"Completely bizarre", said Gizmodo design and technology blog. "Freakishly warm", said the Slatist news blog. "Surf's up", said ABC News. If you were dreaming of a white X-mas pretty much anywhere on the East Coast, you were sorely disappointed this year. It was NYC's warmest Christmas Eve in recorded history. It was warmer than Los Angeles. The temperature in New York was almost as high as it was on the 4th of July. (Gross. Hope you didn't wear your ugly Christmas sweater this year - you probably sweat right through that thing.) Meteorologists basically had a field day and lovingly nicknamed the anomalously-warm-weather pattern the "blowtorch".
This blowtorch was caused by several phenomena: 1) the northward movement of the jet stream in the eastern US; 2) a high pressure system in the western Atlantic, which caused movement of warm, moist air from the south; 3) a strengthened polar vortex, which tightened the ring of circulating cold air in the Arctic; and 4) the strong El Niño (the episodic, anomalous warming of the eastern equatorial Pacific), which can affect weather patterns not only across the US, but also around the world. Important note: the muggy, warm Christmas was driven by weather, which is not the same thing as climate. Just as you absolutely positively cannot "disprove climate change" on a cold, snowy day, you also can't use a warm winter day as proof of climate change either. That said, anthropogenic climate change will likely impact weather patterns and extreme weather events in the long-term.