SOUTH Wales is set for another hot day today, but the weekend is promising torrential rain.
Temperatures in South Wales are expected to be in the mid 20s while across many southern parts of England are expected to top 30C (86F) today - ousting yesterday's 29.2C (84.5F) in London as the hottest of the year - with some parts of the South East reaching at least 32C, perhaps higher.
The Met Office warned of a 60% chance of a heatwave until 8am on Sunday in parts of the country.
But a new weather pattern moving into the UK from the south will bring wet and humid weather tonight, which is expected to erupt into thundery downpours by the early hours of tomorrow.
In a severe weather warning, the Met Office said on its website: "Areas of heavy, thundery showers will develop over England and Wales from Friday night before moving northwards into Scotland during Saturday.
"The location of these is uncertain but where they do form some torrential downpours are possible with frequent lightning, large hail and locally strong gusts. Significant flooding is possible where these do occur from surface water as well as from small, fast responding watercourses.
"The frequent lightning, large hail and strong gusts could also be an additional hazard."
Nick Prebble, a forecaster with weather forecasters MeteoGroup, said while areas of Essex and Greater London could see 32C (89.6F) today, with mid to high 20s across much of England and Wales. He added:"The further north and west you go it will be less warm - the west coast of Wales and Scotland will be about 20C (68F) - which is not bad, but certainly not as hot as in the South East."
The changing weather pattern is being triggered by a plume of unstable air moving into the UK from the south. Despite the lightning and thunderstorms expected tomorrow, it will still be hot.
Mr Prebble said: "Tomorrow will be the worst day of the week, but temperatures in the South East will be 29C/30C (84.2-86F) again, so it will be warm and humid."
Around the UK temperatures yesterday peaked at 25.1C (77.1F) at Hawarden Airport in Wales, with highs of 22.8C (73F) and 23.2C ( 73.7F) in Scotland (Fife) and Northern Ireland (County Down) respectively.
The Environment Agency has said there is a risk of localised flooding over the weekend as a result of intense rainfall.