The Met Office has issued a yellow warning for thunderstorms for much of the south east.
The weather warning came into effect at 09:40 on Monday, and will remain in place until 23:00.
The Met Office has warned that "heavy showers, torrential downpours, and thunderstorms may cause travel disruption and some flooding."
What to expect
There is also a "small chance" that homes and businesses could be "flooded quickly", with damage to some buildings from floodwater, lightning strikes, hail, or strong winds possible.
Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services, the Met Office said.
Spray and sudden flooding could lead to difficult driving conditions and some road closures.
There is also a slight chance that power cuts could occur and other services to some homes and businesses could be lost.