Memphis Tennessee vs. St. Louis Missouri weather, why such a large difference?

I find it amazing that just 220 miles over non mountanious terrain, just futher north could play such a hug difference in temperature readings. Are these two cities in between an area where cold weather goes to die? St. Louis has an average January temperature only two degrees warmer than Pittsburgh! Memphis has a January average that is just one degree cooler than Dallas! We are only talking 220 miles between these cities of Memphis and Saint Louis. I live in Corpus Christi Texas, and a good example of 220 miles north of here is Austin, Texas. Austin has a similiar climate to Corpus Christi, although it is a few degrees colder, there really is not much difference between the two cities climate - both are hot and humid.