The graphic above is the upper air sounding from the Shelby County Airport on the morning of December 24, Christmas Eve. The red line is the temperature trace and the green line is the dew point trace. Where the lines are close together the atmosphere is moist and where the lines are far apart, the air is dry.
The interesting thing I found this morning is the fact that the air temperature increases with height all the way up to the 800 millibar level, or about 6000 feet. There is a very thick warm layer. Now evaporative cooling which occurs when rain from above falls into the drier air below will help to cool off the temperature in that warm layer. The air temperature does not drop below zero degrees until 650 millibars, about 9000 feet or so.
With the surface low forecast to track across Alabama from south to north, it will help to draw in colder air. This still means we face the same dilemma that the south often does when it comes to winter weather – will the cold air arrive before the moisture departs? It’s still a wait and see situation, but the computer guidance that we rely on suggests the moisture will linger enough for flurries across the northern third or so of Alabama – roughly north of the Birmingham area.
By the way, I may not have mentioned it, but I like snow. Sometimes it’s hard to forecast what you think it will do as opposed to what you would like it to do. Merry Christmas.
-Brian-
