Shock maps show hot air hitting Britain from the west leading to highs of up to 26C (79F) on Saturday. The rise in temperatures comes after Hurricane Dorian, which ravaged the Bahamas, joined with Tropical Storm Gabrielle sucks hot air north over the UK.
Experts say the heat will be up to 10 degrees warmer than usual for this part of the year.
Hot air is to cover the length of the UK with temperatures picking up on Thursday in time for the start of the final England and Australia Ashes test at the Oval in south London, where the mercury will hit the mid-20s.
Met Office forecaster Marco Petagna said: “It turns warmer and humid as the remnants of Dorian and Gabrielle feed tropical air to the UK.
“It turns warmer still at the weekend and early next week with high pressure, with 25C possible in the South of the UK.”
The Weather Outlook forecaster Brian Gaze said: “Sunny weather is forecast to see highs up to 21C in Scotland.
“High pressure is expected to bring a southerly feed.”
Netweather’s Jo Farrow said the forecast for the next couple of days were a prelude to the warmer weather hitting later this week.
She said: “We’ve got the remnants of what was Hurricane Dorian and Tropical Storm Gabrielle here. Added into the mix bringing windy weather, particularly in the north western parts and some heavy rain in the north west.
“There’s a bit of extra energy from the tropical systems.
“Hurricane Dorian will strong winds in Ireland and Scotland, along with the west of England as Dorian heads north to Iceland.”
She added: “Tuessday to Wednesday temperatures in southern Britain will head to low 20s. And then there will be a blustery Wednesday with more rain.
“But we’ll see much warmer air and Thursday in south-east we could see temperatrues in mid-20s. Quite a change.”
A Met Office forecaster said: “Rain will move across the whole of the UK, clearing by Wednesday afternoon. Strong winds and blustery showers will continue in the North.
“But Friday and Saturday will be fine for most, apart from further strong winds and blustery showers in the far north.
“Mid-September is likely to be widely settled and dry with light winds and plenty of sunshine, especially in the South.”
Ex-BBC and Met Office forecaster John Hammond of weathertrending said: “Temperatures well into the 20s are quite feasible in the UK after September 16.
“It could well become quite warm. There are indications high pressure will build. And from September 23 to October 6 favours some lengthy and pleasant spells of sunshine.”