A rare January severe weather outbreak is likely across the Southeast U.S. today and tomorrow. As is usual for this time of year, it will primarily be driven by atmospheric dynamics (wind shear, large-scale forcing, etc.) as opposed to the very unstable air we see in Spring severe weather episodes. The dynamic setup on this one is extremely impressive, though, and I will show you a few maps and graphics to illustrate that below.
Water vapor satellite imagery already shows a deep upper-level trough in the wind field digging into AZ and NM. This trough is projected to move eastward and gain a negative tilt by tomorrow morning, which is often associated with severe weather.
Due to divergence ahead of the upper trough, surface pressures are falling rapidly over the Midwest and SE US. Meanwhile, a strong ridge of high pressure is trying to hold on over the Atlantic Coast. This is setting up a pattern of strong low-level southerly flow off the Caribbean and Gulf of Mexico, bringing warm and humid air slowly back into the Southeast.
The air that will be in central Alabama tomorrow is currently just north of the Leeward Islands in the Atlantic Ocean, near the Dominican Republic, according to computer trajectories.
This would normally provide a very unstable air mass. However, we need warm, humid air at the surface and cold temperatures aloft for that, and with the strong ridge of high pressure to our east, our upper level temperatures will not be that cold. Below are forecast temperature, dewpoint, and wind vertical profiles for Birmingham tomorrow noon. There is some instability there, but the computer models have large disagreement. The HRRR (left) shows a CAPE of 1100 J/kg, while the NAM, with a slight warm nose aloft (circled), shows almost zero.
How far north the unstable air makes it will play a large role in how intense the storms will be. The GFS model, that has been around a long time but updated constantly, goes down the middle of these two, so that is a good guess of what will happen. It shows CAPE greater than 1000 J/kg near the Gulf Coast, with values decreasing to near 800 around MGM and 500 at BHM.
Normally, these levels of CAPE would not support a severe weather outbreak. However, the dynamic nature of the upper-trough shown above, and the rapid atmospheric adjustments happening downstream from it, will create extreme wind shear over Alabama tomorrow, some of the worst I have seen in a while. It will be windy tonight and tomorrow at the ground, with winds gusting to 30 mph at times. Up at only 3,000 feet, winds will be 60-70 mph, and at 5,000 feet around 85 mph! This is very rare. The rapid increase in wind speed with height, combined with the winds changing direction with height, will produce helicity, or circulation in the winds flowing into storm updrafts that can be tilted into the vertical by the storms and produce rotation, and potentially tornadoes. A map of the 0-1 km storm-relative helicity is shown below. Only 150 m2/s2 is necessary for tornado development, and we will have values 400-500 m2/s2 from about Clanton northward, and values near 300 m2/s2 all the way to Mobile and Pensacola.
A good indicator of the right combination of instability and wind shear for tornadoes is the EHI, shown below for noon tomorrow. It indicates that despite the best wind shear over north Alabama, the best instability overrides that, placing south Alabama in the highest risk for long-track, large tornadoes. This area of EHI will move eastward during the afternoon.
Models are fairly consistent in bringing an intense line of storms into west Alabama around 10 am tomorrow morning, and this line will move rapidly across the state. It should arrive in BHM by 1 pm. With the lack of significant instability, individual supercell storms out ahead of the line (that are the most dangerous in terms of violent tornadoes), are unlikely north of I-20, but a couple of these could form in areas like Selma, Montgomery, Greenville, Troy, or Enterprise. We can’t rule it out north of I-20, especially if temperatures get into lower to middle 70s before the line comes through. So, tomorrow will be a day where everyone will need to have a plan for a Tornado Warning.
Even in the squall line, all this wind shear could produce multiple tornadoes over north and central Alabama, plus straight-line winds of 50-70 mph. This outbreak will be nothing like all the tornadoes during the afternoon of April 27, 2011, but if you recall, we had an intense squall line move through during the early morning hours that day, producing widespread damaging winds and several tornadoes (including two in the BHM metro area). I have talked to some of my colleagues at UAH and the NWS, and we all agree this squall line has the potential to be like that one. Let’s hope it is not.
Bottom line, have a severe weather plan and a source of weather information. Plan for the worst but hope for the best. You need a NOAA Weather Radio and/or a phone app properly set up to alert you of warnings, and access to local TV or radio. Since it is Saturday, most people will be home. If a tornado warning is issued for your location, go to the lowest floor, near the center of the building, away from windows, doors, and outside walls. If you have a basement, that is best. Pick the part of the basement that is underground. If you don’t have a basement, get in a closet or bathroom in the center of the house. Get under something sturdy if possible like a table or workbench. If you have bicycle, baseball, or football helmets, wear them. Many people are killed by flying debris hitting them in the head.
UAH will be doing full VORTEX SE research operations out across north Alabama tomorrow, with wind profilers, mobile radars, lidars, etc. I will be helping coordinate that from SWIRLL operations center on UAH campus, so I will not be able to do much in the way of blogs or updates tomorrow.
Dr. Tim Coleman
Coleman and Knupp, LLC