SQUAW VALLEY — More than 500 top racers from U.S. Ski and Snowboard Association programs will compete alongside America’s best when Squaw Valley hosts the Nature Valley U.S. Alpine Championships today through Sunday. Athletes will battle for titles in super G, giant slalom, slalom and combined, as the downhill already was contested. Olympic gold medalist and Squaw Valley Ski Team product Julia Mancuso highlights the field along with Ted Ligety and Mikaela Shiffrin, as well as Squaw Valley racers Marco Sullivan and Travis Ganong and Truckee native Stacey Cook. Mancuso has eight Olympic and World Championship medals and a record …