Where the showers are at their heaviest there could be surface water flooding and minor travel disruption.
The Met Office's chief forecaster said: "A slack area of low pressure will move slowly across southern Britain today, bringing unstable air and the potential for thundery showers.
"The light winds mean that the downpours will be slow-moving as well as intense in some regions, with the possibility of 15mm or more falling in less than an hour."