Extreme temperature gradient-- Common in late winter-- especially in La Nina winters.
I've been watching a storm for about a week that is another two part event. Part one is underway-- from MN to NY a big swath of snow with some sleet and ice. Max snow will be over a foot in some places. This is riding along an arctic front that is sliding DUE north to DUE south. A second piece of energy rides along and some of the latest data pushes snow ALL the way down to Charlottesville-- maybe a couple inches, while at the same time NO layer of atmosphere here is even below freezing.
I'm glad I'm not forecasting from DC to PHILLY-- some places in that region have a shot of 6-8 inches of snow and some may get none. The model data is split..but the ECMWF has consistently been south. As a result, I'd bet that much of the DC metro region gets 2-5 inches of snow with someone making a run at 8 inches. (Maybe Leesburg??) This is not set in stone and some fluctuation in the data is likely.