The Met Office has issued a thunderstorm warning for much of Scotland, bar the west coast, and large swathes of northern England and the Midlands, including East Anglia.
It is in place until 9pm tonight, and meteorologists warn there could be 30mm of rain in one hour and 50mm over a two to three-hour period.
The average amount of rainfall in July for the UK as a whole is 70mm, and in England it is 55-56mm.
The heavy showers could cause flooding and travel disruption, with warnings of fast-flowing or deep floodwaters which may cause a danger to life.
The yellow level thunderstorm alert warns homes and businesses they may be flooded quickly, with damage to buildings by water or lightning strikes.
Those places could also suffer delays or cancellations to train or bus services.
Forecasters have warned of difficult driving conditions, road closures or power cuts.
The Met Office warns: "There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater and lightning strikes.
"There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost."