Some of this is from an earlier post, while some is new.
James has been talking dynamic cooling and the upper-level low. I figured I’d go a little bit into what these are and how they are causing snow today to our west, and may cause snow across Alabama this evening. The upper-level low is when a wave in the upper-air flow gets cut off, or makes a complete loop. These often move slowly, unlike open waves in the flow that keep moving. Here is the low as it formed yesterday and as it is expected to progress today on a 500 mb chart (about 18,000 feet).
The solid lines are the heights of the 500 mb pressure surface, and the wind is typically parallel to these lines. As the temperature in the atmosphere cools, it becomes more dense, and the pressure changes more quickly with height. So, when cooling occurs, the height of the 500 mb surface typically gets lower. This is why cold air is usually associated with a trough and warm air is associated with a ridge.
One thing that can cause a low to cutoff is dynamic cooling. This occurs when upward motion in the atmosphere causes it to cool. Below is a loop of atmospheric temperature and dewpoint profiles with height through tomorrow morning. Notice the temperature at 500 mb (about 17,000 feet) drops 10 degrees C (18 degrees F) in just a few hours.
Some of the cold air is also brought in by the wind. In this case, with Arctic air not in place, the cold air has to get down to the ground partly through the precipitation itself. Temperatures aloft are much colder than they are at the ground, and snow falling aloft brings cold air down with it, and causes cooling as it melts below the freezing level. The heavier the precipitation, the more cooling occurs. Eventually, the freezing level can get down close enough to the surface that snow doesn’t have time to melt before reaching the ground, and then the temperatures at the surface really start to cool, due to melting snow.
If the snow is heavy enough, initially warm surface temperatures can be overcome by large amounts of snow falling, cooling the air and accumulating faster than it can melt.
The 12Z models still differ on how far north in Alabama the cold air will get. They do agree that the heavy band of precip now in Mississippi will move into north, and probably central Alabama, late this afternoon and this evening, and will be weakening by then as the surface low moves away. The best guess right now for heavy snow in Alabama would have to be extreme western counties (Pickens, Tuscaloosa, Sumter, Greene, Lamar, Marion, Fayette, Winston, Franklin, Colbert, Lauderdale, Lawrence). But, this is still a very difficult forecast, and 1″ of snow is still possible in Birmingham also.
Another concern I have is roads freezing tonight and early tomorrow morning. Another 1-2″ of rain fell over Alabama last night and this morning, and some precip will fall this afternoon and evening. Temps will likely drop to freezing tonight, especially with the heavy snow pack to our NW, so bridges may ice in the morning even if we don’t get snow.
