Me, too.
WN looks for markets that are underserved and/or over-priced, but another thing they look at is how will any new city fit into their overall current network. I think right now they seem to be looking for new cities that they can link to MDW and BWI - 2 airports where Southwest still has enough gates to accommodate more growth.
Even with the new flights from MDW to both BOS and LGA, MDW will be down to 209 daily departures by the fall, yet they have something like 29 gates at MDW.
Even with the flights from BWI to both BOS and LGA, BWI will be down to 158 daily departures by the fall. They currently have 26 gates in BWI, which is too many for the number of departures they have now. That's probably why WN is looking to give up 6 of those gates when their lease at BWI expires in June.
ATL, CLT and CVG have all been popular choices in other "where will the next new city be?" threads, so for each of those three cities I looked at the O&D info to BWI & Chicago (including both MDW and ORD). I looked at distance, daily passengers and average fare computed on a per-mile basis based on DOT's Q2 2008 Consumer Airfare Report.
For the sake of simplicity, I've assumed that any new city would have an equal number of flights to both MDW and BWI - like BOS will.
ATL-BWI - 576 miles
1,625 daily pax - avg. fare 29.8 cents/mile
ATL-MDW - 606 miles
3,105 daily pax - avg. fare 30.1 cents/mile
5 round trips in each market would give WN 10 roundtrips with an average stage length of 591 miles.
Both routes have a healthy number of daily travelers, but neither route is particularly overpriced.
CLT-BWI - 361 miles
803 daily pax - avg. fare 44.4 cents/mile
CLT-MDW - 599 miles
1,276 daily pax - avg. fare 32.4 cents/mile
5 round trips in each market would give WN 10 roundtrips with an average stage length of 480 miles.
The CLT-BWI/MDW fares are a bit higher per mile than the same routes out of ATL, but still not terribly overpriced.
CVG-BWI - 430 miles
95 daily pax - avg. fare 79.3 cents/mile
CVG-MDW - 264 miles
430 daily pax - avg. fare $1.19 per mile
5 round trips in each market would give WN 10 roundtrips with an average stage length of 347 miles.
Not nearly as many daily passengers out of CVG to BWI and MDW, but then again, look at the average fares. Plus choosing CVG as the next new city means you can do your 10 daily roundtrips to/from MDW & BWI with fewer aircraft than the same routes out of CLT or ATL due to an overall lower avg. stage length.
So my guesses, in order would be
1. CVG
2. CLT
3. ATL
The only thing I'm not sure about is whether or not MDW can really handle any more flights. As I understand from this December 2008 post on FlyerTalk from a WN ramper at MDW, even though there are plenty of gates for additional flights, the ground infrastructure at MDW is strained already. Not sure if that is still the case, (or if it will be the case come this fall.)
