Update 11:30 EST: Schedules on Southwest just opened up to 8-May-2009 (MSP hasn’t shown up quite yet, should shortly).

Late Wednesday evening Southwest Airlines filed new airfare between 56 destinations and Minneapolis (hub city of Northwest Airlines — newly merged with Delta).

Southwest had previously announced flights between Minneapolis and Chicago Midway airport to begin in early March with connections to the rest of Southwest route network.

The newly filed airfares have a travel start date of 8-March-2009.

We are likely to be in for some serious fireworks on prices out of Minneapolis in the coming days for Spring travel — the new Southwest non-stop route between Minneapolis and Chicago (Midway) has a rock bottom price of $69 one-way (21 days advance purchase required).

Currently the cheapest one-way airfare on Northwest, American and United (MSP-CHI) is is $426 one-way (they only file cheaper airfares currently requiring a roundtrip).

The cheapest roundtrip airfare on these legacy airlines MSP-CHI (not including the holiday only specials recently filed) is $376 roundtrip (or $188 one-way equivalent).

The legacy airlines will quickly have to restructure their airfares, by offering one-way fares instead of the 2 night minimum stay roundtrip fares they currently file — likely matching Southwest’s new price points.

Northwest in the past has hit upstart hub competition with slashed prices as well as putting on new flights with larger single aisle aircraft like the Boeing 757 — they have already signaled they are ready for the battle.

As of this morning Southwest only allows booking through March 6, 2009 — we are likely to see the schedules extended shortly by Southwest to allow bookings on these new routes and prices.

I’ll be posting a comprehensive airfare rundown later today.