A yellow weather warning is issued, with parts of the UK set to see up to a month’s rainfall in a few hours.
This video can not be played
To play this video you need to enable JavaScript in your browser.
Thunderstorms, winds and hail are sweeping across the UK and could cause flash flooding, the Met Office has warned.
Warm, humid air this week has caused the storms to develop, it said.
A yellow weather warning for England, Wales, Scotland and Northern Ireland is in place for Sunday and will be extended into Monday in some places.
BBC Weather’s Matt Taylor said some of the worst storms could produce a month’s worth of rain in a few hours.
But the hot weather will continue, with temperatures reaching highs of 29C (84F) in some areas.
There is a chance homes could be flooded quickly as some areas could see up to 80mm of rainfall in an hour on Sunday, the Met Office said, and the storms could also lead to power cuts.
On Sunday, the warning is in places across Wales, most of England and the west of Northern Ireland from noon until late evening.
A yellow weather warning is also in place for rain across northern England and the south of Scotland, with a risk of spray and flooding, which could lead to difficult driving conditions and some road closures.
The Met Office have also issued further yellow warnings for heavy rain and the risk of flooding in the north of England a large part of Scotland for Sunday night and Monday morning.
Heavy rainfall has already hit some parts of England this weekend as 21.5mm fell in Ringley, near Manchester, overnight, as well as 18.6mm in Charlwood, Surrey.
Mr Taylor warned more rain was on the way, as he said: “Some parts of the UK could see over a months worth of rain fall in just a few hours today and tonight, leading to flash flooding and disruption in places.
“Due to the nature of thunderstorms, there could be huge variations in weather conditions over a short distance. Whilst some areas stay dry and humid, others close-by could experience the severe storms with torrential rain, hail and frequent lightning.
“Thundery rain will develop more widely this evening across northern and eastern England, before heading into Scotland.”
However, he said that not everyone will see the storms, with conditions “highly variable over just short distances, and many areas remaining dry”.
Late on Sunday, and into the night, the thundery rain could affect more of northern and eastern England, as well as eastern Scotland, he added.
Image source, Ani Caul/BBC Weatherwatchers
The stormy forecast follows a week of high temperatures, where many parts of the UK officially experienced a heatwave.
The heatwave has caused some to experience heavy hay fever and worsened asthma attacks.
People have complained on social media that their hay fever symptoms are worse than usual this year.
Grahame Madge, a spokesperson for the Met Office, said this was a result of the hot, dry weather over the last few weeks.
“Pollen season is certainly with us,” he said. “The fact we’ve had very dry conditions means that grasses can release pollen on mass into the air column.”
Heatwaves are becoming more likely and more extreme because of human-induced climate change.
The world has already warmed by about 1.2C since the industrial era began, and temperatures are expected to keep rising.
Despite the storms, the heat is set to continue. The highest temperature of the year so far was at Chertsey Water Works in Surrey on Saturday where highs of 32.2C were recorded.
A hosepipe ban was issued in Kent and Sussex on Friday after South East Water said it had no choice after demand for drinking water reached “record levels” in June.
Up to 4,000 customers are without water or have been experiencing low pressure since Monday due to supply issues.