Scorching temperatures forecast for today and tomorrow - with Saturday potentially matching the hottest day of the year so far - could be scuppered by heavy thundery showers, the Met Office has warned.
While the dramatic drop of around 11C into Sunday is the biggest dip in the UK for 16 years.
Parts of the country are set to hit 25C this afternoon, hotter than top tourist destinations including Monaco and Corfu.
And Saturday could just shade it by reaching 26C and matching the highs felt on April 10 in Cornwall - the hottest day of 2020, Met Office forecaster Greg Dewhurst told the Mirror Online.
However, stormy showers across East Anglia, southern England, south Midlands and Wales over both days could hamper the mood slightly.
Regardless of the heat, many beaches and open spaces are likely to be largely empty as people stay at home due to the coronavirus lockdown.
The south-east will be the warmest area today and could see 25C in London while Scotland will reach highs of 17C.
Northern Ireland will see the mercury rising to 18C, and northern England and Wales could each see peaks of 21C.
Met Office forecaster Oli Claydo said: "It will be a bright and warm day for most on Friday.
“There will be some early morning fog in the Midlands and west of England, and scattered showers in Scotland, but that should clear by mid-morning.
"By the afternoon it is likely to be clear and pleasant for all, and could be pretty hot for most.
“But this will start to change on Saturday as a cold front will move from Scotland, bringing scattered showers in parts."
The temperature could drop as much as 11C by Sunday as Arctic air sweeps in from the north of the UK to bring rain and colder temperatures, with possible wintry showers in northern Scotland.
Mr Claydon added: "There is expected to be a drastic drop in temperature on Sunday, falling as low as 15C in London, and there could even be a chance of snow in hilly areas of Scotland, but it is unlikely to set."
Referring to dip, Mr Dewhurst added: "It's not too often we see changes in temperature like that. Probably every 15 or 20 years or so. We get more subtle changes in the UK, usually."
He then confirmed the last time such a day-to-day drop was seen in Britain was in July 1994.
"It's generally because the UK is not a continent like America or Europe - which see these drastic changes fairly often. We are an island so it's less common," he said.
"But it's a big drop. Everyone will feel the cold," he added.
UK 5 day weather forecast
Today:
Early showers over southwestern UK fading leaving sunny spells. Northern Scotland chilly and cloudy with rain at times. Southern UK fine, warm and dry. Elsewhere, warm sunny spells and scattered afternoon showers, some heavy and thundery.
Tonight:
Northern Scotland remaining cloudy with rain at times. Elsewhere, clear spells with daytime showers fading this evening and one or two mist or fog patches about by dawn.
Saturday:
Northern Scotland chilly, cloudy and windy with some rain. Southern UK fine, warm and dry. Elsewhere, warm sunny spells and scattered afternoon showers, some heavy and thundery.
Outlook for Sunday to Tuesday:
Warm with sunny spells in south Sunday, few showers and windy. Some rain across central areas moving south. Becoming much colder from the north, with sunshine and showers in north.
