Here is the list of "alternative airports"...
Independence Air Cities and Acceptable Southwest Alternatives
Washington Dulles-IAD (BWI), NY LaGuardia-LGA (ISP), Newark-EWR (ISP or PHL), White Plains-HPN (ISP), Boston-BOS (MHT AND PVD), PBI (FLL AND PBI), Syracuse-SYR (BUF), Rochester-ROC (BUF), Charlotte, NC-CLT (RDU), Chicago O’Hare-ORD (MDW), Greensboro, NC-GSO (RDU)