Damaging Wind Event Across East Tennessee
July 21, 2006
Event Overview
A large mesoscale convective system (MCS) that developed over northern Missouri early in the morning tracked southeast, leaving a large swath of damage across Missouri, southern Illinois, Kentucky, and East Tennessee. As the MCS moved into middle Tennessee, an outflow boundary or gust front preceded the thunderstorms. It was the winds associated with this gust front that caused widespread wind damage over much of the central and southern Tennessee Valley.
Severe weather reports
Across east Tennessee, there were numerous reports of trees blown down in almost every county (Figure 1). The greatest amount of damage occurred across the southern portions of East Tennessee, from the Sequatchie Valley to southwest North Carolina. There were reports of buildings damaged in Hamilton County. In McMinn County, at least 25 roads had to be closed due to trees being blown down. When a 911 dispatcher in McMinn County was asked if any trees were blown down, she laughed and replied "The question is, do we have any left?".
Figure 1. Plot of storm damage on July 21, 2006.
Figure 2 is a plot of the severe thunderstorm warnings (green boxes) that were in effect at one time (910 pm EDT) across the southern portions of East Tennessee. The majority of southeast Tennessee, from Knoxville to Chattanooga, was under a severe thunderstorm warning at this time. All of these counties reported wind damage.
Figure 2. Plot of warnings in east Tennessee at 9:10 pm EDT on July 21, 2006.
Storm Environment
In the early morning hours of July 21, a mesoscale convective system, or MCS, developed over northern Missouri. An MCS is s large organized convective weather system comprised of a number of individual thunderstorms. It normally persists for several hours and may be rounded or linear in shape. As it traveled southeast, it produced damaging winds in Missouri, southern Illinois, and central and western Kentucky. By the afternoon, the MCS had reached the Tennessee border. The air mass over Middle and East Tennessee that afternoon was quite unstable. Temperatures had reached the mid 90s, with some upper 90s in northern Alabama by 4 pm EDT (Figure 3). Dewpoints were also high, between 65 and 70. As the MCS moved southeast into this unstable air mass, there would be plenty of energy available for it to maintain its strength.
Figure 3. Plot of surface observations at 4 pm EDT on a visible satellite image (lightning strikes are displayed in yellow).
The 00Z sounding from Birmingham (Figure 4) would likely have been representative of the air mass over East Tennessee that afternoon. This sounding has several features that indicated an environment that was favorable for damaging wind gusts. First, the lapse rate (rate of temperature decrease with height) is quite steep – nearly 10 ° C per km in the lowest 2 km. Steep low level lapse rates not only indicate good low level instability, but can also enhance downward momentum transfer of winds aloft down to the surface. Second, the profile of the sounding in the lowest 2 km resembles an upside-down V shape, which is called an inverted-V sounding. This describes a sounding in which the temperature rapidly decreases with height while the dewpoint temperature also decreases with height (though not as quickly as temperature). This indicates a high potential for evaporation and cooling as rain falls into the lower levels. The evaporation of rain cools the air, making it more dense and heavier. This speeds up the outflow from a thunderstorm, and can lead to a damaging downburst of wind.
Figure 4. Sounding from Birmingham, AL at 8 pm EDT on July 21, 2006.
Radar data
Figure 5 is a reflectivity image from the radar at Hytop, AL when the MCS was moving into the southern Tennessee Valley. The gust front stretches from west to east, just north of the radar, near the Tennessee/Alabama border. Wind damage occurred as the gust front moved through, even though the thunderstorms trailed the front by several miles.
Figure 5. Reflectivity image from the Hytop, AL radar at 8:39 pm EDT on July 21, 2006.
Figure 6 is a radial velocity image from the Hytop, AL radar, taken at the same time as the reflectivity image above. It shows winds in excess of 50 kts (or 58 mph) in Grundy County and northern Marion County.
Figure 6. Radial velocity image from the Hytop, AL radar at 8:39 pm EDT on July 21, 2006.
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