A yellow weather warning is in place.
The Met Office believes overnight rain will lead into heavier showers and eventually thunderstorms in the afternoon.
Torrential rain is possible, with around 20 mm of rain forecast in less than an hour and 30-40 mm in a couple of hours.
There is also scope for hail and lightning which may cause disruption.
On their website they say:
- There is a good chance driving conditions will be affected by spray, standing water and/or hail, leading to longer journey times by car and bus
- Delays to train services are possible
- Some short term loss of power and other services is possible, with possible damage due to lightning strikes
- Flooding of a few homes and businesses could occur.