A “once in a lifetime” winter storm sweeping through the southern United States on Tuesday dumped snow at levels millions of residents had never seen before. The storm blanketed New Orleans and Houston with record snow that closed highways, grounded nearly all flights and canceled school for more than a million students more accustomed to hurricane dismissals than snow days. About 40 million people, from Texas to Florida, were under some type of weather hazard, including more than 21 million under a winter storm warning, forecasters said. Residents of several coastal counties near the Texas-Louisiana border were under their first