April 8th Scott County Tornadoes

THE TORNADO TRACKED BETWEEN HOWARD DAY AND NORMA ROADS WHERE MANY
LARGE HARD WOOD TREES WERE SNAPPED OR UPROOTED ALONG WITH MINOR
ROOF DAMAGE TO ONE HOME. THE TORNADO WAS AT ITS STRONGEST JUST
NORTH OF HOWARD DAY ROAD WHERE LARGE HARD WOOD TREES WERE SNAPPED
IN A WOODED AREA. 

THE TORNADO STARTED ABOUT 2 MILES NORTH OF NORMA. IT BEGAN SOUTH 
OF STRAIGHT FORK ROAD RAN NEARLY PARALLEL TO THE ROAD FOR SOME 
LENGTH. NUMEROUS TREES WERE WERE UPROOTED NEAR THE START OF THIS 
PATH. AS THE TORNADO CONTINUED NORTHEAST IT DAMAGED A BARN AND THEN 
CAUSED SIGNIFICANT ROOF DAMAGE TO A HOME BEFORE CROSSING STRAIGHT 
FORK ROAD AND STRIKING A HOME. THE HOME HAD SIGNIFICANT ROOF DAMAGE 
AND A STAND ALONE GARAGE WAS COMPLETELY DESTROYED. A 14 THOUSAND 
POUND CAMPER TRAILER ADJACENT TO THE GARAGE WAS ROLLED ABOUT 30 
YARDS AWAY. FARTHER DOWN THE VALLEY, A DOUBLE WIDE MOBILE HOME WAS 
DESTROYED NEAR STRAIGHT FORK CHURCH ROAD. THE TORNADO THEN CROSSED 
OVER STRAIGHT FORK CHURCH ROAD AND DISSIPATED BEFORE CROSSING 
HIGHWAY 63. THE DAMAGE TO THE GARAGE, CAMPING TRAILER, AND MOBILE 
HOME WAS CONSISTENT WITH EF-2 WINDS OF UP TO 120 MPH.

THIS TORNADO WAS A NON-CONTINUOUS PATH. NO STRUCTURAL DAMAGE OCCURRED BUT
MOST OF THE DAMAGE WAS TO TREES. THIS DAMAGE WAS INACCESSIBLE BY AUTOMOBILE. 

The Enhanced Fujita (EF) Scale classifies tornadoes into the following categories:

EF0 Weak 65-85 mph	EF1 Moderate 86-110 mph	EF2 Significant 111-135 mph	EF3 Severe 136-165 mph	EF4 Extreme 166-200 mph	EF5 Catastrophic 200+ mph

Photos

Near Norma, TN Area

EF-2 Damage on Straight Fork Rd. (NWS)	EF-2 Damage on Howard Day Rd. (NWS)	EF-1 Damage on Straight Fork Rd. (NWS)	EF-1 Damage on Howard Day Rd. (NWS)

Environment

In the upper-levels, a negatively tilted shortwave trough moved into the area with a 60 to 80+ kt jet across the southern Plains/Mississippi Valley. This setup put East Tennessee in the exit region of the jet, which helped to increase upper divergence. Height falls also helped to steepen lapse rates and increase instability. This is all seen with quasi-geostrophic ascent in mid/upper-level differential vorticity advection. At the 700mb level, a mid-level dry slot is seen, which also helped to enhance lapse rates. Near the boundary layer, increasing moisture resulted with strong convergence along the Plateau.

Figure 1: 300mb Heights, Winds and Divergence	Figure 2: 700-400mb Differential Vorticity Advection	Figure 3: 700mb Heights, Winds, RH, and Temps

Figure 4: 300mb Heights, Winds and Divergence	Figure 5: 850mb Convergence

At the surface, a convergent boundary is clearly seen along the Cumberland Plateau within the surface wind pattern. This is also noted with increasing low-level moisture. These factors helped increase surface-based destabilization/forcing and added to an environment conducive for organized convection/supercells. It is also noted that boundary can help increase low-level vorticity, a key metric in tornadogenesis. 

Figure 6: Surface Winds, MSLP, Td's, and Temps	Figure 7: Surface Winds and Mixing Ratios	Figure 8: Surface METAR Plot

The aforementioned increasing instability is clearly seen below in MLCAPE with multiple other environmental parameters delineating the environment conducive to tornadogenesis. LCL heights were near 1,000m or less with research showing that anything less than 1,000m for this parameter being conducive to tornadoes. Also, the destabilization at the surface helped to increase low-level lapse rates, which provided favorable 0-3km (inflow) CAPE. Only about 50 J/kg is typically desired for tornadogenesis with 100+ J/kg being in place over the northern Plateau. Also, supercell composite parameter (SCP) values were 4 or higher across much of the area with a value of only 1 or greater being desired for supercellular convection. When looking at the significant tornado parameter (STP), values were barely reaching to 1 in the area of interest. This is still supportive of tornadogenesis but was not suggesting widespread and/or long-track tornadoes. 

Figure 9: MLCAPE	Figure 10: LCL Heights	Figure 11: 0-3km CAPE

Figure 12: 0-3km Lapse Rates	Figure 13: Supercell Composite Parameter	Figure 14: Significant Tornado Parameter

Radar Imagery

Below is a 4-panel of radar data depicting the supercellular storm of interest for this event. A clear hook echo can be seen in the reflectivity imagery, which is also the location of the mesocyclone in the storm-relative velocity image. With normalized rotation (NROT), a value of 1.00 or greater indicates strong rotation, which is clearly seen in the image. Correlation coefficient, a detection of the consistency of particle sizes, is commonly used as an indicator of debris lofted into the air, usually seen as lower values surrounded by higher values. As this image was from fairly early in the event, it is difficult to see debris. Plus, at this distance from the radar, the sampling is several thousand feet above the surface. 

Figure 15 (from top-left going clockwise): Base Reflectivity, Storm-Relative Velocity, Normalized Rotation, and Correlation Coefficient