Thunderstorms and flooding could hit parts of London and the South East today, forecasters warn.
Power cuts are likely and some communities could be cut off if roads are disrupted by the weather.
The Met Office has issued an amber warning for thunderstorms for 11am on Wednesday, lasting until 10pm tonight.
It covers much of London as well as West Sussex, East Sussex, Kent and Essex.
The Met Office warned the public:
Flooding of homes and businesses is likely and could happen quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds
Fast flowing or deep floodwater is likely, causing danger to life
Where flooding or lightning strikes occur, delays and some cancellations to train and bus services are likely
Spray and sudden flooding may lead to difficult driving conditions and some road closures
Some communities are likely to become cut off if roads flood
Power cuts are likely to occur and other services to some homes and businesses could be lost
It comes as roads in parts of southwest England were flooded on Tuesday on the second day of heavy rain and thunderstorms.
One of the reasons for the concern is that the heavy rain follows weeks of drought conditions - when the ground is too dry, it can't absorb the rain fast enough, so the water runs over the land and floods it.
The Met Office said that Monday marked the eighth consecutive day with the temperature hitting 30C (86F).