Temperatures are expected to drop over the weekend as rain and wind brings an end to the June heatwave.
The Met Office said there is a risk of flooding in parts of Scotland and Northern Ireland on Saturday due to “heavy showers and prolonged rain”.
Showers are also forecast for much of England and Wales, with temperatures expected to be between 19C to 21C (69.8F).
Met Office meteorologist Aidan McGivern said “most places” will see rain at some point during the day, but it will become “more persistent” in western Scotland and Northern Ireland.
He said: “Further south there will be some dryer interludes, some sunshine even. It is in the south where the best of the drier weather will be at times.
“For all areas it is going to be breezy and it will feel much cooler, with temperatures 10C or so lower than on Friday afternoon.”
The Met Office has issued a yellow weather warning for rain for most of Scotland and Northern Ireland between 9am and 10pm on Saturday.
It said up to 20mm of rain could fall per hour in the areas covered by the warning, with up to 40mm falling over a “few hours”.
Sunday is forecast to see rain and gusts of up to 50mph hit coastal areas in north Wales and north west England, while remaining “breezy” further inland, Met Office meteorologist Craig Snell said.
Temperatures in the south of England are expected to be between 20C to 21C (69.8F) on Sunday, while the mercury could drop to 14C to 15C (59F) in the north and in Scotland.
The change in weather comes as the UK experienced its hottest day of the year so far on Thursday, with the mercury rising to 33.4C (92.1F) at Heathrow Airport in west London.
While temperatures remained high in London on Friday, with 31.2C (88.16F), recorded at Kew Gardens in the west of the capital, parts of northern England were hit by thunderstorms, rain and hail.
Photographs posted on social media showed people in Leeds and Sheffield holding handfuls of large hail stones after they fell on Friday evening.
Meteorologist Mr Snell told the PA news agency: “Usually in the winter when we have hail it is quite small, but this time of the year, because the heat gives the thunderstorms more energy and helps keep the hail stones up in the clouds for longer, they get to grow more and then fall from the sky.”