun and stops it from getting any hotter, but we’ll see, there’s definitely potential.”
Wales could be as hot as 22C (71.6F) during the evening, as football fans watch their nation take on Turkey in their second game of Euro 2020.
However, Mr Partridge warned that thunderstorms are beginning to build and develop across the English Channel, and will reach southern areas of the UK by around 7pm.
He said: “This will bring heavy rainfall, loud thunder and frequent displays of lightning for much of England and south Wales until about midnight.
“The rain will be pretty heavy with up to 30mm falling in quite a short time frame.
“It will be a horrible night for many, especially when you’re trying to get some sleep, remaining quite hot and humid in London and the South East, at around 17-18C, combined with the damp weather.”
Thunderstorms are expected to continue until the weekend.
England’s much-anticipated Euro 2020 clash with Scotland at Wembley Stadium on Friday evening could be hit with bands of “heavy thundery showers”, forecasters say.
However, most of Scotland, Northern Ireland and northern parts of Wales are expected to escape the wet weather, with these areas avoiding the yellow weather warnings.
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