For the third year in a row, the United States has set a record for winter warmth, federal scientists reported yesterday. With an average temperature of 38.4 degrees Fahrenheit, the three-month period of December 1999 through February 2000 was the warmest winter season in the last 105 years in the contiguous 48 states, the scientists said. That mark slightly surpassed the previous record of 37.8 degrees, set a year ago.Collection: Winter