Above is a loop of 500 mb heights (lines) and vorticity (colors) over the next 36 hours. You can see the upper low cutting off from the main flow, or forming a full circle, to our west tonight. Satellite imagery indicates this may already be happening. At the same time, much colder air is moving into AL at low levels. Temperatures will drop to near 40 by sunrise, and then hold steady or fall into the 30s by afternoon.
With the dynamic cooling associated with the upper low and the colder air coming in anyway, temperatures aloft will drop very quickly. For it to be cold enough for snow, we typically look for 850 mb temperatures below freezing (that starts in BHM around noon tomorrow), and 1000 to 850 mb thickness of 1310 or lower (cold air is more dense, and this number gives us an idea of the average temperature over the lowest 4,000 feet). That number drops below 1310 around 2 or 3 pm tomorrow, and will stay there through Tuesday. Temperatures will be VERY COLD just off the surface, in the 20s as low as 3,000 feet or lower. So, by tomorrow afternoon in west Alabama and tomorrow night in east Alabama, it will be cold enough for snow to fall.
The 2 main things that create lift in the atmosphere and precip are warm air advection (warm air taking over cold air) and positive vorticity advection at upper levels (more red moving in in the loop, aka upper level disturbance). We will have light precip tomorrow afternoon and night, but the heaviest precip should come through north and central Alabama early Tuesday morning. We’ll have plenty of disturbances rotating around the cold air, but we reach the middle of the cold core (and, therefore, start having warm air advection) by then. The best combo of warm air advection and positive vorticity advection is indicated by models over NW Alabama, but the models are always suspect in upper level lows. That also means confidence in the forecast is lower than normal!
(Note by Tuesday morning we are at center of cold air and have warm advection)
Bottom line…expect rain to start mixing with a little snow west to east tomorrow afternoon and evening, then some snow tomorrow night. How much and where is hard to pin down…NW Alabama most likely, but that could change. With that much cold air aloft, we could even get a few convective snow showers that would be heavier, temporarily causing accumulation on grassy areas, cars, etc. But, the ground is warm after all the 70s last week, and any significant accumulations are unlikely, as are any travel problems. But, with this system being so cold and intense, the forecast could change, and somebody could have travel issues Monday night and Tuesday…especially at higher elevations and/or in NW Alabama (Florence, etc.) It will be fun to watch…
NOTE: Don’t forget to bundle up the family tomorrow also. We’re going from 70s Saturday to wind chills in the 20s at times during the day tomorrow and Tuesday.