Thunderstorm warnings are in place for parts of southwest England and Northern Ireland, with some areas at risk of flooding, lightning strikes and hail.
The Met Office says up to two inches of rain could fall over a few hours in Cornwall, Devon, Dorset, Somerset and in western parts of Northern Ireland on Tuesday.
Two yellow weather warnings for thunderstorms are in force until 9pm today.
The warning in Northern Ireland began at 8am, while the one covering much of South West England starts at noon.
Truro, Plymouth, Exeter, Taunton and Dorchester are all affected by the warning in England.
In Northern Ireland, Londonderry/Derry and Armagh are among the areas forecast to be hit by storms.
The Met Office warns "sudden flooding" and lightning strikes could lead to travel disruption with some train and bus services cancelled and road closures.
There is also a chance of power cuts to homes and businesses.
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Tap hereMet Office forecaster Amy Bokotan said: "The downpours could be slow moving, and that's the main trouble - the wind's quite light at the moment so when they do form, they could be quite slow moving and intense."
She added: "Where you see them they could be quite intense and dramatic, but not everywhere's going to see them and some places just down the road could be completely fine and shower and thunderstorm-free."
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It comes ahead of an area of low pressure approaching the UK from the east on Wednesday - which will bring heavy, prolonged rainfall to eastern parts of the country.
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