New York has set a record for the warmest December 13, with some residents heading out in shorts and sandals just 12 days before the usually frigid Christmas holiday.
Midday temperatures in Central Park climbed to 19degC, according to the National Weather Service.
On Saturday (local time), which was also unseasonably warm, thousands of people participated in the city's annual "SantaCon" bar crawl, which sees revellers dress up as various holiday characters, with some Santa Clauses turning out in only a t-shirt and red trousers or even going bare-chested.
The mean temperature for New York on December 13 is 3degC, with the highest previous recorded temperature for the day at 18degC in 1923.
Data records were first kept beginning in 1869.
Temperatures are expected to remain well above seasonal average throughout the week, the National Weather Service said.
A heat record was also set on November 6 when temperatures reached 24degC at LaGuardia Airport and 23degC in Central Park.