The UK is bracing itself for summer storms, with some areas set to see half a month’s rainfall in the next 24 hours.
Lightning, hail and gusty winds are forecast for large swathes of the country on Sunday evening, with more coming into force on Monday.
In London, daytime showers are expected to turn heavier on Sunday with temperatures dropping to around 14C.
Moving into Monday, scattered showers are expected across the capital, some of which may be heavy with a risk of thunder.
Yellow weather warnings for rain and thunderstorms will remain in place for large swathes until Monday evening.
This means heavy showers and scattered thunderstorms may cause some localised flooding and transport disruption on Monday.
Dozens of flights were cancelled from London’s Heathrow Airport on Saturday and more than 500 were delayed, according to flight tracking data.
Data from FlightAware reveals that Heathrow recorded 590 delays in total yesterday and 77 cancellations. Today there have been a total of 462 delays and 11 cancellations.
Where flooding or lightning strikes occur, there is a chance of delays or cancellations to train and bus services, Met Office has said.
Eastern parts of Northern Ireland are covered by a 24-hour weather warning, from 6pm on Sunday until 6pm on Monday, where heavy rain is expected to cause some disruption and the possibility of flooding.
Simon Partridge, Met Office forecaster, said: "It looks like Northern Ireland is the place where we could see the most rainfall and certainly the most impactful rainfall.
"They could see 50 to 75mm of rain within 12 to 18 hours."
🌧️ Heavy showers will linger for a time this evening, with the chance of thunder in some areas
— Met Office (@metoffice) July 20, 2025
Rain will then ease in the south, giving way to late sunshine, but heavier downpours will persist across parts of the north pic.twitter.com/U8Tu7lJ70r
The country records an average of 89mm of rain in July, meaning more than half a month's rainfall could hit Northern Ireland in less than a day.
Flooding is also possible, the Met Office said, and parts of England and Scotland may also see 20-40mm of rain in just two hours on Monday.
From 3am until 9pm on Monday, the south and east of England are covered by a yellow thunderstorm warning.
By 11am the same warning will be in place for the Midlands, northern England and Scotland.
Eight flood alerts are in place across England and 15 are in place in Scotland where flooding is possible. The Met Office has warned of a chance of delays and some cancellations to train and bus services.
There is also a small chance of power cuts and a small chance of fast-flowing or deep floodwater causing danger to life.
"Certainly a significant amount rainfall in a couple of hours, which will bring some local issues," Mr Partridge said.
"You will see quite a lot of water on the roads and difficult driving conditions.
"Hopefully nothing more significant than that, it certainly won't be an issue for any of the rivers because they are quite low."
Low pressure and humid, moist air across the country will cause the heavy, thundery showers, but even within the warning areas some places will stay dry, the Met Office said.
"There is a good chance of seeing some lightning and some hail, with some gusty winds as well," Mr Partridge said.
"One bit of good news is it has been very muggy overnight. Tonight will be less muggy across the southern half of the UK."
The rest of the week is set to be changeable, with showers still likely on Tuesday and heavy, thundery showers possible in the south-east of England on Wednesday.
Temperatures this week will hover around average for the time of year, the national weather service said.
