ATLANTA, GEORGIA — A powerful winter storm that dumped heavy snow and glazed roads with ice across much of Texas and Oklahoma lumbered eastward into Southern United States Friday, making for dicey travel and a rare snow day for many students. Arkansas and North Carolina mobilized their National Guards for tasks such as helping stranded motorists, as governors in multiple states declared states of emergency. School was canceled for millions of children from Texas to Georgia and as far east as South Carolina. The storm piled up more than a year’s worth of snowfall on some Southern cities. As much