Just ahead of the holiday season, the State Department has issued an updated travel advisory for Mexico -- especially the northern states of Tamaulipas, Nuevo Leon, and Chihuahua.  The new warning urges American citizens against making non-essential trips to those areas, stating that the highways are still considered unsafe for travel.  As in the previous warning, the updated alert points to the threat of carjackings, and notes that gunbattles between rival cartels, and between cartels and the Mexican military, can break out at any time -- on major highways, as well as in neighborhoods, and in public areas, even during the day.  The latest warning replaces one issued in February.