This will make flooding more likely and severe as the dry ground will not be able to absorb as much water.
The amber warning covers a stretch of the south coast, London and Cambridge, and is in force from 04:00 BST to 11:00 on Saturday.
Between 20 and 40mm of rain could fall within an hour in this area, the Met Office has warned, which could accumulate to 70-100mm in just a few hours.
It said homes and businesses are likely to be flooded, which will happen “quickly”, while this amount of surface water will make driving difficult and may lead to road closures.
Lightning strikes, hail and strong winds may also cause train and bus cancellations.
A yellow weather warning for thunderstorms is in place for the rest of eastern, central and northern England until 21:00 on Saturday.
Amber warnings indicate there is an increased chance severe weather could affect people’s day-to-day lives, including a potential danger to life. Yellow warnings are less severe.
The last amber warning over London was in January 2024, when Storm Henk hit parts of central England and Wales, according to the Met Office.
Thunderstorms develop when warm and humid air exists below much colder air in the atmosphere. This destabilises the air, allowing clouds to form and produce heavy rain – and storms.
The thunderstorms will develop initially over northern France but they will be allowed to “grow” as they move north over the eastern half of the UK on Saturday.
Residual parts of the storm will linger for the rest of the weekend in some places.
A yellow warning for rain covers eastern Scotland from 16:00 on Saturday until midday on Sunday, while a yellow rain alert comes into effect for Cornwall and much of Devon from midday on Sunday until the early hours of Monday.
Last week’s heatwave brought travel disruption, a number of water-related deaths and hosepipe bans being declared for millions living in Yorkshire, Kent and Sussex.
One might think a heavy dose of rainfall would help reduce these drought conditions, but because the rain will be very heavy in localised areas, it will run off the dry, baked earth rapidly, perhaps overwhelming local sewers and waterways.
A substantial recovery in reservoir and groundwater aquifer levels would require a more sustained spell of wet weather.
Yorkshire’s hosepipe ban is expected to last until winter.
Thunderstorms following a heatwave in the summer of 2022 brought flash flooding to London and the surrounding areas, flooding roads and Tube stations.
The rainfall also caused cancellations and delays at Gatwick Airport.
