I generally get my weather directly from the National Weather Service website, where in addition to the text forecast I make use of the "Hourly Weather Graph" and radar to decide the mode (bicycle, subway) and timing of my commute. When something big may be going on, I also read the detailed "Forecast Discussion," which includes the thinking behind the forecast, information about models not in agreement, possible changes as events develop, etc. I certainly did so over Christmas weekend, with travel planned for my immediate and extended family. But I couldn't remember when the forecast changed.
So I went through the archived "Forecast Discussion" from the NWS, and found the following. As late as Christmas Eve, the storm was expected to basically miss most of the NY metro, excluding eastern Long Island and Connecticut. On Christmas morning, the possibility of a 6 to 9 inch snowstorm was forecast, and a “Winter Storm Watch” was issued. Someone should have been watching, because Christmas afternoon it was suddenly upgraded to a “Blizzard Warning.” It was still expected to be a powerful but not devastating storm in NYC until just after noon on Sunday, the day the storm hit, when the snow accumulation totals were updated. Bottom line: this storm put out more fakes than Michael Vick, so it is no surprise that a stunned city and MTA shanked a punt to Desean Jackson. The four “Forecast Discussions” follow in full: bold and italics were added by me. If you bore easily, just read those parts.