Torrential rain and winds of up to 90mph are forecast to hit large parts of southern Britain and Ireland in the morning.
A forewarning of the bad weather ahead came earlier when a tornado towered over the coast of south Wales after sweeping up the Bristol Channel.
The powerful funnel of wind - reaching up to 2,000ft high and stretching nearly 30ft wide - was seen near Cardiff and Barry.
It lasted for about 20 minutes and was spotted by many people, including airtraffic controllers at Cardiff airport.
Witness Nigel Evans said the tornado, technically called a waterspout because it formed over water, appeared during a sharp thunderstorm.
"It came up quite quickly," he said.
"The sky was clear and then it was very dark.
There was a flash of lightening and thunder."