Warnings for thunderstorms and heavy showers have been issued by forecasters for Sunday, covering much of England, east Wales and southern Scotland.
The Met Office say there is a “small chance of flooding and disruption” in some areas as a result.
The yellow warning urging people to “be aware” came into force from 04:00 BST, covering large swathes of southern England, the Midlands and the north and expires at 23:59.
Forecasters say thunderstorms and heavy rain will move northwards over the course of the day but not all areas covered by the warning will be affected.
Further heavy showers and thunderstoms are predicted overnight, with another yellow Met Office warning issued from 00:01 BST to 23:59 on Monday.
The Met Office said there is a chance of flooding, adding that “frequent lightning strikes and hail will be additional hazards” in southern and central England.
BBC Weather are forecasting heavy showers and thunderstorms will move northward through parts of south-west England, the Midlands, east Wales and into areas of northern England.
Later on Sunday, some parts of southern Scotland could be affected.
In England, the warning area on Sunday is from Exeter to West Sussex and stretching up the country to the far north in Cumbria and Northumberland.
In Wales, the warning is in place for eastern areas from Bridgend, up the spine of the country to Denbighshire in the north.
Southeastern parts of Scotland, including Edinburgh are also covered in the yellow warning.
The most intense downpours will see frequent lightning and gusty winds.
But pinpointing exact locations of these showers is quite tricky, and it would be worth staying up-to-date with the latest forecast as it may change, BBC weather says.
Away from the heavy showers and thunderstorms there will be some sunny spells.
Feeling very warm and humid for many but especially so in south-east England where temperatures could reach 29C on Sunday afternoon.
Elsewhere, temperatures will be more widely in the low to mid-20s Celsius.
The warning for Monday covers a large area of England, east and north Wales and up into southern and eastern Scotland.
The Met Office states that “while many places will miss them, thunderstorms may cause flooding and disruption”.
It added thunderstorms could bring 60-80mm of rain in an hour in one or two places. Frequent lightning strikes and hail may cause an additional hazard.