A cold start, particularly in Scotland and Northern Ireland, will be replaced by noticeably warmer weather over this weekend.
The Met Office said temperatures across the UK will rise to at least the high teens, but showers and strong winds will hit northern and western areas on Sunday.
It comes after heavy rain caused rail disruption across south-east England and Scotland earlier this week.
More than 500 properties in Wales were also without power on Wednesday after stormy weather, the BBC reported.
The autumn equinox will take place in the northern hemisphere on Saturday, which marks the end of summer and the start of autumn.
Craig Snell, forecaster at the Met Office, said: “Most of us will get off to a bright start [on Saturday] – there will be a few showers in the east, but they should generally move away as the day goes on.
“It is going to be a bit of a chilly start out there, especially across Scotland and Northern Ireland.
“We could see a touch of frost here, so could potentially get below freezing in the more sheltered spots.”
Mr Snell said other areas would also wake up to mid-single figures, but plenty of sunshine would remain for most of the country during the course of the day.
A band of wind and rain will then spread across Northern Ireland, Scotland and western parts of Wales and England through Saturday night, leading to “a very different day” on Sunday.
Heavy rain and strong winds are forecast across northern and western areas, with a yellow rain warning issued for parts of Scotland.
It should remain fairly dry and bright in the Midlands, south-east England and East Anglia, the forecaster added.
Despite the wet weather in some areas, Sunday will be a fairly mild day nationwide as winds blow in from the South West.
Temperatures in the South East could reach up to 21C, and may climb to 18C in the North.
Mr Snell added: “After a chilly start to the weekend tomorrow, by the end of the weekend it will certainly be feeling much different.”
The south-westerly flow will continue into the first half of next week, with above-average temperatures for this time of year and “no major signals of anything significantly cold coming through”.