A yellow weather warning for thunder has been issued across southern England by the Met Office.
The warning, which includes West Sussex and Surrey, begins at 12:00BST on Friday and finishes at 20:00, before another takes its place from 01:00 to 23:59 on Saturday.
While the Met Office says many places will remain dry, thunderstorms and heavy showers could cause some disruption in other places.
The warning also covers Hampshire, Dorset, Devon, Cornwall and the West Midlands.
Where thundery showers do fall, they will bring frequent lightning, gusty winds and potentially some large hail, as well as brief spells of heavy rain, the forecaster said.
It has warned that 15-25 mm of rain fall may fall in an hour, with a very small chance of 30-40 mm falling in three hours.
Driving conditions may be affected by spray, standing water and/or hail, leading to longer journey times by car and bus and delays to train services are possible.
People in areas prone to flooding are being advised to prepare an emergency flood kit.
The Met Office warnings people not to not shelter under or near trees, or other structures which may be struck by lightning
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