The Met Office has issued fresh weather warnings as stormy weather is set to batter Britain with gusts of up to 50mph.
The yellow warnings now cover the north of England, as well as parts of Scotland and Northern Ireland.
Downpours are expected throughout Sunday and into Monday until 3pm, with the rain particularly heavy over high ground.
Up to 120mm of rain could be dumped on Cumbria and Lancashire, while 40 to 50mm of rain is likely to fall quite widely.
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Elsewhere, there are likely to be scattered showers with some sunny spells.
Northern Ireland, northwest England and southwest Scotland should also brace for strong gales, with gusts of 50mph forecast in exposed places.
The Environment Agency has also warned of possible flooding in Nottinghamshire and Loughborough.
The stark contrast with the searing heatwave earlier this week is due to a low pressure system bringing heavy rain, thunderstorms and plummeting temperatures.
It comes after a day of rain and chilly temperatures on Saturday, which saw 24mm of rain in Sennybridge in Wales.
In marked contrast to earlier in the week, the mercury is only expected to reach a high of 22C today in London.
Giving the latest forecast, a Met Office meteorologist said: "Looking ahead into Sunday, it's going to be an unsettled day, there will be heavy rain particularly across the northwest of the UK.
"A chilly start to Sunday morning, outbreaks of heavy rain across the north and the west of the UK, a rainfall warning in force right through Sunday and into Monday morning.
"There's risk of flooding as we go through the day in places, best of the bright skies across southern England and Wales but also a scattering of showers here.
"Windy and cool for everyone, temperatures reaching 20 to 21c at the best so much cooler than it has been.
"Through Sunday evening we continue with the heavy rain across northwestern parts of the UK, difficult driving conditions at times too, drier elsewhere with some clear skies as we go into Monday morning.
"The outlook is for a fairly unsettled week to come, cooler than of late as well and often breezy."
UK 5-day forecast
Today:
Rain across parts of central and northwest Britain will be heavy at times over hills. Elsewhere, sunny spells and scattered showers, perhaps heavy and thundery across far north far Scotland. Windy in south and feeling rather cool.
Tonight:
Rain persisting in parts of central and northwest Britain accompanied by strong winds. Elsewhere, isolated showers and clears spells, but staying windy in south.
Monday:
Wet and windy across central and northern areas again for much of the day, but clearing later. Elsewhere, breezy with bright or sunny spells and feeling rather cool again.
Outlook for Tuesday to Thursday:
Rain or showers for most, though not as heavy or as widespread as preceding days. Drier by for many by Thursday. Cool generally, though warmer at times in the south.
