The Met Office issued a yellow weather warning which is in effect from midday to 9pm.
A spokesman said: “While many places will stay dry, heavy showers and thunderstorms may cause flooding and travel disruption in a few places.
“There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
“Spray and sudden flooding could lead to difficult driving conditions and some road closures.
“There is a small chance that some communities become cut off by flooded roads.
“Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
“There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.”
Following hours of cloudy skies, there is an 80 per cent chance of heavy rain from 4pm and a 60 per cent chance of thunderstorms at 5pm, though this is expected to ease into heavy showers by 6pm. Temperatures will range from 13C to 19C.