Thunderstorms could bring a month's worth of rain to parts of the UK, before the hottest temperatures of the year so far bring a balmy end to June.
Some parts of Scotland could see 80mm of rain in 18 hours on Monday and the Met Office has issued weather warnings amid fears of flooding and travel disruption.
Meanwhile, central and eastern spots in England may get more than half a month's rainfall in just an hour, the forecaster said.
Yellow weather warnings, covering Northern Ireland, much of England and eastern parts of Wales, advise there could be damage from floodwater, lightning strikes, hail or strong winds.
High humidity on Monday and Tuesday is being blamed for sparking the thunderstorms, and will result in muggy conditions for many, with overnight highs of 19C possible throughout the week.
The mercury is forecast to rise to 33C in the south east of England by Friday.
The best of the weather will be in western Wales and the south west of England, Met Office forecaster Simon Partridge said.
He said: "Thunderstorms will be fairly well scattered and not everyone will see them but again if you do catch one they're likely to be pretty hefty downpours.
"Overall through much of England and Wales tomorrow generally speaking it's a reasonably dry day with some bright sunny spells but there's that risk of thundery showers which can break out anywhere in those areas.
"The most reliable places to stay dry and see the best of the sunshine really is western Wales and south-west England on Monday."
Up to 40mm of rain is forecast to fall in parts of Northern Ireland and north-eastern parts of England.
Temperatures are likely to reach 28C in London and 21C in Wales and Northern Ireland, but will be lower in Scotland with a high of around 14C on Monday.
The highest temperature of the year so far was 28.8C in Norfolk on June 2.
The June record is 35.6C, which occurred in Southampton in 1978.