A Met Office spokesman warned of a "shock to the system" as the warm weather unusually gives way to freezing temperatures.
The spokesmen said: "It will get much colder as we head towards Friday and the weekend and the wind changes further north, we may see some sleet and wet snow in the northern areas."
"It will be a shock to the system because it has been quite moderate recently and there will be a considerable decrease in temperature."
The spokesman continued: "There will be no snow further south, but there will be a noticeable decrease in temperature, perhaps as low as 8C or 9C, a big drop considering that temperatures were as high as 21C in parts of the south during the end of week.
"Temperatures will be below average, compared to the average at this time.
"Some places will see snow or sleet and others will not."
This morning, Met Office meteorologist Steven Keates also warned of snow during the weekend.
Mr. Keates also told the inhabitants of the Orkney and Shetland Islands to prepare for the "strong" winds of the gale force to crash on the islands today and to sweep the northern tip of Scotland.
Mr. Keates said: "The force of the wind will continue to cross the northern tip of Scotland with the storms probably for a while."
Elsewhere, conditions should be softer, with the "best" sun in Wales, the Midlands and the South.
Mr. Keates said: "Half of Wales, the central region and the southern regions will hardly see a cloud in the sky for much of the day."
Mr. Keates said that temperatures will be around 13 ° C today, much colder than the highs of 21 ° C that many experienced this weekend.
But as we move into the weekend, bitter temperatures could bring snow in the northernmost parts of the United Kingdom.
Mr. Keates said: "You can be aware that towards the end of the week, things are getting a little colder.
"We could see some snow in our northern hills by the time we get to the weekend."
Predictions for extreme cold during the next two weeks could make the British prepare for a nearly unprecedented white Halloween.
Bookmakers are already betting on spooky snowfalls with Coral offering 1-2 of evenings in the winter rains this month and 2-1 on the coldest Halloween ever recorded.
Spokesman John Hill said: "The odds suggest that we will have snowfall before the end of this month, as temperatures will drop by the end of October."
"We have been betting that this Halloween is the coldest in the United Kingdom, where the odds are falling over time after the threat of snow."