...WINTER WEATHER ADVISORY IN EFFECT FOR MOST OF OUR AREA FOR TOMORROW...
The long-term cold air pattern has already started taking shape over the southeastern U.S. this week. Due to the long-wave nature of the upper trough bringing the cold air to the eastern U.S., it will be much slower to move east or lift out due to atmospheric dynamics. So, the cold air is here to stay for at least another 10 days.
It is cold but not miserable outside today. It is currently 47 degrees here at the UAH SWIRLL remote facility in Trussville. However, a reinforcing shot of cold air is on its way, and it will move in tonight. Check out the colder temperatures right now over Tennessee, Kentucky, Arkansas, and Missouri (20s and 30s), and the north winds bringing them this way.
Radar already shows precipitation over south Alabama, but much of it is not reaching the ground, drying up on the way down in the very dry air in place there right now.
As a large upper-level trough approaches tonight and tomorrow, air that is slowed down in the curve at the base of the trough will speed up over Alabama and Georgia, creating upper-level divergence, pulling air upward. This will create more precipitation by morning, and move it farther northwest. The air aloft will be more humid, also, as it comes from the southwest, while cold air seeps in at low levels. With this upper-divergence, enhanced by our being in the right-entrance region of an upper jet, there will likely be enough forcing for precipitation to become heavy enough to reach the ground by tomorrow morning.
This is a difficult forecast (it always is with snow in Alabama!). The air will be too dry over northwest Alabama for any snow, and it will be too warm over southeast Alabama to fall as snow. So, there will be a narrow band, somewhere near I-59, where snow will occur. The high-resolution NAM model places this band as shown below, with about 2" of snow falling in Birmingham, and a maximum of 3-5" from Demopolis to Columbiana to Heflin. Note that with ground temperatures warm and air temperatures starting out above freezing, not all this snow will accumulate, but only part of it. Even in the areas with the most snow, only 1-2" will be on the ground, with much of that in grassy areas and on elevated surfaces like trees, cars, and bridges. Also, notice how narrow the snow band is, and the models have uncertainty with something this precise. It is possible the max could drift north or south, taking Birmingham and Tuscaloosa completely out of the snow, or putting us in the middle of the heaviest band. We are not sure at this point.
The bottom line is that light rain and light snow will begin over central Alabama around 6:00 am tomorrow morning, and it will become cold enough from northwest to southeast very quickly for all snow, especially with the initial snowflakes melting (and endothermic process that uses heat and cools the atmosphere, like ice cubes in your drink). This will not be anything like January 28, 2014, since air temperatures that day were around 19 degrees, while tomorrow it will be 31-35. But, a few slick spots are possible on area roads tomorrow morning through about 1 pm, especially on bridges where the cold air can get underneath them.
Temperatures tomorrow night will drop into the 20s, so any water left on roads could freeze, making travel hazardous all over the area.
Stay tuned to Fox 6 for continued updates. I will have another blog late tonight.
Dr. Tim Coleman