Thunderstorms this afternoon may lead to disrupted road and rail travel, the Met Office warns.
The Met Office has issued a yellow weather warning for thunderstorms in the South East, East and South West of England, as well as parts of Wales.
“A band of thunderstorms will move quickly eastwards” until early evening, possibly causing “localised disruption”, the Met Office warned.
This follows unseasonably warm weather over the new year.
Saturday marked the warmest New Year’s Day on record, reaching 16.3C (61.3F) in London’s St James’s Park.
The latest Met Office warning suggested “a slight chance” of power cuts in the affected area, and cautioned that “sudden flooding”, as well as damage to trees and rooftops by high winds could lead to dangerous driving conditions.
The broad area facing possible disruption includes London, Oxford, Portsmouth and Bath. Heavy rain is already affecting a wide area of Wales, with yellow warnings in place for Cardiff, Monmouthshire and Newport.
“Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services,” the Met Office warned.
On Friday, Britain saw its hottest-ever New Year’s Eve with temperatures reaching 15.8C (60.4F) in Merryfield, Somerset.
The Met Office said the extremely mild spell was driven by a flow of warm, moist air from the Canary Islands.
But manmade climate change is also a factor: warm weather records are being broken nine times more frequently, according to the Met Office – a clear sign of an over-heating planet, according to BBC environment analyst Roger Harrabin.
Globally, climate change has driven temperatures up by 1.1C. In recent days, the US reported dramatic winter wildfires in Colorado – halted by sudden, heavy snowfall – while the state of Alaska recorded its hottest-ever December day at the end of last month.
The Met Office said the weather across the UK was forecast to get cooler from Tuesday, potentially bringing wintry showers and frosts.
BBC Weather’s Tomasz Schafernaker said, following the “extraordinarily mild weather”, it is set to turn colder and temperatures would be “closer to the norm for the time of year” – with temperatures expected to be below freezing in Scotland and Northern Ireland by early Tuesday.
Most of the UK would continue to have “above average” temperatures on Sunday and Monday, he said, warning of low pressure and an approaching weather front bringing “a little bit of sleet and snow” to some northern parts of the UK by Monday.
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