Some areas of the country could see as much as 80mm of rain in just hours on Sunday, the Met Office has warned.
A yellow weather warning for thunderstorms almost entirely covers England and Wales between noon and midnight on Sunday.
Lightning, strong winds and hail are all to be expected, with the forecaster saying London through to the Midlands as well as eastern England, east Wales and lower areas of northern England are most likely to experience disruption.
The warning states that while some places may miss the rain, others could see 30mm in an hour or less and a few spots may even see 60 to 80mm within three to six hours.
Some of the storms have the potential to be quite intense, with high rainfall rates— Grahame Madge, Met Office
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 all together 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.”
