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Weather forecasters are warning of a dramatic break in the current heatwave with heavy rain overnight and early on Tuesday bringing flash floods.
Isolated thunderstorms are expected to hit parts of north east Wales on Monday evening with downpours likely to make driving conditions difficult.
Showers will spread across Wales overnight and the heat will trigger more thunderstorms in the morning.
It is likely to remain humid though temperatures will be slightly lower.
The Met Office has issued a rain warning for England, Wales and southern Scotland.
The yellow alert covering most of Wales, apart from the far west and Pembrokeshire, is issued to give people notice that there may be disruptive storms.
End Quote Behnaz Akhgar, weather presenter BBC Wales
It will still feel warm but not as warm as it has been today"
The first of the downpours are expected to arrive in the next few hours in parts of north east Wales.
The sun-hardened ground is likely to make it easier for rain water to run off into drainage systems more quickly.
People travelling home from work or events like the Royal Welsh Show in Builth Well, Powys, are being advised to be prepared for the change to wet road conditions.
Natural Resources Wales said heavy rain is likely to cause roads to flood and drains, ditches and small streams to overflow.
In a statement, it said: "Those who are camping or caravanning should also keep an eye on the weather conditions and listen to advice from the site owners."
BBC Wales weather presenter Behnaz Akhgar said the first downpours would be replaced overnight by extended showers moving up from the south into the north and east.
"These will intensify as the sun comes up. There is a risk of hail and thunder and some gusty winds.
"The rain will fall heavily in some places, with the risk of localised flash flooding, but a couple of miles down the road there might not be anything.
"It will still feel warm but not as warm as it has been today. It will still be humid, perhaps even more humid than today."
Typically average temperatures in the shade on Tuesday would be 24C (75F) compared to around 28C (82F) on Monday.
The showers should disappear by later afternoon on Tuesday, leading to a settled period once again, she said.
The UK has had its first prolonged heatwave since 2006 - though Saturday was the first day in seven when temperatures did not exceed 30C anywhere in the country.
However, it has not matched a heatwave in the summer of 1976, when temperatures above 32C (89.6) were recorded on 15 consecutive days.