CONCORD, N.H. — Falling temperatures replaced the weekend’s falling snow Monday as bitter cold and gusty winds swept across the eastern United States. The National Weather Service had forecast that temperatures would be more than 20 degrees below normal across the Northeast, with wind gusts up to 30 mph and wind chills approaching minus 40 degrees in northern New York and Vermont. Atop the Northeast’s highest mountain, the temperature fell to minus 23 degrees Monday morning and dropped to minus 31 later in the afternoon, according to the Facebook page for Mount Washington Observatory, in New Hampshire.