Parts of Britain have been warned to expect an ominous mix of thunderstorms, torrential rain, lightning and large hail – posing a potential “risk to life”.
The Met Office has issued an amber rain warning for London and the South East, South West, Wales and the West Midlands from 4pm on Thursday until 6am on Friday.
Early afternoon thunderstorms are expected to become slow-moving and merge together to produce “larger and more persistent areas of thunderstorms”, it says.
This will lead to torrential rain with 30-40mm of rain possible in an hour, with some places seeing as much as 60-80mm in two to three hours.
There is also a possibility of “frequent lightning and large hail”, the Met Office warned.
“Flooding of homes and businesses is likely and could happen quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds,” it said.
“Where flooding or lightning strikes occur, delays and some cancellations to train, bus and air services are likely.”
It warned of “spray and sudden flooding probably leading to difficult driving conditions and some road closures and some communities likely to become cut off if roads flood”.
“Power cuts (are) likely to occur and other services to some homes and businesses could be lost,” it added.
The Met Office also issued yellow rain warnings across Britain on Friday, with Scotland continuing to be affected on Saturday.
Yellow warnings are mostly issued when it is likely that weather will cause low-level impact, such as disruption to travel, or when the certainty of severe impact being caused is lower.
Amber warnings are due to an increased likelihood of impact from severe weather, including travel delays, road and rail closures, power cuts and “the potential risk to life and property”.
The Environment Agency has urged people to check their flood risk, particularly if driving or staying in unfamiliar locations over the half-term break.
It said communities in the South West, Hampshire and the West Midlands are most at risk and warned people not to drive through flood water.
Kate Marks, Environment Agency flood duty manager, said: “Further heavy showers and scattered thunderstorms bring a risk of localised surface water and river flooding on Thursday and Friday.
“The main risk is for the western counties of England, with urban areas most likely to see the impacts of any flash flooding.”
She said that teams from the agency would work “24/7” to operate flood defences, clear blockages in rivers and streams and supporting partners at any incidents of surface water flooding.
“With heavy rain forecast during rush hour, drivers should stay up to date with the latest weather forecast and travel information before making their journey,” she said.
“We remind people not to drive through flood water as just 30cm can move your car.”
Richard Hancox, duty tactical manager for Natural Resources Wales, asked people to “be alert” for heavy rain in South Wales.
“Summer storms are hard to predict so we urge people to listen to the latest weather reports for any potential flooding or disruption where they live or work,” he said.
“If you do see any flooding on the roads please do not risk driving or walking through it.
“Driving conditions could be quite dangerous so please take extra care and allow more time for your journey.”