More heavy rain hit parts of the UK on Monday as the recent heatwave and hottest July in seven years seemed a distant memory.
However forecasters said the hot weather was expected to return later this week, with temperatures up to 30C in the South East on Thursday.
Storms hit the South West this morning and were due to move north during the day.
Paul Knightley, a senior forecaster with MeteoGroup, the weather division of the Press Association, said: "There have been some nasty thunderstorms in the west country, around Bath and Bristol, and they are due to move on to the Midlands, North West, Northern Ireland and parts of Scotland."
A yellow warning of severe weather for rain in Northern Ireland was issued by the Met Office.
It said that heavy, slow moving and thundery showers were expected to develop this afternoon, bringing the risk of some torrential downpours, particularly across northern areas.
It warned the public to be aware of the potential for localised flooding, with standing water and spray on roads also possible.
The Environment Agency had two flood alerts in place in England and Wales, where people should be prepared for possible flooding, on a stretch of the River Trent in Derbyshire, and of the Lower River Eden in Cumbria.
Knightley said: "Some areas will see heavy rain tomorrow, with temperatures a shade below what you would expect, but they will begin to rise on Wednesday, when it may reach 23 to 24C in the South East.
"We can expect Thursday to be very warm or hot, with temperatures up to 30C in the South East, 28 to 29 across the Midlands, and 25 to 27 in the north.
"Friday should still be very warm, and there will be a growing risk of showers and thunderstorms at the weekend."