High temperatures threaten to trigger torrential downpours across a swathe of England, which could see up to 1.2ins (30mm) of rain fall in an hour, according to forecasters.
The prolonged dry period means the rain will fall onto hard ground, making it less easily absorbed into the soil and increasing the risk of flooding.
As well as the threat of travel disruption as people start their summer getaway there is also the chance of power cuts.
Yellow weather warnings are in place across the East Midlands and East of England, London and the South East, and the South West until the end of the day.
Rain is also forecast across Scotland and Northern Ireland throughout Friday.
Despite the risk of heavy showers, temperatures are set to remain in the high-20s across southern parts and could climb even higher going into the weekend.
Next week, parts of the south will see temperatures climbing to 33C (91F) or more.
Meanwhile, United Utilities, which supplies the North West, is seeking permission to take more water from three lakes in Cumbria to safeguard supplies, applying for drought permits for Ullswater and Windermere and a drought order for Ennerdale.
It is introducing a hosepipe ban from 5 August amid concern over low reservoir levels.
The UK as a whole saw only 19.5% of expected summer rainfall between 1 June and 16 July, according to the Met Office.
As of Wednesday, the UK having just 1.85ins (47mm) of rain so far means it is the driest start to summer in modern records which date back to 1961, followed by 2013 with 2.3in (59mm) of rain.
Sky News’ weather producer Jo Robinson said: “Isolated thunderstorms will develop across East Anglia and the South East on Friday afternoon.
“Torrential rain will bring the risk of localised flooding, especially if falling over urban areas.
“The storms will ease and fade overnight.
“Over the weekend, a few heavy showers are possible in the South, but most places will be fine away from the wet North West.
“After a slight dip in temperatures recently, the heat will return by Sunday and it’ll turn even hotter next week, especially in the South.
“On Monday and Tuesday, parts of the south will see temperatures climbing to 32 or 33C (90 or 91F), potentially even higher later in the week.
“There’ll be some really uncomfortable nights as well.
The heat and humidity may trigger isolated thunderstorms, while more general rain will affect the cooler and fresher North West.”