The mild weather over the festive period is “unusual but not unheard of” the Met office has said.
While Sunday saw temperatures of 11C across Northern Ireland, northern Scotland reached an unusual 16.9C at 3am.
“That is exceptionally mild,” a Met Office forecaster told the News Letter.
Other places which reached the mid-teens included North Wales, North East Scotland and North East England.
“However 16.9C is the highest temperature we have ever recorded this late in the year,” he said.
2016 did see a slightly warmer temperature, at over 17C, he added, but that was in early December.
The forecaster also conceded that British winters have seen significantly warmer readings on at least three other occasions since the middle of the 20th century, specifically, in 1958, 1971 and 2003, but those were all in the early months of the year.
The explanation for the high Scottish temperature this year is in part down to the Foehn Effect, which saw a wind coming from the south west hit higher ground and lose its moisture as it rose.
Coming down the other side of the mountain, the air is thus able to warm up much faster than normal, the forecaster said.
For those who dreamt of a white Christmas, however, there is still the possibility of a festive flurry.
The forecaster said Monday would be clear with some rain later and that Tuesday would be largely dry and sunny.
New Year’s Day, Wednesday, will be mostly dry with quite a lot of cloud, while Thursday will be chillier, with highs of 5–6C and Friday dipping to 3C.
“It will be more unsettled by the end of the week and snow or sleet is not out of the question,” he added.