As mentioned earlier, the two main processes in the atmosphere that cause upward motion, and therefore precipitation like snow, are:
1. Warm air moving into the area and replacing cold air. That’s why it often stops raining as the cold air arrives.
2. Upper-level divergence associated with an approaching vorticity maximum (disturbance) aloft.
If these two are countering each other (cold air advection and upper-level divergence), as they were earlier, the precipitation is often dampened. But, as we saw in Memphis last night, the heaviest snow has been occurring where warm air advection is occurring aloft (the air is plenty cold for snow), enhancing the snow. That area of warm advection aloft is now in NW Alabama, and the upper level low is over SW Alabama.
So, snow has picked up over north and central Alabama, and is now rather heavy in parts of west Alabama. Radar:
The snow is falling fairly heavily even here in the BHM Metro area, with the grassy areas completely white here in Trussville. But, the heaviest snow may still be on its way from NW Alabama. The roads just look wet right now in most of the BHM area, but bridges will likely become icy (if they haven’t already) in the next hour or two. And, roads at higher elevations may already be icy now.
Take a look at temperatures:
The freezing line at the ground runs from about Scottsboro to BHM to Selma, and will move SE as snow continues to fall, cooling the lower atmosphere, and NW winds pick up. Road conditions in the BHM area will likely deteriorate by 8 am, so travel is not recommended. If you have to get out, be very careful, leave plenty of room between you and the car in front of you, and when you go across an icy bridge, try not to apply the gas nor the brakes if possible (unless you have to), as these can cause you to lose control of your car.
Later today, it may feel a little blizzard-like, with NW winds gusting to 25 mph, temperatures only in the lower 30s, and maybe even some blowing snow.
