Paul Hennessy/Anadolu Agency via Getty Images(NEW YORK) -- A massive storm has blanketed much of the U.S. with snow, slowing air travel during a peak season and causing tens of thousands of flight cancellations. The holiday meltdown at Southwest Airlines, however, has far eclipsed its competitors. By Wednesday afternoon, the company had canceled more than 2,500 flights planned for the day, which amounted to 62% of the day's total, according to the flight tracking website FlightAware. By contrast, as of Wednesday afternoon, United Airlines had canceled just 11 flights scheduled for the day...