Based on the climate data from 1970-2000, this graphic from the National Climatic Data Center shows the odds of having a "White Christmas" (they define it as having at least 1 inch of snow on the ground). For most of the Tennessee Valley, the probability is less than 5%, though it jumps to 5-10% in parts of Franklin and Lincoln Counties in Tennessee. Click the graphic to enlarge.

hmm 5 to 10%...