Flash Flooding around Knoxville
September 23, 2006


Event Overview

Thunderstorms produced very heavy rain for several hours across much of the central Tennessee Valley in the late morning and early afternoon hours of Saturday, September 23. The storms developed along a boundary that was located between Chattanooga and Knoxville, and as the storms tracked northeast, they repeatedly moved over the same areas. In addition to the flooding, there were also a few severe thunderstorms that blew down trees across the Tennessee Valley and Cumberland Plateau.

Area Rainfall Amounts

The following are measured rainfall amounts from NWS Cooperative Observers in the area of the greatest flooding:
Lenoir City Water Plant: 5.85
Fort Loudon/Tellico Dam: 5.52
Knoxville Water Plant - Tellico Lake: 4.08
McGee-Tyson Airport: 3.06
Watts Bar Dam: 2.97
Watts Bar Lake - Kingston: 2.87
Oak Ridge: 2.75
Radar estimated rainfall amounts were close to the measured amounts, with up to 6 inches estimated in Loudon and Knox counties. A long swath of rainfall amounts from 2.5 to 5 inches stretched from just north of Chattanooga the north side of Knoxville. Figure 1 is an image of radar-estimated rainfall amounts.

Doppler radar estimated rainfall on 23 September 2006
Figure 1. Doppler radar estimated rainfall on 23 September 2006.

Much of the rainfall that was recorded in this area fell within a span of about three hours. This resulted in significant flooding problems. Creeks and streams across Roane, Loudon, and Knox counties overflowed their banks. Numerous roads had to be closed, and people had to be evacuated from their homes. The heavy rain in Knoxville caused the start of the Tennessee-Marshall football game to be delayed due to flooding around Neyland Stadium.

The Environment

During the early morning hours of September 23, storms were moving east across northern portions of Middle Tennessee and central Kentucky. These storms moved into the Cumberland Plateau and blew down some trees in Scott County, TN and Whitley County, KY. Figure 2 is a water vapor satellite image overlaid with lightning strikes as the storms crossed this area. As the line of storms moved east and weakened, the cool outflow from these storms was channeled to the southwest down the Tennessee Valley. Before reaching Chattanooga, the outflow was stopped by a good southwest wind. This set up a boundary located across Rhea, Meigs, and McMinn counties. Figure 3 is an analysis of surface data showing the location of the boundary. To the north of this boundary, the air was quite cool, with temperatures in the 60s and an east wind at Knoxville airport (TYS). To the south of the boundary, the air was very warm and moist, with temperatures well into the 80s and dewpoints in the lower 70s. Chattanooga airport (CHA) was reporting a gusty south wind. The presence of this boundary set up a favored area for shower and thunderstorms to continuously develop.

Water vapor satellite image with lightning strikes at 9 am EDT
Figure 2. Water vapor satellite image with lightning strikes at 9 am EDT.

Surface analysis of theta-e (image and dashed lines), streamlines, and surface observations
Figure 3. Surface analysis of theta-e (image and dashed lines), streamlines, and surface observations.

The Flash Flood Event

The convergence of southerly winds and east to northeast winds along the boundary led to showers and thunderstorms developing in a concentrated area over Rhea, Meigs, and McMinn counties. Strong southwest winds in the middle and upper levels of the atmosphere steered these storms to the northeast. Figure 4 is a water vapor image overlaid with lightning strikes, which shows the line of storms oriented along the Tennessee Valley. The storms were continuously developing in the same place and training over the same areas. Figure 5 is a radar reflectivity image of the storms as they were producing heavy rain over Loudon and Knox counties.

Water vapor satellite image with lightning strikes at 2 pm EDT
Figure 4. Water vapor satellite image with lightning strikes at 2 pm EDT.

Reflectivity image from Morristown Doppler radar at 1:37 pm EDT
Figure 5. Reflectivity image from Morristown Doppler radar at 1:37 pm EDT.

Tornado and Severe Weather Threat

In addition to the flood threat, there was also a threat of tornadoes and severe thunderstorms. The Storm Prediction Center issued a Tornado Watch for much of Middle Tennessee and central Kentucky, which included the Chattanooga area. Figure 6 is a vertical profile of temperature, dewpoint, and winds from Nashville in the afternoon. It shows that there was a very unstable air to the south of the boundary. This instability, along with strong winds through the atmosphere, suggested that the environment was favorable for tornadoes. Fortunately, the tornadoes did not materialize, and the reports of straight-line wind damage were fairly isolated.

Sounding from Nashville at 2 pm EDT
Figure 6. Sounding from Nashville at 2 pm EDT.


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