Households should pack up their valuables and prepare for floods, the Met Office has said, with three million homes at risk of flooding.
In Devon and Cornwall, floods began on Monday afternoon as thunderstorms brought hail and almost three inches of rain in some places, with the weather service warning of danger to life from flash floods.
Forecasters said there would be an “incredible deluge” over the next few days, which will hit ground baked hard in the heatwave and could cause flooding within an hour.
A yellow warning for thunderstorms is also in place across the UK on Tuesday and in the south of England on Wednesday, but meteorologists said it was not clear exactly where the storms would strike.
Roads were flooded in Launceston, Cornwall, and in Devon, where a road was closed after a multi-vehicle crash in torrential rain, with more serious impacts expected throughout Monday evening.
Ian Fergusson, a BBC weatherman, posted on Twitter:
This looks very treacherous now in parts of #Devon. Some torrential rain rates, plus lightning risk. Some stretches of M5 look likely to be awash... amber warning remains in force until 8pm. Latest radar & lightning (via @Netweather ) pic.twitter.com/lXWbchFjD6
— Ian Fergusson (@fergieweather) August 15, 2022
Grahame Madge, a Met Office spokesman, said: “If people know that properties may have flooded before it might be the time just to be ready – have a clear up, put any valuables at a higher level.
“It could be as simple and as fundamental as that, so that if you do have to move quickly you are already halfway prepared. It’s incredibly challenging to identify where you are going to get the most extreme events.”
Environment Agency estimates suggest that more than three million households in England are vulnerable to surface water flooding, with another 300,000 in Wales and Scotland also at risk.
Flooding can unfold “very quickly”, Mr Madge said, adding: “We could get situations changing within a couple of hours or even an hour.”
Anyone taking a journey by car should be “prepared to get stuck” and expect flooded roads and run-off from fields, including soil.
Summer thunderstorms have the potential to develop in individual spots over large areas of the country as the land heats up. Humidity, combined with recent hot weather, means they could appear with little warning, meteorologists said.
The west coast of Scotland has already been hit by heavy rain and flooding. In the south of England, the most severe weather is expected to hit on Wednesday.
Local drainage authorities, the public sector bodies that report to the Government, would usually deplete water levels in rivers in preparation for an expected flood – but the difficulty of predicting where exactly heavy rain will fall, combined with drought conditions across much of the country, means that cannot be done this time.
Robert Caudwell, the chairman of the Association of Drainage Authorities, said: “Our members have been holding as much water in their systems as possible to try and prevent things going dry, for the environment and farmers who want to irrigate.
“So the last thing we want to do is what you would normally do if heavy rain was forecast, which is to build a bit more freeboard in by allowing more water to be taken out of the system, which is very difficult when you’re really unsure which areas will get a thunderstorm.”
The image below shows the changing August weather at West Bay, Dorset:
Householders have been encouraged to use water butts, break up the soil in their gardens and dig trenches around plants to catch the deluge, with water more likely to run straight off hard, dry ground, limiting the amount the plant can absorb and contributing to surface water flooding.
Sadiq Khan, the London Mayor, told those at risk of flooding to prepare a “grab bag” with essentials including important documents, electronics, cash and credit cards and medication.
He said: “We learned a lot from last year in July when there was flash flooding caused by a huge amount of rain – two months worth of rain – in just a couple of hours and people's homes, businesses and public transport were flooded.
“Speaking to the Met Office, the Environment Agency and many others, we are concerned that over the next few days we could see a huge amount of rain in a short period of time which could lead to flash flooding.”
Last year, videos captured Pudding Mill Lane station, in east London, completely underwater.