A yellow weather warning in place for Sunday was cancelled and replaced with an alert lasting from midnight until midnight on Monday June 24.
The Met Office said there was a “small chance” of flooding, with lightning strikes and strong winds also posing a risk.
The warning reads: “Heavy rain and thunderstorms may lead to flooding and disruption to travel.
“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.
“There is a small chance of fast flowing or deep floodwater causing danger to life.”
Met Office forecasters said where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
“Spray and sudden flooding could lead to difficult driving conditions and some road closures,” the warning reds.
“There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.”
The Met Office has a range of advice on its website for keeping safe during thunderstorms.
Where there’s a risk of lighting, it advises people outside to avoid water and find a low-lying open place that is a safe distance from trees, poles or metal objects.
The Met Office advises motorists to keep their windows wound right up when driving during a thunderstorm and stay inside their car, as the vast majority of cars have a metal roof and frame which will conduct current to the ground.