The morning low was 1° below zero, New York's earliest date for a sub-zero reading. The high rose to 11° in mid-afternoon and then stayed at 10° degrees for the rest of the day.
This was the third morning in a row in which the low was 17° degrees, 14 degrees below average. These were the coldest readings of the month.
With a high of 30°, today was the first day of the season with a high of 32° or colder.
Today's high of 31°/low of 22° (ten degrees colder than average) was the first day completely below freezing since Feb. 10, when it was 29°/22°. This morning's low would end up being the coldest reading in December. It was the "mildest" minimum temperature in December since 1984 when December's coldest reading was 25°.
An Arctic front bringing the coldest weather since the beginning of February triggered a burst of snow at around 4 PM. As the front approached a "Snow Squall Warning" was issued mid-afternoon (although it's unusual for NYC, a similar warning was issued earlier in the year on 1/30). Less than an inch of snow accumulated but it fell in such a brief period of time that it produced whiteout conditions for 10-15 minutes. By midnight the temperature was down to 21°, on its way to 16° by daybreak on the 19th.
Yesterday had extraordinarily dry air in place (humidity in the afternoon bottomed out at 14°), and the difference between the air temperature and dew point late in the afternoon was 48 degrees (air temperature of 57°, dew point of 9°). Today, with a chilly and damp air mass in place, there was just a one degree difference between the two (air temperature of 43°, dew point of 42°)