…Another outbreak of severe weather, including tornadoes, possible on Thursday…
Here we go again. There is more uncertainty with this system than with the one last week, but it still looks threatening. First of all, for today (Tuesday), we have a system moving through with rain, and some thunderstorms. But, the best instability for any strong storms will be near the Gulf Coast, and no severe weather is expected north of a line from Monroeville to Opp to Enterprise.
The above full-disk image from GOES is fascinating, in that it shows that daylight is fairly evenly distributed in the northern and southern hemispheres (we just passed the equinox), the sun just came up in Hawaii, and there is a second upper-level system (circled) in the southwest US, headed our way.
A significant upper trough is currently centered over southern California, and it will move out into the Southern Plains by Thursday morning. Here is where some of the uncertainty comes in…there has been some computer model disagreement on the strength of the upper system coming in. Compared to last week, this upper system does not look as intense over OK/AR/MO/KS on the American models. But, the European model has a stronger system. Even with the weaker upper system, there will not be a wedge of cool air coming in from the east this time to weaken most storms as they get north and east of Tuscaloosa, and the wind shear in this system will be high. The American GFS model this morning has come in with a classic severe weather setup, with a surface low in Tennessee by Thursday afternoon.
The NAM, showing the weakest solution of the three major models, is still indicating a setup over north and central Alabama for tornadoes. As the upper system approaches, falling pressures will lead to a strong low-level jet stream, transporting warm, moist air from the Gulf into Alabama. This will produce unstable air over Alabama. With the surface low so close by and deepening as it moves across to our north, wind shear values will be very high.
Between 3 pm and 10 pm on Thursday afternoon, CAPE values will increase from west to east, to 1,000 to 2,000 J/kg. In addition, storm-relative helicity values, that indicate wind shear for storm rotation, will be quite high, between 300 and 500 m2/s2. These, in combination with high ground-level relative humidity, will produce a significant risk for tornadoes. This is measured by the Significant Tornado Parameter (STP), that used all three of these values. Below is a map of the STP for late Thu afternoon and Thu evening. Any value over 1 is sufficient for tornadoes, and we have some values up near 5 in some places/times.
Note that the greatest threat for tornadoes will be over Mississippi and the northwest 1/2 of Alabama, mainly north and west of I-59, including Birmingham, Tuscaloosa, Huntsville, Florence, Jasper, Cullman, Hamilton, Fayette, and Demopolis. The greatest threat for supercell tornadoes will be in the late afternoon and early evening hours. By late evening, the storms will likely form into a line, but still have a risk of 70 mph winds and tornadoes as they move through. Expect the main line of storms in the BHM area between 11 pm and 3 am.
One other factor that could keep supercells from forming in the unstable air out ahead of the main line is a small cap of warm air aloft, as shown on the computer-model generated temperature profile below. The red line shows how temperature changes with height. The jump over to the right near 800 mb (about 7,000 feet) is a warm area. When thunderstorms build, they do so primarily due to buoyancy, or by the air in the storm updraft being warmer than the air around it. If the storms encounter warm environmental air, it could suppress them until the main line comes through during the late evening. However, if this warm cap does not appear, or the sun comes out and warms the surface temperature into the 70s so that the thunderstorm air just goes right through the cap, it won’t matter and we’ll have supercell storms with possibly large tornadoes like we did last Wednesday. Just another uncertainty right now.
This system will bring storms with heavy rain, strong winds, and possibly hail and tornadoes, Thursday night. It is possible that intense supercell storms will form earlier, between 2 pm and 7 pm, and produce large hail and significant tornadoes. It could get rough. We don’t know yet. The farther west you are, the larger the risk. We will know more as the system develops and gets closer, but every severe weather system is different. I will be watching this very closely. UAH plans to deploy groups to collect weather balloon and surface data, as well as Doppler wind lidar, wind profiler, and mobile Doppler radar data, across north Alabama. Another blog from me tomorrow evening.
Dr. Tim Coleman
Consulting Meteorologist
Coleman and Knupp, LLC
Coleman Knupp and Dice Weather Blog 