…TORNADO WATCHES IN EFFECT FOR LOUISIANA…MISSISSIPPI…AND WESTERN HALF OF ALABAMA…
The cold front has moved east this afternoon, and the low pressure area along it that is causing all the wind shear for storm rotation has moved just SE of Memphis. Take a look at the air swirlling around the low at the large scale, and the huge temperature difference across the front. 73 in Columbus, MS; 39 in Little Rock.
UAH SWIRLL is fully deployed with mobile radar, wind profilers, and Doppler Lidar in NW Alabama as part of VORTEX SE, and is releasing weather balloons from campus in Huntsville every 2 hours. Here is the latest. Note the cap of warm air at 700 mb is almost gone compared to what it was on the NWS Birmingham balloon data this morning. That cap has prevented supercells from forming out ahead of the main line of storms so far, and if we make it through the atmospheric changes that we have found to occur during the day-to-evening transition, it looks like we will avoid significant storm development ahead of the main line. That is good news, as lone supercell storms have a higher risk of tornadoes.
Even still, VAD wind profiles from the NEXRAD Doppler radars show that computer models underestimated the wind shear in this system. Note that winds at 4,000 feet at Columbus, MS (KGWX radar) are 55 knots (64 mph), 10-15 knots higher than predicted. This means storm-relative helicity, wind shear for rotation in storms, is also higher, around 500 m2/s2 (as opposed to the forecast 300 m2/s2).
This is causing storms within the QLCS, aka squall line, to rotate. Tornado Warnings have been issued for a significant area of rotation in the QLCS in NE MS that has now moved into NW AL, and a new area of rotation has prompted a Tornado Warning. These warnings are now in effect for Lauderdale, Colbert, Franklin, Morgan, Lawrence, and Limestone Counties, including Florence, Decatur, and Athens. A tornado has been confirmed in Colbert Heights, AL. Note that the entire line of storms has swirlled into a large MCV with a small eye near Rogersville.
Further south, in the storms approaching Hamilton, Tuscaloosa, and Jasper, there is not as much rotation at this time, so fortunately there the main risk is lightning and straight line wind. Those storms are further south and in a more unstable air mass, but clearly the wind shear and its dynamic forcing are trumping the instability, at least in north Alabama. However, tornadoes have been reported in several locations in central and southern MS, as far north as McComb, where the air is more unstable, and Tornado Warnings are in effect now in storms south of Jackson, MS moving toward Meridian. We may have to deal with a few of those in west-central Alabama before 7 pm, so be ready in Pickens, Sumter, Greene, Hale, and Tuscaloosa counties. With the higher-than-expected helicities (see below for current), we have to be on the lookout for small tornadoes within the line of storms too.
Bottom line…things look a little more scary than they did in my last blog due to the unexpected increase in wind shear. However, with the low instability over the northern half of the state, it is taking huge wind shear to spin up any tornadoes, and it looks like the shear will decrease somewhat as the line moves toward Birmingham as the low pressure area moving into Tennessee gets farther separated from the storms. It is odd, from a meteorology perspective, to see some storms with high instability and low shear, then others with low instability and high shear, both producing tornadoes, but the ones in the middle, where the mixture should be best, doing nothing. This means we will be looking at the storms very carefully the next 3-4 hours.
Dr. Tim Coleman
Consulting Meteorologist
Coleman and Knupp, LLC
Coleman Knupp and Dice Weather Blog 