My first Fall, 4 hours down I-55, in Blytheville, when I was in the USAF. That was THE hottest place in the Summer! I remember in late September, we had a day with temps in the upper 80s and it was humid. I woke up one morning after a front had gone through overnight and it was just like you said.....It was about 55 but the contrast made it feel MUCH cooler. After that?? It was on! It got cooler and cooler till January......IT WAS FREEEEEEZING!!!