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), the precipitation is often dampened. Take a look at 850 mb winds and temperatures late this evening:
Notice the 850 mb 0 C isotherm (the rain/snow line) is already in NW Alabama. The coldest 850 mb temps are in Arkansas. Here, cold air is still moving in, so that is suppressing the precipitation somewhat. Here is a chart of temperature advection.
Notice in Tennessee where the heavy snow has been this evening (up to 8″), there is warm advection! We have the upper-level disturbance nearby already. Here is the 500 mb chart.
Strong upper-level dynamics.
My read on this is that the light drizzle over central Alabama will changeover to snow in the next 2-3 hours as temperatures become colder. Once it gets cold and the cold air advection stops slowing down the precipitation (as the upper low gets further south), precipitation will pick up some, maybe after 3 am. 1-2″ still looks good for BHM.
