The UK could see snow in the coming days, as the temperatures plummet to -1C in a sudden “cold snap”.
The Met Office predicts a sharp decline in heat as mid-October approaches, signalling the end of an “Indian Summer”.
Heavy rain is expected across parts of the UK before the weather turns much cooler by the weekend.
England and Wales could experience a wet Friday, whilst up north, there will be a combination of showers and sunshine.
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The rain will increase as the week passes by, both in heaviness and frequency.
Met Office Deputy Chief Meteorologist, Brent Walker, said: "As we head through second half of this week cold air will push southwards across the country and there is a risk that showers over mountains of Scotland could turn wintry. By the weekend we expect all regions of the UK to be in the cold airmass and overnight frosts are possible.
"With high pressure continuing to dominate our weather early next week, it will start largely fine, settled, and cool by day, with cold nights and a risk of rural air frosts in places. Any early morning mist or fog should clear quickly and there could be a few showers possible around some coasts at times."
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Met Office meteorologist Clare Nasir said: "A wet evening and a wet night into Thursday morning. But clearer skies further north that will allow temperatures to dip to single figures and close to freezing across rural spots.
"So Northern Ireland could see lowest temperature around minus one degree Celsius. The glens of Scotland perhaps down to around zero."
However, Nasir added central southern England could cling onto double figures.
The national weather agency has also issued a yellow weather warning for rain from 6am today until 6pm this evening, stating: "Heavy rain is likely to cause some disruption to travel."
Temperatures will drop in a sudden "cold snap"
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The Met Office also said: "The heaviest rain is expected to be across Wales on Wednesday, with some central and southern parts of England and Wales potentially having a very wet day on Friday.
The drastic change in temperature could come as a relief to many, as temperatures have been unseasonably high this autumn.
Temperatures soared over the weekend to 26C – the hottest temperature in five years recorded in October.
Autumn has so far been "unseasonably warm", as the average temperature for October is normally 16C.