When it comes to winter weather forecasting in the Tennessee Valley many times you don't know what to expect until the night before. To get snow in North Alabama you need the perfect setup. You typically need a surface area of low pressure to the south. The closer this area of low pressure is to the Gulf of Mexico the better. You also need a good supply of arctic air moving south. There is also a process through dynamic cooling from a cold core upper level low pressure system that can bring down the snow even though temperatures to start the day are not even close to freezing at the surface. Some storms systems in the Tennessee Valley are a combination of both.
The one thing we are trying to do this far out is get closer to a more accurate solution for the Tuesday/Wednesday time frame next week and find some forecast model consensus. Depending on the exact track and strength we could end up with just cold rain or we could see some accumulation of snow on grassy areas. Other areas of the Southeast could be dealing with stronger thunderstorms. Regardless of what the setup looks like right now the system is still nearly a week away and a lot of fine details will not show up for several more days. Here are some graphics that show you what we are dealing with as we track the evolution of this developing storm system. There are two primary long range forecast models we use. 1. The GFS (American model) 2. The ECMWF (European model)
Three things we need to keep in mind as we go forward. 1. Models have a hard time dealing with the extend of Arctic Air that is on the move. 2. Models have difficulty with storm systems coming out of the Desert Southwest and parts of Mexico. 3. Any error on the model run early in the run will only grow exponentially with time. Meaning if the beginning model solution is not accurate the forecast days that follow are useless.
Here is the European model solution for Tuesday December 18th at 4pm.
The rain/snow line in the dashed purple line is across NW Alabama with the surface low near Montgomery. This low placement isn't bad for snow in North Alabama. The blue arrows represent the jet stream. This would give us a rain changing to snow from west to east situation. The changeover would be due to evaporative cooling from the cold core low. Strong thunderstorms for SE Georgia and north Florida.
Below is the GFS (American model solution)
The rain/snow line is almost identical. The time frame is moved up to 1pm Tuesday. The surface low placement is not good at all for snow in North Alabama. It is closer to Chattanooga. We would need a lot of moisture and some strong dynamic cooling to change the cold rain to snow. The surface low is stronger than the European model.
The closer the solutions by these two models the better. That will increase the accuracy and the confidence of forecasting this system. If we can get a slower developing storm system that is farther to the south we might be in business for some snowfall. Ground temperatures are warm for this time of year so chances are if we get any snow it would likely only stick to grassy areas. But it is still possible the Arctic area will make it farther south that the models think. As I mentioned earlier they have trouble with Arctic air on the move.
Stay tuned but don't go getting your bread and milk just yet. I'll keep you updated with each model run. I will focus on the morning and evening model runs.
WAFF 48 Storm Team
We Track Storms