By Tony Owusu The U.S. Department of Transportation called the situation at Southwest “unacceptable” as the airline canceled thousands more flights on Tuesday. Ask any Southwest Airlines (LUV) – Get Free Report holiday traveler, and they’ll tell you the airline deserved nothing but lumps of coal from Santa for Christmas. Extreme weather plagued holiday travelers across the country last week, delaying and stranding tens of thousands of passengers traveling to see their families and friends. While the weather affected every major carrier in the U.S., Southwest Airlines’ (LUV) – Get Free Report p…