Met Office meteorologist Craig Snell told the PA news agency that temperatures are set to return to average in January following a mild late December.
Normally the UK sees average daytime temperatures of 7-8C in the South and 3-4C in the North in January, he said, with night-time temperatures of 2-3C in the South and 0C in the North.
"(After New Year's Eve) there is a trend for temperatures to return nearer to normal", he said.
"That's not surprising as temperatures are way above average."
He added that northern England and Scotland could even see frost in the new year.
It comes after parts of Yorkshire and Scotland had a white Christmas after unexpected snowfall.
Mr Snell added: "For this time of year, if we see any clearer slots at night then that does give some risk for frost, particularly across the northern half of the UK."
The UK is experiencing an unusually warm end to December, and forecasters believe New Year's Eve could be the warmest on record, with temperatures set to reach 15C in some parts of the country.
Met Office meteorologist Greg Dewhurst said: "The record is 14.8C on New Year's Eve and that was in 2011. Temperatures look like they'll be 14-15C (57.2-59F) so it is possible that temperatures could be that value."
Mr Dewhurst said the weather throughout the week will be "on the mild side", adding: "We're going to see across the whole of the country, through the rest of this week, temperatures that are above average for this time of year.
"The average temperature in the UK around this time of year should be around 7-8C (44.6-46.4F).
"Going forward, we're looking at highs of around 12-14C (53.6-57.2F), possibly locally 15C in one or two spots, so it's going to be well above average.”