DURHAM, N.C. — Southerners shoveled, scraped and plowed their way Thursday out of a snowy deep freeze that caused a standstill across much of a region accustomed to mild winters.At least 15 people died, including a baby in a car that slid off an icy street outside New Orleans, and a 6-year-old boy who sledded onto a roadway in Virginia.Authorities across the South urged drivers to stay off treacherous roads. Louisiana highways remained closed much of the day and New Orleans residents were avoiding showers to restore pressure to a system plagued by frozen pipes. Atlanta was slowly returning to normal after being frozen in its tracks by about an inch of snow.All this raises a familiar question: Why do severe winters seem to catch southerners unprepared?