The scarves, hats and gloves have been temporarily set aside while the unseasonably mild weather causes mercury levels to rise.
According to the Met Office, the hottest December in Northern Ireland was in 1994 when 16C was recorded at Murlough in Co Down.
By 9am on Wednesday, temperatures at Magilligan in Co Londonderry had hit 14.8C, rising to 15.8C by lunchtime — warmer than Paris, Madrid and Bordeaux.
Elsewhere, it was slightly cooler in Castlederg, Co Tyrone, which saw highs of 14.1C and at Murlough where it was 13.2C.
A Met Office spokesman attributed the unusual spell to an area of high pressure bringing settled weather to the east and colder air currently across parts of the USA and Canada which has been strengthening the jet stream.
The mild conditions are expected to remain for a few days but Wednesday looks like having the highest temperatures, the spokesman added.
Among those making the most of the exceptional conditions was John Miskimmon, 34, who was browsing the array of food and gifts at Belfast's bustling Christmas market wearing just a T-shirt and trousers.
He said: "I am loving this weather. It's great to be able to walk about in a T-shirt and not have to worry about a coat.
"I might run back and put my shorts on it's so warm."
At the nearby Victoria Square shopping centre which was decked out with twinkling lights and decorated trees, pensioners Gwen McMillan, 71, and Muriel Graham, 72, from Lisburn in Co Antrim were also glad to have left their coats behind.
Ms McMillan said: "It is a bit strange, but it's very pleasant."
Meanwhile, retired health service worker John Gorman switched his indoor winter training regime for a run through Ormeau Park in south Belfast.
The 67-year-old said: "I normally train indoors but because it is so nice I thought I'd take it outside and go for a run through the park. It's unbelievably mild.
Jogger Siobhan McCormack, 41, said there was a real contrast with the wintry conditions experienced in recent weeks.
She said: "A few weeks ago we were out running with our gloves on, snoods wrapped around our heads, and coats.
"We are really enjoying the change in temperature. Although it is a bit odd."
Cook Brendan McCann, 40, who was walking his dog Marley through Ormeau Park, said: "It was colder in July."
Elsewhere in Ireland, temperatures were also unusually high for the time of year.
At Finner near Ballyshannon in Co Donegal the readings topped 15.3C while at Malin Head, also in Donegal and Dublin's Pheonix Park it was 15C.