A major winter snowstorm expected to dump 6 to 12 inches of snow, with up to 18 inches possible in spots, across much of northern New Jersey on Tuesday is expected to start as early as early as 2 a.m. and make for a treacherous morning commute. Winter storm warnings start as early as 11 p.m., though precipitation will initially begin falling as rain, the National Weather Service said. It will change over to snow in northwestern New Jersey around 2 a.m., the weather service said. Snow will continue to overspread the state in the pre-dawn hours, and by about 6 a.m. there should be a transition t…