Bitter cold and snow are sweeping nearly half the country on Friday, with 18 million Americans under cold-weather alerts as they wake up ahead of the holiday weekend. Ten million more people are under winter weather advisories across parts of the northern plains and upper Midwest, as well as the Great Lakes, into the Appalachians, and upstate New York, heading into the weekend. Cities under winter alerts include Fargo, North Dakota; Sioux Falls, South Dakota; Des Moines, Iowa; Green Bay,