Severe Squall Line across East Tennessee
May 5-6, 1999
During the late afternoon and evening of May 5th 1999, severe thunderstorms developed along a cold front across the Mississippi River Valley of eastern Arkansas and western Tennessee. These severe thunderstorms formed into a squall line that moved across middle Tennessee where a tornado developed in Perry county that killed several people. This squall line then moved into east Tennessee around midnight, then decreased below severe limits as it moved into southwest Virginia and the mountains along the North Carolina and Tennessee border. There was widespread wind damage, mainly trees and power lines blown down, across east Tennessee south and west of a Tazewell to Morristown to Gatlinburg line. Also during the event, there was heavy rain across most of the Morristown County Warning Area (CWA), with most rainfall totals between one and three inches. This lead to minor urban and small stream flooding across parts of the CWA.
Even in the night, there was plenty of instability as measured by the Convective Available Potential Energy (CAPE or in the sounding B+) in the Nashville 00Z (8 p.m. EDT) sounding. Also contributing to strength of the Squall Line was the wind shear in the atmosphere. Wind shear can be described as a turning of the wind with height, an increase in wind speed with height or some combination of both. Again from the 00Z Nashville sounding, the hodograph shows the wind shear that was over Nashville at 8 p.m. EDT. The radar site at Hytop, Alabama (KHTX) also showed this wind shear in the VAD wind profile (VWP) from 11:04 p.m. EDT. The VWP indicated 20 knot winds at 2,000 feet from the southeast, 50 knot winds at 6,000 feet from the south southwest, and 60 knot winds at 12,000 feet from southwest. While the radar at Morristown, Tennessee (KMRX) also indicated some wind shear, it was not as strong as the Nashville sounding or the Hytop radar.
Nashville sounding at 8 p.m. EDT on May 5, 1999.
Nashville hodograph at 8 p.m. EDT on May 5, 1999.
Two important radar signatures that can be detected with some squall lines are the "bow echo" and the "line echo wave pattern (LEWP)". This squall line exhibited both of these features. At 12:45 a.m. EDT, the base reflectivity image from the Morristown radar, indicated a large bow echo from Morgan county through Roane county and into Meigs and Rhea counties. The apex of the bow echo (over western Roane county) is where the strongest winds are usually located. Strong to severe winds, a severe wind is 58 mph or greater, were occurring along the squall line at this time. With most squall lines, the strongest winds occur along the leading edge of the storm. This is commonly known as the "gust front", and may be the first part of the squall line that you experience. Finally in this reflectivity image, a severe storm was detected across western Knox county, ahead of the squall line. This storm was the reason why some people in the Metro Knoxville area had two rounds of severe weather.
KHTX VAD wind profile at 11:04 p.m. EDT on May 5, 1999.
The LEWP can be seen in the base reflectivity image from the Hytop radar at 12:32 a.m. EDT. The LEWP is a location where there is a particularly strong storm within the line. At this point, there is better inflow of the moist and unstable air into the line than any other place. This is the part of the line that would most likely produce hail and possibly even a tornado. The Hytop radar indicated some rotation in the storm in the 12:32 a.m. EDT Storm Relative Motion (SRM), but the lowest that the rotation ever reached was 8,000 feet. In this product, green indicates targets moving toward the radar (the white circle to the left of the "KHTX"), and red indicates targets moving away from the radar. From this image, we can detect a couplet of inbound (green) and outbound (red) targets over Marion county. This rotation within the storm is called a Mesocyclone.
KMRX base reflectivity image at 12:45 a.m. EDT on May 6, 1999.
KHTX base reflectivity image at 12:32 a.m. EDT on May 6, 1999.
KMRX storm relative velocity image at 12:32 a.m. EDT on May 6, 1999.
Click here to see the Morristown Radar loop (size: 762K) of the squall line. The loop runs from 11:27 p.m. EDT to 2:18 a.m. EDT.
Click here to see the Hytop Radar loop (size: 869K) of the squall line. The loop runs from 11:39 p.m. EDT to 2:47 a.m. EDT.