The Met Office has issued a series of yellow weather warnings around the UK as heavy rain is set to bring the current warm sunny weather to an abrupt halt.
Most of the country is facing weather warnings today and tomorrow, with some areas set to experience thunderstorms while others will be hit by heavy downpours.
A warning for thunderstorms has been issued across parts of Northern Ireland from 12pm until 9pm today, with the Met Office saying that scattered downpours may cause disruption and flooding.
Another yellow warning for thunderstorms covers all of southern and central England as well as Wales - and remains in place from 12pm until midnight on Sunday.
Forecasters have also issued a yellow warning for rain across northern England and southern Scotland from 7pm until midnight on Sunday as a large area of heavy rain will move northwards.
The Met Office said: "Whilst some thunder is possible, heavy rain is likely to be the main hazard with 15-25 mm of rain likely widely in 3 hours, and with 40-60 mm possible in places.
"One or two locations could see 80 mm or more, although this is a very small likelihood in any given place. Some localised flooding and disruption is possible as a result."
A separate yellow warning for rain has been issued across northern England and Scotland from midnight until 12pm on Monday.
It reads: "Whilst isolated thunder is possible at first, heavy rain is likely to be the main hazard with 15-25 mm falling widely in 3 hours and 40-60 mm possible in a few places.
"One or two locations could see 80 mm or more but this is a very low likelihood for any given spot. Some flooding and disruption is possible as a result."
The storms will follow this week's humid heatwave, which has caused some to experience heavy hay fever and even worsened asthma attacks.
Grahame Madge, a spokesperson for the Met Office, said: "What we're seeing is a change of air mass from last week.
"We've now got much more moist air, that's coming up from warmer latitudes in the Atlantic so it's quite humid.
"Heat and humidity are the key ingredients for thunderstorms. There's a warning out for Northern Ireland today, but tomorrow that extends to essentially the whole of England and Wales, apart from some areas around the Lake District."
He added: "Some of the storms have the potential to be quite intense, with high rainfall rates.
"Some locations may well miss them altogether but all we can do now is indicate that there is a risk across both countries."
He added: "We will keep the yellow warning closely monitored and if we need to escalate that for local areas then we will."
Members of the public have taken to social media to complain that their hay fever symptoms are worse than usual this year. Mr Madge put this down to 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.
"Around 70% of hay fever sufferers are allergic to grass pollen, and will likely have symptoms continuing from mid June into July during grass season."
The heat and humidity has also led to a surge in the number of people presenting at hospital with lung problems. More than five million people in the UK have asthma, leaving them more vulnerable to the weather conditions.
"People with asthma who have hay fever can experience much more severe symptoms during thunderstorms," said Emma Rubach, head of health advice at Asthma + Lung UK.
"This is because thunder breaks pollen into smaller particles, which can then be inhaled deep into the lungs, causing wheezing, coughing, a tight chest and shortness of breath.
"If there is also higher pollution in the atmosphere - which tends to be the case when the weather is warm - this is an added threat."
UK 5 day weather forecast
Today:
Sunny spells and scattered showers at first. During the day showers will break out more widely, especially across England, Wales and Northern Ireland, turning very heavy and thundery in places. Feeling warm and humid.
Tonight:
Heavy and thundery rain continues across northern areas, but becoming drier elsewhere with clear spells and patchy low cloud. Another warm and humid night, particularly in eastern England.
Monday:
Heavy rain in Scotland will gradually clearing northwards. Any low cloud further south clearing to leave another day of sunshine and showers, perhaps turning thundery at times. Feeling humid.
Outlook for Tuesday to Thursday:
A mix of very warm sunshine and scattered showers, these perhaps thundery, particularly in the northwest. Becoming drier in the south east, but still warm and often feeling humid.
