The probability of a white Christmas

The National Oceanic and Atmospheric Administration created a map of the Lower 48 States showing the probability of a white Christmas. Remarkably, the southern area where snowfall by the 25th of December historically is least likely, is almost perfectly demarcated by the so-called Missouri Compromise Line, the parallel running at 36°30′ north (and forming the border between North Carolina and Virginia, Tennessee and Kentucky, Missouri and Arkansas, also running close by the border between Oklahoma and Kansas, New Mexico and Colorado, and Arizona and Utah). South of that line, chances of a white Christmas are mostly below 5%, with a few 5-10% patches thrown in. Only the Rocky Mountain range in New Mexico significantly break this pattern. Those Rockies further north are practically the only areas outside of northern Minnesota, Wisconsin, Michigan and Maine with over 90% likelihood of Christmas snow.