Spartanburg, SC Tornadoes

A potent October shortwave traversed the southern U.S. on October 23rd, bringing with it a round of severe weather to the Southeast U.S. The Storm Prediction Center outlook had a 5% contour for tornadoes (Figure 1; probability of a tornado within 25 miles of a given point), citing very strong low-level shear and weak to moderate buoyancy.

Fig.1: SPC tornado outlook issued at 16Z 10/23/2017.

Convection took on a linear storm mode in South Carolina, with a surge in a line heading toward Spartanburg (Figure 2). The NOAA/CIMSS ProbTor model increased to over 80% by 18:20Z, owing to increases in low and mid level azimuthal shear, and a jump in total lightning density, while residing in an environment with excellent 0-1km storm relative helicity (Figure 3).

Fig. 2: ProbSevere (all hazards) contours, NWS warnings, MRMS MergedReflectivity, GOES visible reflectance. The ProbTor was the highest hazard probability for this storm; 88% at this time. 

The first tornado reported was at 18:54Z, coincident with the first NWS tornado warning. The first tornado damaged permanent and mobile homes, downed power lines, and destroyed a horse barn. Later, the storm produced another tornado between downtown Spartanburg and I-26, snapping large trees, as ProbTor was about 45%. After dropping below 20%, ProbTor again increased to 40% at 19:36Z, when another tornado was reported near the North/South Carolina border.

Fig. 3: Time series of ProbTor (thick red line), with constituent predictors. NWS warnings and preliminary LSR reports are on bottom axes.

In this case, ProbTor may have been able to provide an earlier "heads up" to forecasters monitoring the storms for potential tornadic activity. ProbTor is best utilized in conjunction with current warning practices and interrogation of base radar data. If a velocity couplet is outside of the ProbTor object polygon, ProbTor will be underestimated, as may have been the case for a second tornadic storm in North Carolina on this day.