The UK will see a north-south split in terms of weather over the weekend – the last before more stringent coronavirus restrictions come into effect.
The Met Office has issued a weather warning for western Scotland where as much as 150mm of rain could fall in hilly areas, while temperatures could go as high as 25C (77F) in southern England and increase further into next week.
Prime Minister Boris Johnson will bring in the so-called "rule of six" from Monday in England meaning groups of more than half a dozen cannot meet socially, while tougher new Covid-19 lockdown measures will be imposed in the West Midlands from Tuesday.
Parts of Scotland will be hit by torrential downpours that could bring almost a month's worth of rain in just one day, with a yellow weather warning in place from 6pm on Saturday to Sunday evening.
But the south of the country will have a largely fine and dry weekend with long spells of autumn sunshine, according to the Met Office.
Met Office forecaster Bonnie Diamond said the "tropical continental air pushing up from a southerly direction in combination with light winds" will see temperatures of up to 25C on Sunday and then 29C (84F) by Monday.
The top temperatures expected on Tuesday could hit 31C (88F) and remain into Wednesday, but there is the possibility of some thunderstorms mixed in.
"It definitely feels like one last blast of summer even though we are, meteorologically speaking, in autumn," she added.
The September heat follows on from an August heatwave which saw temperatures top out at 34C for six days running last month.