Knox County (south Knoxville) Tornadoes
May 15, 2003
On May 15, 2003, two tornadoes touched down in south Knoxville as a large supercell (a thunderstorm with strong rotation) moved southeast across East Tennessee. This storm formed over the northern Cumberland Plateau during the middle of the afternoon, and entered Morristown’s County Warning Area in Morgan County around 3 p.m. EDT. Straight-line wind damage to several trailer homes and golfball size hail was reported near Sunbright at 3:43 p.m. The wind damage was determined to be the result of a phenomenon known as a ‘gustnado’.
(Radar reflectivity loop from 3:20 p.m. until 4:18 p.m. EDT)
This supercell storm continued to move southeast across Anderson County (producing quarter size hail and winds up to 70 mph) before entering Knox County around 4:45 p.m.. As it entered Knox County, the storm intensified and began to show indications of becoming tornadic. A Tornado Warning was issued at 4:48 p.m. for Knox county with radar reflectivity images at 4:50 p.m. revealing a well-defined 'hook echo' (a classic tornado signature). Hook echoes on the back edge of storms can indicate that rain is being wrapped around a strong rotation. The storm relative velocity images at this time also indicated developing rotation. Storm relative velocity images show the velocity of the wind (in relation to the radar) with the motion of the storm subtracted out. The green areas on these images indicate wind moving toward the Morristown radar with the red areas indicating wind moving away from the radar. When these two colors are bright and next to each other in a tight couplet, we can infer that there is rotation.
The first tornado was reported at 5:10 p.m. near the Lyons Bend area (traveling around a mile on the ground), while the second and final tornado was reported at 5:15 p.m. south of John Sevier Highway near Apache Trail. (tornado track map) This second tornado traveled 1.3 miles and lifted near the intersection of Martinmill Pike and Tipton Station Road. Radar reflectivity images continued to reveal indications of a 'hook echo' at 5:10 p.m. and 5:15 p.m.. Also, the storm relative velocity images at 5:10 p.m. and 5:15 p.m. showed signs of strong rotation (as evidenced by the bright red and green couplets). Both tornadoes were rated F1 on the Fujita tornado intensity scale. This supercell storm continued moving southeast into Blount and Sevier Counties (radar reflectivity image at 6:00 p.m.), with quarter to golfball hail reported in both counties. At 6:00 p.m., tennis ball size hail was reported with another storm over Loudon county.
(Radar relectivity loop from 4:30 p.m. to 5:30 p.m. EDT)
(Radar storm relative velocity loop from 4:30 p.m. until 5:30 p.m. EDT)
The weather pattern this day was not a ‘typical’ severe weather pattern for East Tennessee, since the upper-level winds were from the northwest instead of the more typical southwest direction. These northwest winds were observed between 10,000 and 20,000 feet above the ground at speeds of 20 to 40 mph. Between the surface and 4,000 feet, the winds were from the southwest around 20 mph. Typical upper-level (10,000 to 20,000 feet) wind speeds on tornadic days would be around 50 to 100 mph, with winds of 30 to 40 mph around 4,000 feet. At the surface, the winds were from the southwest and gusting to around 25 mph which is fairly typical of tornadic days, but the direction would most likely be out of the south instead of the southwest.
The turning of the wind directions from the surface to around 10,000 feet (from southwest to northwest) was important in causing this storm (and others on this day) to develop rotation. It was this rotation that helped lead to the two tornadoes in Knox County and the extremely large hail that was reported elsewhere. Other factors which helped severe thunderstorms to develop this day included a “short-wave” (or upper level storm) which moved across the Tennessee Valley. This short wave moved across east Tennessee during the afternoon, when peak daytime heating resulted in a moderately unstable atmosphere (primed for rapid thunderstorm development).