Warnings for rain and thunderstorms have been issued for much of the UK after the hottest day of the year so far over the weekend.
Storms have begun to move in, with heavy downpours and lightning observed across North Wales, the Bristol Channel, Northern Ireland and some parts of the Scottish highlands.
People have been warned to expect some disruption, especially to travel.
The Met Office has also warned of possible flooding and power cuts.
There are three new yellow warnings for thunderstorms in place for Sunday.
One covers most areas of the west of the UK, including the majority of Wales, where thunderstorms are expected between 12:00 BST and 22:00.
The second is for the western half of Northern Ireland between 11:00 and 19:00.
The third is for western parts of Scotland between 14:00 on Sunday to 04:00 on Monday.
The Met Office has also announced warnings for England's south-east and Northern Ireland.
The alerts warn people to expect travel delays and possible flooding.
For England and Wales, the warning is in place throughout Monday, but stretches into Tuesday morning for Northern Ireland.
The clouds rolling in may impact the chances of seeing the Northern Lights, which are expected to make another appearance for a third consecutive night on Sunday.
Saturday was the warmest day of the year so far for all four nations, the Met Office said, but there are expectations that that record will be usurped on Sunday.
Temperatures reached at 25.9C (78.6F) in Herstmonceux, East Sussex on Saturday, with northern Scotland not far behind with readings of of 25.7C in Cassley.
London was expected to top the temperature charts on Sunday, with highs of 27C expected in the capital.
Temperatures will be dropping going into next week, BBC Weather presenter Simon King said.
"For the week ahead, it's going to be much more unsettled than the weather we've had over the last few days. Not quite as warm, with temperatures coming down into the mid to high teens for most of us," he said.
The Met Office forecast highs around 15 to 20C across the country on Monday.
It expects there will be further spells of rain through Tuesday and Wednesday, but with some periods of sunshine in between.
