It has been predicted that Arctic Air will move through the UK this week.
The Met Office also forecast a drop in temperatures towards the end of next week.
Meteorologist Sarah Kent said: "We are seeing temperatures fall well below average at the end of next week after starting much smoother."
"This will be a shock to the system, since some people will have had temperatures in the teens at the beginning of the week, it will be time to take off hats and jumpers.
"During the second half of next week, we are likely to see a winter climate with snow on highlands in Scotland and northern England developing."
Bookmakers predicted that October could be the coldest Halloween recorded in the UK.
John Hill of Coral 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."
What is the weather forecast in the United Kingdom this week?
Temperatures are set to plummet from 21C to 7C, as Arctic Air will arrive in the country on Friday.
The Met Office warned that icy air could cause snowfall in some areas of the north.
Met Office meteorologist Steven Keates said the Orkney Islands and the Shetland Islands will have "strong" force winds that will crash into the islands.
He said: "The force of the wind will continue to move through the extreme north of Scotland with probable winds for a while."
Mr. Keates's additional temperatures will average around 13 ° C in the first half of the week.
However, this will be much colder than the slightly warmer 21C highs that spread across the UK this weekend.
The weekend will see bitter temperatures and potential 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."