Torrential downpours and thunderstorms are sweeping in from the South early today, bringing the risk of flooding as some areas may receive as much as a month's average rainfall over the next 36 hours.
MeteoGroup has said rainfall totals in England and Wales are expected to be between 30 and 50mm today and Friday, with the risk of intense thundery downpours. Average August rainfall in central and eastern England is only about 50mm.
Senior forecaster Gareth Harvey said: "There are some quite active thunderstorms in the English Channel and they will move towards the south coast during the night. There is quite a cluster of storms heading for Hampshire and the Isle of Wight.
"These showers will push north through Thursday, with some really heavy downpours in places. It will reach Scotland, with heavy rain in the east - parts of north-west Scotland might escape most of it."
The Met Office has issued severe weather warnings for Wales and all English regions except the North East today, and Wales and all English regions on Friday.
Warm air moving in from the south today is expected to make it feel very humid across central and southern areas of the UK, with heavy rain.
A spokesman said: "As we have already seen this summer, this type of weather situation has the potential to cause sudden, localised surface water flooding and hazardous travelling conditions."
The weather system is expected to clear away to the east during Saturday, with drier, fresher weather following everywhere, with some sunshine.
The AA warned drivers to be careful.
Paul Watters, head of roads policy, said: "Flash-flooding in the summer can be hit-and-miss but potentially dangerous for anyone caught up in it.
"The AA advises drivers to keep an ear out for local weather warnings and traffic news, and be prepared to encounter flooding.
"Flooding close to where you live may present less of a problem as most drivers know which are the hazardous stretches of road and can avoid them.
"Those on holiday face a more precarious challenge and asking local people where they might run into flooding is a wise move."
The AA warned motorists only to drive through water if they knew it was not too deep - generally this would mean no more than halfway up the wheels.
Also they should not try driving through fast-moving water, such as a ford - the car could easily be swept away.
A spokesman said: "If you are driving and become stuck in flood water, it's generally best to stay in the car and telephone for help rather than try to get out - unless the water is shallow, stationary and you can see the ground beneath the water.
"If you return to find your car standing in flood water it's best to leave it and telephone for help or wait for the flood water to subside, rather than try to get to it and move it - unless the water is shallow, stationary and you can see the ground beneath the water."