Four flood warnings and 25 alerts are in place across Britain with forecasters predicting up to six inches of rain in 3-4 hours in parts of the country.
Tuesday is set for highs of 37C as the latest scorching heatwave continues, but thunderstorms are moving across all of the UK - with temperatures to plummet later in the week.
The Met Office's countrywide yellow weather warnings remain in place until Friday morning, with a small chance of fast flowing or deep floodwater causing danger to life.
While the Scottish Environment Protection Agency has issued 19 flood alerts, which add to the one in place via Natural Resources Wales and the Environment Agency's five for England.
Added to that, four flood warnings are in place across Lancaster and Ulverston in the north west.
An EA flood report states river levels have risen at the Fitz Bridge and Burrow Beck river gauges as a result of localised thunderstorms yesterday.
Consequently, "flooding of property is possible", it added.
Met Office chief meteorologist Steve Ramsdale said the worst of the thunderstorms are likely to occur in the north west of England, North Wales and the West Midlands.
"Here, exceptional rainfall totals could be seen of 60mm in an hour with a very small chance of 150mm of rainfall in three or four hours," he said.
While EA flood duty manager Caroline Douglass warned: "Isolated flooding is also possible more widely across England today through until Thursday due to further heavy thunderstorms."
According to BBC weather charts, London will see a scorching starting temperature of 21C today and "sunshine will just shoot up again like a rocket", according to Beeb forecaster Tomasz Schafernaker.
"Generally speaking, there is going to be a lot of sunshine across the UK on Tuesday and just hit and miss thunderstorms here and there," he continued.
Wednesday could see temperatures get even hotter before a major plummet of 12C by Friday.
"After Wednesday, that really hot day, Thursday is going to be the day when those temperatures really start to come down across the south east, East Anglia and central parts of England," added Mr Schafernaker.
UK 5 day weather forecast
Today:
An area of heavy rain and thunderstorms moving north across parts of Scotland this morning. Otherwise, sunny spells for most areas, and although a lot of places will remain dry, scattered thunderstorms again developing. Very hot again in the south-east.
Tonight:
A warm night in prospect for most areas, especially southern UK. Some heavy showers and thunderstorms in the north, whilst becoming mostly dry in the south.
Wednesday:
Sunny spells for most areas, and although some places will remain dry, scattered thunderstorms again developing. Very hot in the south.
Outlook for Thursday to Saturday:
Increasingly cloudy across England, Wales and Northern Ireland, warm (south-east UK locally hot Thursday), and humid with areas of rain and thunderstorms developing. Scotland mostly dry, bright, with average temperatures.
