Former hurricane Gert will arrive laden with warm, humid air, pushing the mercury into the 80s across the South from tomorrow.
Tuesday is shaping up to be the hottest day of August, with temperatures tipped to hit 82F (28C).
But Scotland and the north of England face cooler conditions with almost a month’s rainfall expected.
Rain arrived in the South-west on Sunday with showers forecast to sweep north through the early hours of today.
Gert will continue to dominate the weather until Thursday when Britain can look forward to a drier, calmer weekend.
The North and North-west face the heaviest downpours as temperatures start to climb further south, according to the Met Office.
Forecaster Craig Snell said: “Gert has been absorbed by another area of low pressure in the Atlantic. It started to affect the South-west on Sunday.
“This system will move up the country bringing a nationwide split from the start of the week.
“Rain is forecast in Scotland and the North while the South turns warm and humid.
“We expect the heat to peak on Tuesday with temperatures of around 77F or 79F in the South and the chance of 81F or 82F where there are breaks in the cloud. Thundery downpours are forecast through Wednesday with temperatures only dropping slightly in the South.
“Much of the region will escape any downpours over the coming days although it will feel warm and muggy.
“Showers could turn heavy and thundery in the North on Wednesday with the South largely staying dry for the week.”
Gert was originally forecast to pass over the North Atlantic missing Britain entirely but has since veered southwards.
AccuWeather forecaster Jordan Root said up to two inches of rain could hit parts of northern Britain.
He warned regions in the firing line to prepare for flash flooding and travel disruption over the next couple of days.
He said: “Gert will continue to lose tropical characteristics as it travels over the colder waters of the North Atlantic and merges with another area of low pressure.
“The combination of these two storm systems will yield a soaking rainfall, but a limited wind threat.
“The biggest concern will be bouts of rain that will spread to Scotland and northern England on Monday and Tuesday. Motorists will need to use caution and be on the lookout for standing water on roadways.”
Exacta Weather forecaster James Madden said: “High pressure will begin to build in across the country to bring a late taste of summer.
“The switch to warmer weather is likely to continue into the start of autumn although there could be some cooler more unsettled conditions at times.”