The Met Office has issued a yellow thunderstorm warning for a stretch of the south east that includes Brighton and Hove.
Thunder, lightning and heavy rain are forecast from 7am until noon along with the possibility of large hail and strong winds.
The official forecaster said: “Thunderstorms are expected to develop in the English Channel on Wednesday night and move east during Thursday morning.
“While many thunderstorms will remain offshore, some may also develop over south east England.
“Ten millimetres to 15mm of rain is likely to fall across the warning area fairly widely, with isolated accumulations of 25mm to 30mm, mainly over three hours on Thursday morning.
“However, a few places may see these higher accumulations fall in an hour, particularly towards the south coast.
“Frequent lightning, hail up to 3cm in diameter and squally winds are additional hazards.
“There is a good chance driving conditions will be affected by spray, standing water and/or hail, leading to longer journey times by car and bus.”
The Met Office added that there was a chance of some damage to a few buildings and structures from lightning strikes, large hail or strong winds.
Delays to train services were possible, the Met Office said, and some short-term loss of power and other services was possible.
And the forecaster said: “There is a small chance of flooding of a few homes and businesses, leading to some damage to buildings or structures.”
The area affected stretches from the Isle of Wight to the Thames estuary.
It’s not morning any more, but there’s a big, wet thunderstorm over Hove right now (3.15 pm)