Because last time we had a winter storm watch, we got *so* much snow...
But anyway -- right now, there's 3-5 inches predicted for Tuesday, with snow starting overnight on Monday, according to the National Weather Service. The winter storm watch notes that if the storm track changes, the Providence-Boston corridor is likely to get more rain and less snow.
Additionally, Todd Gutner at WBZ-TV indicates the uncertainty of predicting snowfall totals with this storm, noting the lack of reliable cold air for Tuesday.