Thunderstorms will bring a wet start to the week for some parts of the UK, but sun-worshippers can expect to see the hottest temperatures of the year so far by the weekend.
The mercury could rise to 33C in the south east of England by Friday, while overnight highs of 19C throughout the week will mean muggy conditions for many.
High humidity on Monday and Tuesday will spark thunderstorms and weather warnings are in place for Northern Ireland, Scotland and much of England.
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."
As much as 80mm of rain could fall in Scotland, while 40mm is forecast to fall in parts of Northern Ireland and most 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.