The Met Office has issued a yellow weather warning for heavy rain across parts of Wales and England.
Meannwhile, temperatures could drop below freezing as a cold snap moves in - leading to potential snowfall.
Earlier this week, the UK was enjoying the end of a spell of unseasonably warm weather for October - but forecasters are warning unsettled and colder conditions will see temperatures plunge with the risk of overnight frost and snow.
It comes after the warmest October day in five years was recorded on Sunday, as temperatures hit 25.8C (78.4F) in Kew Gardens, west London - just days after major flooding in Scotland.
The Met Office said the rain warning applies to Wales and much of central and southern England and will be in force from 9pm on Thursday until midnight on Friday.
Around two inches of rain may fall and bring flooding to homes, make road conditions more hazardous and impact public transport, it warned.
Temperatures in English cities will drop to as low as 2C (35.6F) when the colder air moves in at the end of the week, but it could fall to as low as -3C (26.6F) in rural northern areas.
Sky News weather producer Chris England, said:
"Heavy rain moving through England and Wales on Friday will clear by the weekend, with a cold northerly wind setting in.
"That'll bring much colder conditions, with quite extensive overnight frosts in the north and daytime temperatures in mid-single figures.
"There will be increasingly wintry showers in the north as well, with snow on the hills and perhaps to lower levels in the far North East by Saturday evening, but most places will be dry by the end of Sunday."
The Met Office said the cold conditions will be a "shock" for some areas.
Spokesman Grahame Madge said:
"On Wednesday night, there may potentially be some frosts and much colder than average conditions, which might be a shock for northern parts of Britain because we've had this relatively warm air across most of the UK.
"There will be an increasing tendency towards cooler conditions, however in the southern parts of England and Wales we've got the prospect of heavy rain coming in.
"We're saying there is a significant rain warning from 9pm on Thursday through to midnight on Friday, and we can expect 20 to 30mm in some places in a couple of hours.
"Even up to 50, 60 or possibly up to 70mm across the high ground of Wales, these are significant amounts.
"Once that warning expires, we've got another shift in our weather pattern where we're getting a more northerly flow coming in.
"That will introduce much cooler conditions, six degrees below average, which given the fact a few days ago we were talking about conditions six or so degrees above average, will feel like a marked contrast.
"We've got the prospect on Saturday of early frost in some sheltered northern areas, even some snowfall over the Scottish mountains," he added.
(c) Sky News 2023: UK weather: First snow of winter and heavy rain on way as temperatures plunge below f
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