I think it was April 2012 0r 2013 (?) that we had an epic all day sleeter in Greenfield. It was a late season SWFE that had little local weather media attention but was decent impact between RT 2 and VT/NH.
Because you need an ideal combination of cold air drain, moisture and lift which usually only occurs over a relatively small area.
edit: a stalled front is also helpful. It's one of the more elusive weather events.