Mother Nature Network: More than a month before the official start of winter, residents of parts of the United States are being blanketed with snow and hit with frigid temperatures. But the infamous polar vortex that sent Americans shivering last January isn't the only weather system responsible for this unseasonably cold weather, experts say. The early onset of winter has manifested itself in different ways across the country, dumping more than 2 feet (0.6 meters) of snow in parts of Michigan and sending temperatures...