Flood warning as 'month's rain to fall in two days'
As much as one month's rainfall could soak the country in just two days, forecasters warned.
The heavy downpours, due to hit on Thursday, could cause flash flooding in England and Wales, MeteoGroup said.
A spokesman said 48mm of rain could fall on Thursday and Friday - the average rainfall for August is around 50mm.
The Met Office said warm air moving in from the south on Thursday is expected to make it feel very humid across central and southern areas of the UK.
Paul Watters, the AA's head of roads policy, warned drivers to be careful and to listen out for local weather reports before making a journey.
"Flash-flooding in the summer can be hit-and-miss but potentially dangerous for anyone caught up in it," he said.
Motorists were told to only to drive through water if they know it is not too deep - generally no more than halfway up the wheels.
And they should not try driving through fast-moving water, such as in a ford as the car could easily be swept away.
But the wet weather is not expected to last into the weekend which has a brighter forecast.