Reuters: As one of the warmest U.S. winters moves into the new year, nothing will change the pattern unless the polar vortex unexpectedly invades the country from the Arctic, meteorologists said. With home heating demand running 20 percent below normal, January through March would need to be as cold as early 2014 to make this an average winter, according to federal data. Last year the fast moving current of air encircling the Arctic, called the polar vortex, moved down from Canada, causing the coldest U.S....