The Yellow weather warning is in place from 12.30 today until midnight.
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, leading to temporary surface water flooding in a few areas, together with minor disruption to travel.”
A warning is also in place for Wednesday, with further slow-moving bands of heavy rain likely to continue; again it will be a minorityof places which see the larger downpours, but where it occurs, there is the risk of localised minor flooding.
Upwards of 30 mm could fall in a few places, but conditions are expected to ease during the afternoon.