A yellow warning for lightning has been issued across southern parts of England for Saturday afternoon and evening, following soaring temperatures on Friday.
The UK recorded its hottest day of the year on Friday for the third day in a row with temperatures reaching 32.7C in Santon Downham in Suffolk.
Temperatures dropped on Saturday and the Met Office warned storms could rumble over the southwest and into southern England on Saturday afternoon until 7pm.
Forecasters are predicting heavy rainfall but fear the rapid pace of the lightning is "more likely to be a hazard," with the most frequent strikes expected in parts of Cornwall and Devon.
Residents have been told to prepare for some short-term loss of power or other utility services and potential damage to buildings.
No weather warnings have so far been issued for Sunday.
As a result of an influx of cooler Atlantic air, the mercury was predicted to drop on Saturday across Scotland, Wales, Northern Ireland and the majority of England, potentially by as much as 15C in some parts.
On Sunday, rain showers are predicted for southern parts and the North while other regions are expected to have sunny spells.
A flood warning for a tidal area of the Thames in London was lifted on Saturday morning.
A flood alert for the Churchill Barriers in the Orkney Islands was also removed on Saturday, by the Scottish Environment Protection Agency.
By mid-afternoon on Saturday, Scotland had no flood alerts or warnings in place.