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Severe Thunderstorms on December 28, 2005
Event Overview:
A line of severe thunderstorms moved across east Tennessee and southwest Virginia in the afternoon and evening of December 28th. These storms were associated with a strong upper level low pressure system and a cold front at the surface (see the surface analysis below). As the system moved across the Mississippi Valley region, it produced very little precipitation. But as it encountered a warm and moist air mass over the Tennessee Valley region, enough instability was generated for severe thunderstorms to develop. Farther south, numerous tornadoes were reported in central and southern Georgia.
Severe Weather Reports:
A total of 27 severe weather reports (wind damage or hail of penny size or larger) were received from across east Tennessee and southwest Virginia. The National Weather Service office in Morristown issued 31 severe thunderstorm warnings.
A detailed listing of severe weather reports that occurred across the region with this event can be found at the following link: http://www.spc.noaa.gov/exper/archive/events/051228/index.html
Storm Environment:
The severe weather over the Tennessee River Valley was somewhat unexpected. A slight risk of severe thunderstorms was forecast by the Storm Prediction Center for Alabama, Georgia, and South Carolina, and most of the severe weather was expected to remain well to the south. A band of weaker showers and isolated thunderstorms had moved through east Tennessee in the morning, and clouds were expected to remain over the area through the day. However, the clouds dissipated enough in the early afternoon to allow temperatures to rise into the 60s across the valley. A southwest flow up the valley also allowed dewpoint temperatures to rise into the 50s. The showers in the morning also helped provide low level moisture to the area.
In the image of surface data above at 1 p.m., the surface cold front was located from western Indiana across middle Tennessee and through western Alabama. The front was not very strong, as temperatures difference across the front was about 10 degrees across Tennessee. There was a good push of dry air behind the front, as dewpoints across the South were near 60 in Georgia, and in the 30s and 40s in Arkansas and Mississippi.
The strong upper level low played an important role in developing the severe thunderstorms. Very cold temperatures aloft were observed with the system. When these cold temperatures aloft move over warm air near the surface, an unstable environment is created. This can be seen in the following image, which is a vertical profile of temperature and dewpoint at Nashville on Wednesday morning.
As the upper low moved east, temperatures around the 500 mb level (just below an altitude of 6 kilometers) decreased by about 10 degrees, while the low level temperatures warmed in the afternoon. This caused a rapid cooling of temperatures with height, which results in an unstable environment.
The wind profile also shows that winds were turning with height and increased rapidly above the surface. This created low-level wind shear, which also contributed to the severe thunderstorm development. Farther south, the shear and instability were even stronger, and thus the storms across Georgia were able to produce several tornadoes.
Satellite Data:
The above satellite image taken in the early afternoon shows the upper low centered over western Kentucky. The band of showers that moved across east Tennessee in the morning is located in western North Carolina at this time. The thunderstorms that would produce severe weather in the area are just beginning to develop over middle Tennessee. Note the lack of cloud cover over the Tennessee Valley, which allowed temperatures to warm quickly.
This satellite image is from the afternoon, when the severe thunderstorms were moving across the Tennessee Valley.
Radar Data:
In the radar image above, the showers that crossed the area in the morning have moved into North Carolina. The thunderstorms that would later produce severe weather are beginning to develop in middle Tennessee. The spin of the upper level low pressure system can be clearly seen over southern Indiana and Kentucky.
The line of severe thunderstorms was moving through the Tennessee Valley at this time. Notice that the storms in Georgia are more cellular in nature, while the storms in east Tennessee and eastern Kentucky have formed along a line. This is due to the higher shear over Georgia. The formation of individual cells (called supercells) is often more favorable to producing tornadoes than storms that form in a line.
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