If any airline could change the competitive environment out of Charlotte Douglas International Airport, it’s Southwest Airlines. For a city its size, Southwest has a very meager presence at CLT, just six flights a day, seven in summer, to four destinations: Baltimore and Chicago Midway each twice a day, Houston Hobby once a day most of the year going to twice a day in the summer, and Dallas Love Field once a day. The good news is that Southwest will announce Charlotte-Nashville service tomorrow.