The 4,500 Jewish teenagers from all over the world — from Australia to Slovakia to Singapore to Brazil to France to Los Angeles — who had come to Brooklyn for a weekend of inspiration and learning through the Chabad movement, intended to return home on Sunday. A blizzard that paralyzed the city, shutting airports and leading officials to ban cars from the streets, required a sudden change in plans. Instead, young people who had never seen snow before had friendly snowball fights outside 770 Eastern Parkway, world headquarters of the Chabad-Lubavitch movement, and competed in teams to see who could...