Showers moving in from the Atlantic will lash southern and eastern areas of the country on Sunday, which will also be hot and humid.
Forecasters say the “tropical” conditions will continue until Wednesday, with temperatures climbing into the high twenties.
Greg Dewhurst, a Met Office forecaster, told the PA news agency: “A changeable picture will emerge over the coming days with it being on the warm and humid side for many.
Seized flags should not detract from hugely successful Derry Day: Apprentice Boy
17 pictures as Derry Day is commemorated with Apprentice Boys march through Londonderry
Field Marshal on top of world again
Poppy wreaths on Bogside internment bonfire branded ‘disgraceful’
Retro: Romantic wedding held of Rathlin Island (August 1898)
“It will be warm and muggy and there will be plenty of cloud as high pressure builds on Monday, the day starting sunny and dry before turning wet and windy in north-eastern and western parts of the UK.”
Temperatures in London and the South East could then push as high as 29C on Tuesday, before turning sunnier and cooler from the middle of the week.
Mr Dewhurst went on: “There’s going to be a very muggy feel, more so at night, so expect an uncomfortable time sleeping, with temperatures in the late teens.”
He said an area of “tropical maritime air” moving in from the south west will hang over the UK over the coming days.
Another forecaster earlier told PA an “odd rumble of thunder may be heard in isolated areas”.