Early on Friday the 7th, a classic spring severe weather setup began to unfold across the area. A strong upper level storm system (see an upper-air map) was positioned across the central Plains with an associated band of very strong winds aloft extending into Arkansas and Missouri. This system was forecast to shift southeast with time and approach the mid Mississippi and Tennessee Valleys Friday evening. At the same time, a strong surface low pressure system was located across Kansas (see a surface map from 7am 4/7/06) with a trailing dryline extending through the Southern Plains. This surface low was forecast to develop east southeast and to eventually track across northwest Tennessee by the late night hours. The combination of these two features helped to bring a surge of gulf moisture northward into the Tennessee Valley and also create favorable wind shear conditions for supercell thunderstorms and tornadoes.
Supercell thunderstorms began to erupt rapidly across western Tennessee and Kentucky during the late morning hours on Friday. Several of these supercell thunderstorms continued to gain strength and wind energy as they tracked eastward toward the Nashville Metropolitan area by the early afternoon hours (see a radar image from 4:30pm Friday afternoon). However, storms were slower to develop across the Tennessee Valley. The primary reason was that a pocket of warm air about 5000 feet above the ground, commonly referred to as a “cap” or thermal inversion, was inhibiting the growth of thunderstorms. This cap began to erode by late in the afternoon as moisture began to increase across the region and the upper level trough began to bring in cooler temperatures above the surface.
Thunderstorms began to erupt across northern Mississippi shortly after 4 PM CDT Friday afternoon, and many of these became severe very quickly (see a radar image from 5pm Friday afternoon). The first tornadic supercell moved into northwest Alabama around 5 PM, but it would not be the last. All totaled, eight severe storms moved across the Tennessee Valley region between 5 PM and midnight, several of which were long lived supercells.
Click the thumbnail to view a larger version.
|A map of the conditions around 15-20,000 feet (500mb) at 7pm on 4/7/06 shows the strong upper-level system over Kansas, and a band of higher winds extending from western Texas into the Ozarks of Missouri and Arkansas. (Map from the Storm Prediction Center.)||Infrared satellite imagery from 3:30pm CDT on April 7. At this point, the coldest cloud tops (the shades of blue indicating -50 to -60 degree Celsius) are over Tennessee, where numerous tornadic supercells are already in progress. However, the "bumpy" clouds over northeast Mississippi indicate that thunderstorms are beginning to develop in thar area.|
|A surface map from 7am CDT on the morning of April 7th. The system centered north of Topeka, Kansas would eventually move east to impact the Tennessee Valley. (Map from the Hydrometeorological Prediction Center.)||A surface map from 7am CDT on the morning of April 8th. By this time, the main cold front had moved east of the area, and the warm, muggy airmass partly responsible for the storms had been replaced with cooler, drier air. (Map from the Hydrometeorological Prediction Center.)|
|A view of several supercell thunderstorms raking across middle Tennessee, from the Hytop, AL radar at 4:30pm on the 7th. Note the lack of storms across north Alabama, thanks to a pocket of warm air above the ground commonly referred to as a "cap".||By 5pm CDT, supercells had been firing across northern Mississippi for nearly an hour. This radar image, taken from the Columbus, MS radar at that time, shows the first round of supercells starting to move into northwest Alabama.|
|Infrared satellite imagery from 7:15pm CDT on April 7. Note the swirl in the clouds over Kansas indicating the upper-level low, and the cold cloud tops (-60 to -65 degrees Celsius!) indicating thunderstorms over Mississippi, Alabama, and Tennessee.||Visible satellite imagery from around 5pm CDT on April 7. The swirl indicating the upper-level low is even more apparent in this image. You can also see tiny "bubbles" in the centers of the clouds; these are often called "overshooting tops", indicating very strong thunderstorms.|