Bitter gales from Siberia and the Arctic region will make temperatures feel close to -11C (12F) in the North and -6C (21F) elsewhere.
Forecasters warned to expect a dramatic change from the recent mild, wet and windy conditions with the cold set to last for weeks.
The big freeze is due to arrive after the weekend after more rain has hit parts of the UK already battling with severe floods.
The Met Office has issued severe weather warnings for heavy rain in the South-west tomorrow with more than an inch likely to fall in parts.
Forecaster Dan Williams said: “It is getting calmer though there is some more rain about on Thursday, some quite heavy in Wales.
“This will spread northwards and there is still some wind about, around coastal regions these could reach gale-force.
“We are then going to move away from the milder conditions and it is going to feel colder with an increased risk of frosts, for and wintry showers.”
The Environment Agency tonight still had 181 flood alerts and 103 more serious flood warnings in place with many rivers at dangerously high levels.
It said levels along the Avon in Wiltshire, Hampshire and Dorset, the Stour in Dorset, the Parrett in Somerset and the Severn are still rising.
Communities along the River Thames in Surrey, Berkshire and Oxfordshire are also at risk of flooding over the next few days.
Paul Mustow, flood risk manager at the Environment Agency said: “Teams continue to work to work around the clock maintaining flood defences, clearing watercourses and deploying pumps and temporary defences to protect communities at risk.
“The risk of flooding continues this week, with communities in the south west and south east urged to stay safe and sign up to free flood warnings.”
Jonathan Powell, forecaster for Vantage Weather Services, said freezing air from Siberia and North-eastern Europe is due to flood in after the weekend.
He said a stubborn area of high pressure threatens to wedge itself over the UK leading to weeks of freezing winds, snow and ice.
He said: “Cold air from the east is going to sweep in meeting the milder air from the west currently keeping temperatures above average.
“Depending on how this battle plays out we are likely to see a dramatic change in the weather with temperatures plunging bringing widespread snow, ice and frost.
“The entire country could see wintry showers, with central and eastern parts most at risk, and this could shape up to be a very severe winter snap.
“Depending on the stubborn nature of the high pressure the entire country could shiver for weeks in a severe bitter winter blast.”
James Madden, forecaster for Exacta Weather, said rain over the weekend is likely to turn to snow with the North braced for a “significant” deluge.
He warned the extreme cold and snow threatens to hold out into next month with much of the country in for weeks of winter misery.
He said: “Rain is also likely to turn to snow in some places throughout Sunday and into Monday, especially across some parts of Scotland, Northern England, and some parts to the west of the country and Wales.
“Some significant snow is likely across parts of Scotland, with the potential for some moderate falls of snow in parts of Northern England, even some parts of the far south may be at risk of seeing some non-significant wintry showers.
“The remainder of January is likely to see a dominating theme of rather cold conditions that are also likely to persist into February.
“The cold conditions will also be accompanied by widespread snow showers and some lasting accumulations of snow across the country.”
Netweather warned of a chance of snow even as far south as London by the end of next week with the North facing showers from the weekend.
Forecaster Paul Michaelwaite said: “Looking ahead into next week, the forecasting models have started to show colder air from the east making a move toward the British Isles.”
The Weather Channel’s Leon Brown warned of more wet weather over the next few days before the “first spell of winter” arrives.
He said up to four inches of snow could fall over high ground of the North with around two expected elsewhere.
He said: “Another band of rain will try to push east late Sunday to Monday, but this time will become blocked by high pressure building over Scandinavia.
“There is a growing risk that the rain may turn to sleet and wet snow from the Midlands northwards on Monday, especially above 150 metres, but perhaps even at lower levels.
“Next week then looks colder with the first real spell of winter with winds from the east.
“Expect night frosts and wintry showers in the east as high pressure builds over Scandinavia and the North Sea.”
Communities face a massive clean-up after weeks of heavy rain and gales have flooded homes, and left swathes of the country under inches of water.
The death toll from the storms has hit seven while more than 1,700 homes and businesses have flooded since the beginning of Christmas.
High winds over Christmas left 250,000 homes without power with some families waiting days for electricity to be restored.
Environment Secretary Owen Paterson this week said the Government was working with councils and the insurance industry as the clean-up begins.
Prime Minister David Cameron said some energy companies did not have enough staff working over Christmas to deal effectively with power failures.
Speaking during Prime Minister’s Questions yesterday he said: “No matter how good the preparation there are always lessons to learn and I think there are lessons to learn on this occasion.
“On the positive side, the Environment Agency warning service worked better than it has in the past, the flood defences did protect up to a million homes over the December and Christmas period.
“But there are some negatives there and we need to learn lessons - particularly some of the energy companies did not have enough people over the holiday period for emergency response.”
