Image 1: Severe weather reports collected on April 7, 2006

 

Image 2: Water vapor satellite image and 700 mb data plot at 12Z April 7 2006

Event Overview
During the afternoon and evening hours of April 7, a tornado outbreak occurred over West and Middle Tennessee, northern Mississippi, and northern Alabama. Severe weather occurred as far north as Lake Erie and as far south as northern Louisiana. There were 12 deaths and at least 60 injuries from the tornadoes in Tennessee, most occurring around Gallatin, just northeast of Nashville. The storms moved east into East Tennessee in the evening hours, producing many reports of large hail and wind damage (see image 1 above), but no confirmed tornadoes.

Storm Environment
One of the important ingredients needed for a tornado outbreak is a supply of warm and dry air from the Mexican Plateau in the midlevels of the atmosphere, with cool temperatures above and plenty of moisture below. The water vapor satellite image (image 2 above) shows a flow of dry air (brown and yellow colors) coming from the southwest into the Tennessee Valley. The data plot from 700 mb (in image 2 above) also shows the midlevel dry air, with dewpoint depressions between 20 and 30 degrees (lower left number in the plots; larger numbers indicate drier air). The large low pressure system over Kansas would move southeast, and provide colder temperatures aloft, further increasing the instability.

A profile of temperature and dewpoint from Nashville that afternoon (image 3 below) showed that layer of dry air in the midlevels. The presence of this warm, dry air beneath cold air leads to a rapid decrease of temperatures with height, making the atmosphere very unstable; i.e., the air can rise rapidly when lifted. When the lower layers of the atmosphere are moist, as was the case on this day, the instability can be even greater.

Also important to tornado formation is wind shear, or the turning and increasing strength of winds with height. The profile of winds (image 3 below) shows that the winds just above the surface are quite strong, between 30 and 40 kts, and are quickly turning from south to southwest with height.

So why did the thunderstorms produce tornadoes in Middle Tennessee and not in East Tennessee? The most likely reasons are that the low level wind shear in the Tennessee Valley was weaker, and there was less instability. Image 4 below is a plot of surface observations from the mid-afternoon hours (19Z). Notice that the winds in West and Middle Tennessee, Mississippi, and Alabama were from the south or south-southwest. The winds in East Tennessee, especially at Knoxville and Tri-Cities, were from the southwest. Just this slight difference in wind direction can affect the ability of tornadoes to form. Also notice that dewpoint temperatures (lower left number in the data plots) are in the lower to middle 60s in West and Middle Tennessee, Mississippi, and Alabama. In East Tennessee, they were in the middle to upper 50s. The green contour highlights dewpoints that are 60 degrees. Thus the air in East Tennessee was not as moist (and thus not as unstable) as it was farther west. The thunderstorms also formed in West and Middle Tennessee in the afternoon, so by the time they reached East Tennessee in the evening, much of the surface heating provided by the sun had been decreasing, leading to less instability.

Image 3: Vertical profile of temperature, dewpoint, and winds from Nashville at 18Z April 7 2006.

 

Image 4: Surface data plot at 19Z April 7 2006. The bright green contour highlights dewpoints of 60 degrees.

Satellite Data
An interesting feature that was noted in the satellite imagery was the presence of what is called an “enhanced-v” signature (image 5 below). The arrows in the image below point to these signatures. The "enhanced-v" appears in an infrared imagery, and resembles a V-shape or boomerang-shaped area of cold brightness temperatures, and is bordered by an area of warm brightness temperatures downwind. The origin of this signature is thought to lie in the circulation forced by the overshooting top. As air approaches from downstream, it flows around the overshooting top as if a solid obstacle (like water in a stream flowing around a boulder, for example). Downwind of the overshooting top, the air subsides and warms, producing the warm wake characteristically observed in tandem with the enhanced-v. If an thunderstorm has an updraft strong enough to produce this signature, it nearly always produces severe weather and often tornadoes.

Radar Data
Image 6 below is a reflectivity image of the storm that produced a tornado in Sumner County near Gallatin, resulting in several deaths and many injuries. Note the hook signature on the southern end of the storm. This is where the tornado was located. Also note within the hook there is an area of higher reflectivity (white). This may be due to debris from the tornado.

Image 7 below is a radial velocity image taken at the same time as the reflectivity image. This is what a tornado looks like to a Doppler radar. The tornado is located just west-southwest of Gallatin and northeast of Hendersonville, where the bright green colors are right beside the red colors. The radar is located at the black circle at Green Hill. The bright green colors indicate winds that are moving toward the radar, and the red colors indicate winds moving away from the radar. By seeing the green and red colors directly beside each other, this is a signature of the counter-clockwise rotation of a tornado. Most of the time, the radar can only see the storm well above the actual tornado, and only shows the circulation higher in the storm, called a mesocyclone. But because this storm is so close to the radar, it can actually see the rotation of the tornado itself.

Image 5: Infrared satellite image from 23Z April 7 2006, with arrows pointing to the enhanced-v signatures.

 

Image 6: Reflectivity image at 1922Z April 7 2006.

 

Image 7: Radial velocity image at 1922Z April 7 2006.