A powerful and expansive winter storm is forecast to sweep across large portions of the United States from Friday through Monday, bringing a dangerous mix of heavy snow, sleet and freezing rain to millions across the South, Midwest and Northeast. The system, known as Winter Storm Fern, is expected to impact more than 180 million people as it stretches from New Mexico and Texas to parts of New England, creating hazardous travel conditions and raising the risk of widespread power outages.