ATLANTA, GEORGIA — Flight cancellations piled up and state officials warned of continuing dangerous roads Saturday in the wake of a winter storm that closed schools and disrupted travel across parts of the southern United States. A storm that brought biting cold and wet snow to the South was moving out to sea off the East Coast on Saturday, leaving behind a forecast for snow showers in the Appalachian Mountains and New England. But temperatures are expected to plunge after sundown Saturday in the South, raising the risk that melting snow will freeze and turn roadways slick with ice. “I