East coast snow storms are notoriously difficult to predict. I'm not surprised that even with modern technology they still can't get it perfect. In addition to the dynamic nature of the low-pressure circulation interacting with the coast and the gulf stream (like a hurricane) you've got the all-important freezing temperature line. It's even worse than "a line that might shift" though, because if the cold air intrudes under the warm you get freezing rain, not snow.
I grew up in that region (DC area) and it was always like this. I have no envy for those forecasters.
One of my fondest memories is of the 1978 storm. Hit in the afternoon, 2" predicted. 6PM, forecast increased 6-8". Next morning? Most of us had 24", some hit 36". I wonder if modern tech could have done better.
More often than not though, it seemed like DC always got cheated out of snow when I was a kid. Rain, sleet, snow that got rained on and turned into a soggy mess. Beautiful snow that you could play in on your day off was just all too rare.