An intense and dynamic winter storm system is affecting the Southeast U.S. today. Most of the rain and storms are west of the MS River or north of I-40 right now. Much of the heavy precip in Missouri and Illinois is snow! But note the storms exploding over SE Texas, Louisiana, and SW Mississippi. That will be the big trouble over LA/MS/AL today. Check out the current surface map (annotated).
So, it is 70 degrees in Muscle Shoals, AL but 42 in Memphis and 28 and Springfield, MO. Very strong cold front. The surface low that has formed near the LA/AR border (note the wind flow swirlling around it) could be a big troublemaker for us today. Upper air charts below show a jet to our north at 500 mb (18,000 feet), and divergence due to atmospheric adjustment south of the jet is causing the surface low. Also, at 850 mb (about 5,000 feet), note the strong southerly flow developing, transporting warm moist air into the Southeast U.S.
The map below shows the trajectories of air that will be located at various points by 2100 GMT today (3 pm CST). Note that the air in Birmingham, AL comes from SW Florida between Tampa and Ft. Myers.
So, as the afternoon goes on, temperatures from central and south MS into north and central AL, especially with the sunshine we are getting, will warm into the 70s, and dewpoints will slowly climb into the mid 60s, setting up a prime lower atmosphere for thunderstorm development. One thing that will hold back storms over eastern MS and most of AL through late afternoon is a cap of warm air aloft, shown on this morning’s National Weather Service weather balloon at Calera, AL.
With time, as the cold front approaches, upward motion and cooling aloft, along with warming at the surface, will erode that cap. That will happen quickly over LA and western MS this afternoon, so storms will likely develop in that area quickly over the next 3 hours, and SPC has issued a Tornado Watch over that region. It will take longer over east MS, and maybe until the cold front actually arrives this evening in central AL, to fully become unstable. By then, it will be dark, surface temperatures a tad cooler, so the air over AL will not become as unstable as that in MS. The greatest risk for tornadoes today will be over Louisiana and Missisippi. There is a chance a few isolated supercell storms with a tornado threat could form west of I-65 over Alabama between 3 and 6 pm, but the widespread severe weather in Alabama will arrive after sunset.
As the cold front moves into Alabama this evening, the air will become more unstable. CAPE values will reach 500-800 J/kg, nothing like the 1500 J/kg in south MS, but enough for severe storms. The concern is the wind shear associated with the surface low. The wind will change direction and increase in speed quickly with height, creating 0-1 km SRH values near 300 m2/s2 over central Alabama. This wind shear provides the energy for storm rotation. A good combination parameter for wind shear and instability, the EHI, is shown below for 00, 03, and 06 GMT (6 pm, 9 pm, and 12 am CST).
Note the EHI decreases overall as it moves into Alabama this evening, mainly due to the wind shear pulling away to the north but the instability staying south.
The bottom line here is that there will be a threat for tornadoes, some significant, from Louisiana into Mississippi this afternoon and early evening. As the dynamics pull away to the north and it gets dark, storms will likely grow into a squall line as they move into Alabama around 7 pm and into BHM by 9 pm. However, with the high wind shear, straight-line wind damage and isolated tornadoes will likely occur along the line across west and central Alabama this evening, and in west Alabama, a couple of supercells could form ahead of the line with a higher tornado threat in places like Hamilton, Tuscaloosa, and Demopolis.
This is a dangerous weather situation, especially in Louisiana and Mississippi, but even into Alabama. Review your tornado safety plan wherever you will be. If a Tornado Warning is issued, go to the lowest floor of the building you are in, get in a small room near the center of the building (away from walls, doors, garage doors, and outside windows), get under something sturdy if possible like a desk or table, and if you have helmets (baseball, football, bike, etc) wear them.
I will have another update on this blog around 5 pm CST.
Dr. Tim Coleman
Consulting Meteorologist
Coleman and Knupp, LLC
Coleman Knupp and Dice Weather Blog 