Forecasters are increasingly certain that the largest snowstorm in two years is headed for Massachusetts on Friday, with the only real question remaining just how much snow the state will see, National Weather Service meteorologist Charlie Foley said today. As of now, the storm is expected to slam the state in true nor’easter fashion beginning Friday morning and wrapping up by 1 p.m. Saturday. The powerful storm will leave 10 to 14 inches of snow across most of the state, including Boston, the weather service said.